Lenses sentence example
Concave Lenses are used in the treatment of myopia or shortsight.
In practice, however, it is not found that the presence either of a decidedly greenish-yellow colour or of numerous small bubbles interferes at all seriously with the successful use of the lenses for the majority of purposes, so that it is preferable to sacrifice the perfection of the glass in order to secure valuable optical properties.
All of them wore eerie red contact lenses.
The term Anatomy, originally employed in biological science to denote a description of the facts of structure revealed on cutting up an organism, whether with or without the aid of lenses for the purposes of magnification, is restricted in the present article, in accordance with a common modern use, to those facts of internal structure not concerned with the constitution of the individual cell, the structural unit of which the plant is composed.
The great reflecting telescope at Dorpat was manufactured by him, and so great was the skill he attained in the making of lenses for achromatic telescopes that, in a letter to Sir David Brewster, he expressed his willingness to furnish an achromatic glass of 18 in.Advertisement
To Brewster is due the merit of suggesting the use of lenses for the purpose of uniting the dissimilar pictures; and accordingly the lenticular stereoscope may fairly be said to be his invention.
The lateral eyes are in Limulus " compound eyes," that is to say, consist of many lenses placed close together; beneath each lens is a complex of protoplasmic cells, in which the optic nerve terminates.
The lateral eyes of Scorpio consist of groups of separate small lenses each with its ommatidium, but they do not form a continuous compound eye as in Limulus.
Plate glass is, nevertheless, considerably used for the cheaper forms of lenses, where the scattering of the light and loss of definition arising from these fine striae is not readily recognized.
If the glass is very badly annealed, the lenses made from it may fly to pieces during or of ter manufacture, but apart from such extreme cases the optical effects of internal strain are not readily observed except in large optical apparatus.Advertisement
From the large masses great lenses and mirrors may be produced, while the smaller pieces are used for the production of the disks and slabs of moderate size, in which the optical glass of commerce is usually supplied.
He also had some knowledge of the properties of concave and convex lenses and mirrors in forming images.
On the strength of similar arrangements of lenses and mirrors the invention of the camera obscura has also been claimed for Leonard Digges, the author of Pantometria (1571), who is said to have constructed a telescope from information given in a book of Bacon's experiments.
About the same time Francesco Maurolico, or Maurolycus, the eminent mathematician of Messina, in his Theore y nata de Lumine et Umbra, written in 1521, fully investigated the optical problems connected with vision and the passage of rays of light through small apertures with and without lenses, and made great advances in this direction over his predecessors.
In his later book, Dioptrice (1611), he fully discusses refraction and the use of lenses, showing the action of the double convex lens in the camera obscura, with the principles which regulate its use and the reason of the reversal of the image.Advertisement
He also demonstrates how enlarged images can be produced and projected on paper by using a concave lens at a suitable distance behind the convex, as in modern telephotographic lenses.
Concave lenses should never be used for work within the far point; but they may be used in all cases to improve distant vision, and in very short-sighted persons to remove the far point so as to enable fine work such as sewing or reading to be done at a convenient distance.
The setting of both lenses symmetrically would, under such circumstances, be very tedious.
The inverting telescope, composed of two, convex lenses, was a later invention; still it is not impossible that the original experiment was made with two convex lenses.
Until Newton's discovery of the different refrangibility of light of different colours, it was generally supposed that object-glasses of telescopes were subject to no other errors than those which arose from the spherical figure of their surfaces, and the efforts of opticians were chiefly directed to the construction of lenses of other forms of curvature.Advertisement
James Gregory, in his Optica Promota (1663), discusses the forms of images and objects produced by lenses and mirrors, and shows that when the surfaces of the lenses or mirrors are portions of spheres the images are curves concave towards the objective, but if the curves of the surfaces are conic sections the spherical aberration is corrected.
He was well aware of the failures of all attempts to perfect telescopes by employing lenses of various forms of curvature, and accordingly proposed the form of reflecting telescope which bears his name.
When in 1666 he made his discovery of the different refrangibility of light of different colours, he soon perceived that the faults of the refracting telescope were due much more to this cause than to the spherical figure of the lenses.
He argued that the different humours of the human eye so refract rays of light as to produce an image on the retina which is free from colour, and he reasonably argued that it might be possible to produce a like result by combining lenses composed of different refracting media.'
After devoting some time to the inquiry he found that by combining lenses formed of different kinds of glass the effect of the unequal refrangibility of light was corrected, and in 1733 he succeeded in constructing telescopes which exhibited objects free from colour.Advertisement
Like Gregory and Hall, he argued that, since the various humours of the human eye were so combined as to produce a perfect image, it should be possible by suitable combinations of lenses of different refracting media to construct a perfect object-glass.
Adopting a hypothetical law of the dispersion of differently coloured rays of light, he proved analytically the possibility of constructing an achromatic object-glass composed of lenses of glass and water.
Stereoscopic lenses can never be powerful systems, for the main idea is the recognition of the depth of objects, so that only systems having a sufficient depth of definition can be utilized.
Johann Zahn, in his Oculus Artificialis Teledioptricus (1685-1686), described and figured two forms of portable box cameras with lenses.
These prisms may be combined with concave lenses, which correct the myopia, or, since a concave lens may be considered as composed of two prisms united at their apices, the same effect may be obtained by making the distance between the centres of the concave lenses greater than that between the centres of the pupils.Advertisement
Until recently these spherical lenses were numbered in terms of their focal length, the inch being used as the unit.
All objects, therefore, which lie beyond a certain point (the conjugate focus of the dioptric system of the eye, the far point) are indistinctly seen; rays from them have not the necessary divergence to be focused in the retina, but may obtain it by the interposition of suitable concave lenses.
The weakest pair of concave lenses with which one can read clearly test types at a distance of 18 ft.
Such cases should be treated with convex lenses, which should be theoretically of such a strength as to fully correct the hypermetropia.
This condition may be cured completely, or greatly improved, by the use of lenses whose surfaces are segments of cylinders.Advertisement
They may be used either alone or in combination with spherical lenses.
Here, in order to fulfil the purposes of the previous models, the distance of the centres of the lenses from each other should only slightly exceed the tangent of sun's diameter X focal length of lenses.
Savary dwells on the difficulty both of procuring lenses sufficiently equal in focus and of accurately adjusting and centring them.
In this construction the lenses are much closer together and the diaphragm for the eye is much farther from the lenses than in Ramsden's eye-piece.
This lens is divided and mounted like a heliometer objectglass; the separation of the lenses produces the required double image, and is measured by a screw.Advertisement
He attacked Leon of Modena's anti-Kabbalistic treatises, and as a result of his conflict with the Venetian Rabbinate left Italy for Amsterdam, where, like Spinoza, he maintained himself by grinding lenses.
He appears soon to have found that single lenses of very short focus were preferable to the compound microscopes then in use; and it is clear from the discoveries he made with these that they must have been of very excellent quality.
His skill as a working lapidary was very great; and he prepared a number of lenses of garnet and other precious stones, which he preferred to the achromatic microscopes of the time.
The coloured borders seen in the images produced by simple lenses are due to dispersion.
Hall in 1733 constructed achromatic lenses.Advertisement
Such stones have been occasionally cut as lenses for microscopes, being recommended for such use by their high refractivity, weak dispersion and great hardness.
Essentially it consists in an optical system of lenses and mirrors, or mirrors alone, the upper part of which projects from cover, or from the deck of a submarine, while the observer looks into the lower end, receiving an image of the surrounding country or sea by reflection down a tube.
From the beginning of the 20th century, however, the practical introduction of submarine navigation brought about the development of new elaborate periscopes of great length and provided with an optical system of lenses, which were built into the structure of the submarine.
The two achromatic lenses, C and D, bring the rays to a focus on the plane surface of the large lens, E, forming an image there.
Leonhard Euler in 1747 had suggested that achromatism might be obtained by the combination of glass and water lenses.Advertisement
Kepler, who examined Porta's account of his concave and convex lenses by desire of his patron the emperor Rudolph, declared that it was perfectly unintelligible.
He fitted the lenses in a tube, in order to adjust and preserve their relative distances, and thus constructed his first telescope.
Knowing the theory of his instrument, and possessed of much practical skill, coupled with unwearied patience, he conquered the difficulties of grinding and polishing the lenses, and soon succeeded in producing telescopes of greatly increased power.
Kepler first explained the theory and some of the practical advantages of a telescope constructed of two convex lenses in his Catoptrics (1611).
William Gascoigne was the first who practically appreciated the chief advantages of the form of telescope suggested by Kepler, viz., the visibility of the image of a distant object simultaneously with that of a small material object placed in the common focus of the two lenses.
The triple object-glass, consisting of a combination of two convex lenses of crown glass with a concave flint lens between them, was introduced in 1765 by Peter, son of John Dollond, and many excellent telescopes of this kind were made by him.
Indeed, in its practical form the principle of the instrument has remained unchanged from the time of the Dollonds to the present day; and the history of its development may be summed up as consisting not in new optical discoveries but in utilizing new appliances for figuring and polishing, improved material for specula and lenses, more refined means of testing, and more perfect and convenient methods of mounting.
The only way in which the secondary spectrum can be reduced still further is by the employment of three lenses of three different sorts of glass, by which arrangement the secondary spectrum has been reduced in the case of the Cooke photo visual objective to about I/loth part of the usual amount, if the whole region of the visible spectrum is taken into account.
It is possible to construct a triple objective of two positive lenses enclosing between them one negative lens, the two former being made of the same glass.
Let it be supposed that two positive lenses of equal curvature powers are made out of these two glasses, then in order to represent the combined dispersion of the two together the two 0µ's for each spectral region may be added together to form 0'µ as in the line below, and then, on again expressing the partial z'µ in terms of L'µ (C to F) we get the new figures in the bottom row beneath the asterisks.
Hence it is clear that if the two positive lenses of equal curvature power of o 60 and 0.102 respectively are combined with a negative lens of light flint o 569, then a triple objective, having no secondary spectrum (at any rate with respect to the blue rays), may be obtained.
This case well illustrates the much closer approach to strict rationality of dispersion which is obtainable by using two different sorts of glass for the two positive lenses, even when one of them has a higher dispersive power than the glass used for the negative lens.
Shaping, polishing and figuring of specula are accomplished by methods and tools very similar to those employed in the construction of lenses.
Like every Jew, Spinoza had learned a handicraft; he was a grinder of lenses for optical instruments, and was thus enabled to earn an income sufficient for his modest wants.
His skill, indeed, was such that lenses of his making were much sought after, and those found in his cabinet after his death fetched a high price.
It contains only some small books, some engravings, a few lenses and the instruments to polish them."
If there be refraction at a collective spherical surface, or through a thin positive lens, 0' 2 will lie in front of O' 1 so long as the angle u2 is greater than u 1 (" under correction "); and conversely with a dispersive surface or lenses (" over correction ").
The aberrations can also be expressed by means of the "characteristic function " of the system and its differential coefficients, instead of by the radii, &c., of the lenses; these formulae are not immediately applicable, but give, however, the relation between the number of aberrations and the order.
The total aberration of two or more very thin lenses in contact, being the sum of the individual aberrations, can be zero.
This is also possible if the lenses have the same algebraic sign.
Of thin positive lenses with n= 1-5, four are necessary to correct spherical aberration of the third order.
In most cases, two thin lenses are combined, one of which has just so strong a positive aberration (" under-correction," vide supra) as the other a negative; the first must be a positive lens and the second a negative lens; the powers, however, may differ, so that the desired effect of the lens is maintained.
By one, and likewise by several, and even by an infinite number of thin lenses in contact, no more than two axis points can be reproduced without aberration of the third order.
All these rules are valid, inasmuch as the thicknesses and distances of the lenses are not to be taken into account.
In practice, however, it is often more useful to avoid the second condition by making the lenses have contact, i.e.
Instead of making d4 vanish, a certain value can be assigned to it which will produce, by the addition of the two lenses, any desired chromatic deviation, e.g.
For example, the condition for achromatism (4) for two thin lenses in contact is fulfilled in only one part of the spectrum, since do /dn 1 varies within the spectrum.
Should there be in two lenses in contact the same focal lengths for three colours a, b, and c, i.e.
Archer overcame the difficulty by constructing fluid lenses between glass walls.
In using glasses not having proportional dispersion, the deviation of a third colour can be eliminated by two lenses, if an interval be allowed between them; or by three lenses in contact, which may not all consist of the old glasses.
If a collective system be corrected for the axis point for a definite wave-length, then, on account of the greater dispersion in the negative components - the flint glasses; - over-correction will arise for the shorter wavelengths (this being the error of the negative components), and under-correction for the longer wave-lengths (the error of crown glass lenses preponderating in the red).
Such lenses are often seen formed by drops of fat floating on the surface of hot water, soup or gravy.
Such microscopic examination requires the use of the finest lenses and the application of various staining methods.
In consequence of its low refractive and dispersive power, colourless pellucid fluor-spar is valuable in the construction of apochromatic lenses, but this variety is rare.
In the Copepoda the median eye may undergo considerable elaboration, and refracting lenses and other accessory structures may be developed in connexion with it.
Clear transparent rock-crystal is used for optical purposes and spectacle lenses.
Fused quartz has recently been used for the construction of lenses and laboratory vessels, or it may be drawn out into the finest elastic fibres and used for suspending mirrors, &c., in physical apparatus.
In 1819 he was nominated a commissioner of lighthouses, for which he was the first to construct compound lenses as substitutes for mirrors.
In a small commonplace book, bearing on the seventh page the date of January 1663/1664, there are several articles on angular sections, and the squaring of curves and " crooked lines that may be squared," several calculations about musical notes, geometrical propositions from Francis Vieta and Frans van Schooten, annotations out of Wallis's Arithmetic of Infinities, together with observations on refraction, on the grinding of " spherical optic glasses," on the errors of lenses and the method of rectifying them, and on the extraction of all kinds of roots, particularly those " in affected powers."
It is known that he purchased prisms and lenses on two or three several occasions, and also chemicals and a furnace, apparently for chemical experiments; but he also employed part of his time on the theory of fluxions and other branches of pure mathematics.
For it is easy to see that if, by means of lenses of rock-salt or mirrors, we focused all or nearly all the rays from a small surface on to another surface of equal area, this would not raise the temperature of the second surface above that of the first; and we could not obtain a greater concentration of rays from a large heated surface, since we could not have all parts of the surface simultaneously in focus.
Lateral eyes also are often present which are monostichous with aggregated lenses (Limulus)or with isolated lenses(Scorpio), or are diplostichous with simple lens (Pedipalpi, Araneae, &c.).
Several large object lenses, engraven with his name, are preserved at Florence.
Light from an extended source passes after polarization through two convex systems of lenses, between which the crystalline plate is placed, and is then received in an eyepiece furnished with an analyser.
All the rays through a given point in the first principal focal plane of the anterior system of lenses traverse the plate as a parallel beam and reunite at the corresponding point of the second focal plane of the posterior system, each in its passage being divided into two by the plate having a given relative retardation.
The actual details of the systems of lenses depend upon the object for which the polariscope is intended, and are given for some of the principal types of instruments in Th.
Two lenses only - a planoconvex and a plano-concave - were needed for the composition of each, and this simple principle is that still employed in the construction of opera-glasses.
Convex glass lenses were first generally used to assist ordinary vision as " spectacles "; and not only were spectacle-makers the first to produce glass magnifiers (or simple microscopes), but by them also the telescope and the compound microscope were first invented.
Antony van Leeuwenhoek appears to be the first to succeed in grinding and polishing lenses of such short focus and perfect figure as to render the simple microscope a better instrument for most purposes than any compound microscope then constructed.
Robert Hooke shaped the minutest of the lenses with which he made many of the discoveries recorded in his Micrographia from small glass globules made by fusing the ends of threads of spun glass; and the same method was employed by the Italian Father Di Torre.
Such lenses are termed " lenses for direct vision."
This vignetting can be observed in all lenses.
In a lens with two bounding surfaces in air there is a loss of about 9%; and in a lens system consisting of two separated lenses, i.e.
Losses due to absorption are almost zero when the lenses are very thin, as with lenses of small diameter.
When the pupil regulates the aperture of the rays producing the image the aberrations of the ordinary lenses increase considerably with the magnification, or, what amounts to the same thing, with the increase in the curvature of the surfaces.
For lenses of short focus the diameter of the pupil is too large, and diaphragms must be employed which strongly diminish the aperture of the pencils, and so reduce the errors, but with a falling off of illumination.
In this manner lenses of short focus can be produced having lower curvatures than glass lenses necessitate.
Improvements in glass lenses, however, have rendered further experiments with precious stones unnecessary.
Wollaston altered this by taking two piano-convex lenses, placing the plane surfaces towards each other and employing a diaphragm between the two parts (fig 5).
Doublets, &'c. - To remove the errors which the above lenses showed, particularly when very short focal lengths were in question, lens combinations were adopted.
This construction was further improved (I) by introducing a diaphragm between the two lenses; (2) by altering the distance between the two lenses; and (3) by splitting the lower lens into two lenses.
Axial aberration is reduced by distributing the refraction between two lenses; and by placing the two lenses farther apart the errors of the pencils of rays proceeding from points lying outside the axis are reduced.
The Wilson has a greater distance between the lenses, and also a reduction of the chromatic difference of magnification, but compared with the Fraunhofer it is at a disadvantage with regard to the size of the free working distance, i.e.
These anastigmatic lenses, which are manufactured up to X 40, are chromatically and spherically corrected, and for a middle diaphragm the errors of lateral pencils, distortion, astigmatism and coma are eliminated.
Gullstrand showed how to correct these lenses for direct vision, i.e.
Lenses of various magnifications can be adapted to the carrier and moved about over the stage.
Although we now know how the errors of lenses may be corrected, and how the simple microscope may be improved, this instrument remains with relatively feeble magnification, and to obtain stronger magnifications the compound form is necessary.
The arrangement of two lenses so that small objects can be seen magnified followed soon after the discovery of the telescope.
A series of objectives with short focal lengths are available, which permit the placing of a liquid between the cover-slip and the front lens of the objective; such lenses are known as " immersion systems "; objectives bounded on both sides by air are called " dry systems."
Since glass does not transmit the ultra-violet light, quartz is used, but such lenses can only be spherically corrected and not chromatically.
But, owing to the various partial reflections which the illuminating cone of rays undergoes when traversing the surfaces of the lenses, a portion of the light comes again into the preparation, and into the eye of the observer, thus veiling the image.
The spherical aberrations, however, can be overcome, or at least so diminished that they are quite harmless, by forming appropriate combinations of lenses.
Correction of the spherical aberration in strong systems with very large aperture can not be brought about by means of a single combination of two lenses, but several partial systems are necessary.
Good apochromats often have as many as twelve lenses, whilst systems of simpler construction are only achromatic, and are therefore called " achromats."
Both lenses together form the exit-pupil of the objective behind the eyelens, so that this image, the exit-pupil of the total system or the Ramsden circle, is accessible to the eye of the observer.
The sine of this angle is the numerical aperture for dry lenses.
Three SLD glass elements are employed for effective compensation of color aberration, which is a common problem with super-wide angle lenses.
But it was difficult to make small high power achromatic lenses.
To ensure peace of mind, D&A includes aftercare in the fee for their Contact Lenses By Post package.
Forty Shades of Blue is a textbook Sundance Film Festival favorite - all long lenses, grainy stock and arduously earnest indie angst.
I have astigmatism - can I wear soft lenses?
Some people may not have clear vision wearing soft contact lenses due to certain eye conditions such as severe astigmatism.
Linford Christie sporting the now legendary Puma contact lenses prior to his Olympic bid - the ultimate in cool sportswear items.
Some frame styles are not suitable for bifocal lenses and, on the website, these do not offer the bifocal option.
The use of mirrors instead of lenses was another way to avoid chromatic aberration.
Newton was led by this reasoning to the erroneous conclusion that telescopes using refracting lenses would always suffer chromatic aberration.
Find out how to get started wearing contact lenses.
Before you browse - check out this page - about buying contact lenses online.
Dispensing opticians are also able to fit contact lenses after undergoing further specialist training.
D&A had been supplying contact lenses from The Channel Islands.
This is what happens when you get contact lenses and decide to buy new eyeshadow.
No glasses or hard contact lenses can be worn.
Soft and Gas permeable contact lenses start at £ 49 per pair, with daily disposables at £ 1 per pair.
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We carry an extensive collection of special effects contact lenses including vampire, halloween and glow in the dark contact lenses.
Soft and Gas Permeable contact lenses start at £ 49 per pair, with daily disposables at £ 1 per pair.
Using these make-up tips will help you minimize any distortion caused by your lenses.
Wide angle adaptors can be screwed onto the front of a very few lenses, and have the opposite effect of a range extender.
Great for all general applications they feature fully multicoated lenses, good close focus and twist-type large diameter rubber eyecups for viewing comfort.
We have a wide range of stylish quality eyewear (approx 1000 different frame styles in each store ), contact lenses and accessories.
We will start updating the firmware of these lenses free of charge in the near future.
Its newly designed optical system consists of 17 elements in 13 groups, including one fluorite and two UD lenses.
Affairs, rivalries, & enmities keep the pace frantic, along with dropped lines, missed cues, and lost contact lenses.
Get your clip-on prescription lenses for your ski goggles made up by us.
Remember contact lenses are not suitable for wear when swimming so ask your optician about prescription goggles and diving masks.
Fit lenses to cameras and connect auto iris lenses.
Superb fully coated violet tinted lenses give absolute clarity right to the edge of the sight picture.
Virtual images are often used, for example, in magnifying lenses, where the image must be viable from different angles.
Unlike interchangeable lenses, this one's fixed to the camera for ever.
The lens part of the DMC-FX01 comprises of seven elements in six groups, incorporating four aspherical lenses to generate high optical performance.
Two wheels for interchangeable dichroic color and gobo selections, and three available lenses make the luminaire completely user-configurable.
A ' dissolving pair ', they were shown using magic lanterns with two or three lenses.
Anyone who wears glasses or contact lenses but is unable to read standard newsprint is partially sighted.
They are equally suitable for use in both public and private environments with lenses made of stabilized, vandal resistant polycarbonate.
They know this and respect the lenses of the eyes, however populous the assembly be.
I have presbyopia - can I wear contact lenses?
This proved a really satisfying exercise, especially as out came the Sun, so did the hand lenses and the humorous quips.
Views out to the peripheral retina may be obtained with some lenses.
Using manual iris lenses instead can make a significant saving on a system.
This is why astronomers are developping multi-field spectroscopy where integral field units replace the lenses or slits in a multi-object spectrograph.
Sometimes I use goggles with clear lenses in Scotland in winter to protect from the stinging spindrift.
If anyone is thinking about lenses but feels squeamish, I would encourage them to go for it.
This is because a single strobe usually does not have enough spread to cover lenses wider than a hundred degrees.
In ray tracings, lenses are always reduced to simple lines, which I've marked L in the drawings.
You can wear glasses or contact lenses and still use a lightbox or a light visor.
The system is suitable for most 35mm SLR still and digital stills cameras using wide-angle or standard lenses.
It is beyond doubt that Huygens independently discovered that an object placed in the common focus of the two lenses of a Kepler telescope appears as distinct and well-defined as the 3 Delambre, Hist.
But the difficulties interposed by spherical and chromatic aberration had arrested progress in that direction until, in 1655, Huygens, working with his brother Constantijn, hit upon a new method of grinding and polishing lenses.
Hardness and Chemical Stability.-These properties contribute to the durability of lenses, and are specially desirable in the outer members of lens combinations which are likely to be subjected to frequent handling or are exposed to the weather.
Again, to obviate the necessity for excessive convergence of the eyes so common in hypermetropia, the centre of the pupil should be placed outside the centre of the corrective convex lenses; these will then act as prisms with their bases inwards.
Struve (Description de l'Observatoire ' Central de Pulkowa, pp. 196, 197) adds a few remarks to Steinheil's description, in which he states that the images have not all desirable precision - a fault perhaps inevitable in all micrometers with divided lenses, and which is probably in this case aggravated by the fact that the rays falling upon the divided lens have considerable convergence.
By introducing the concave grating which (see Diffraction Of Light, § 8) allows US to dispense with all lenses, Rowland produced a revolution in spectroscopic measurement.
Early in 1757 he succeeded in producing refraction without colour by the aid of glass and water lenses, and a few months later he made a successful attempt to get the same result by a combination of glasses of different qualities (see Telescope).
The extension of the image away from the axis or size of field available for covering a photographic plate with fair definition is a function in the first place of the ratio between focal length and aperture, the longer focus having the greater relative or angular covering power, and in the second a function of the curvatures of the lenses, in the sense that the objective must be free from coma at the foci of oblique pencils or must fulfil the sine condition (see Aberration).
The earlier arrangement of two lenses of the Huygenian eye-piece (see Microscope) having foci with ratio of 3 to I, gives a fairly large flat field of view approximately free from distortion of tangential lines and from coma, while the Mittenzw ?
In the Ramsden eyepiece (see Microscope) the focal lengths of the two piano-convex lenses are equal, and their convexities are turned towards one another.
In optical systems composed of lenses, the position, magnitude and errors of the image depend upon the refractive indices of the glass employed (see Lens, and above, " Monochromatic Aberration ").
Since the index of refraction varies with the colour or wave length of the light (see Dispersion), it follows that a system of lenses (uncorrected) projects images of different colours in somewhat different places and sizes and with different aberrations; i.e.
Very often such stereoscopic lenses, owing to faulty construction, give a false idea of space, ignoring the errors which are due to the alteration of the inter-pupillary distance and the visual angles belonging to the principal rays at the object-side (see Binocular Instruments).
By compounding two lenses or lens systems separated by a definite interval, a system is obtained having a focal length considerably less than the focal lengths of the separate systems. If f and f' be the focal lengths of the combination, and f2, f2 the focal lengths of the two components, and A the distance between the inner foci of the components, then f = - f,f2/4, f' =fi f27 0 (see Lens).
Eyeglasses with shatterproof lenses may protect the eyes, but most do n't seal out the wind which makes your eyes water.
Polycarbonate is a clear and relatively tough plastic used to make shatterproof windows, lenses and even helmets.
Doublet lenses are used to produce a higher quality image than singlet lenses of which they easily out perform, even in monochromatic light.
See FAQ 's for further information on all our optional extras including transition lenses and sunglass tints.
Special toric lenses are required to correct your vision.
In ray tracings, lenses are always reduced to simple lines, which I 've marked L in the drawings.
These grooves were roughly triangular in cross section with the " peaks " forming little prismatic lenses.
Eyeshadow in contact lenses can cause extreme irritation-if eyeshadow should get in one of your lenses, remove and clean the lens immediately.
Colored contact lenses are one of the hottest trends in celebrity and fashion-forward style.
While colored lenses have been around for awhile, today they are more comfortable than ever, manufactured from state-of-the-art materials, and available in a wide range of styles and colors.
What's also great about the new breed of colored contact lenses is the daily disposable colors.
The lenses come packaged in a sterile blister pack ready to go.
These types of lenses either intensify your color, change your color completely in a dramatic fashion, or brighten the eye color with sparkles or luminescence.
This means a visit to an eye doctor, an eye exam, and a proper fitting for contact lenses.
You should also be wary of online merchants who sell contact lenses without telling you the brand or manufacturer, the lens parameters, and material they're made from.
Wide-angle lenses may give you a larger field of view, but it will sacrifice clarity.
Multi-lens coating is best and will help you avoid fogged up lenses.
The cameras paparazzi and pro-photographers use allow you to switch lenses and pop on other attachments.
You will mainly be concerned about the lenses and keeping them free of dust.
Whether the improvements are basic like quality or lenses or feature-rich, digital cameras don't need to be difficult to learn.
Polarized lenses filter the waves of light by absorbing some of the reflected glare while allowing other light waves to pass through them.
The first polarized lenses were made of a polarizing film that was sandwiched between two flat sheets of glass.
Because glass lenses are so heavy, however, the advent of plastic and polycarbonate materials, which are lighter and thinner, made the glass lenses less popular.
This problem has been solved with modern lenses, as the plastic can be melted and poured into a mold in which the polarizing film has been suspended.
Polycarbonate lenses are created in a different manner, since polycarbonate lenses are injection molded and the heat from the process would destroy the polarizing film.
For polycarbonate lenses, the polarizing film is applied to the front of the lens and covered with a scratch-resistant coating.
This process means that polarized polycarbonate lenses are the thinnest and lightest polarized lenses available.
All polarized lenses have some color, as it is possible to polarize light only when the film is colored.
Depending on the material used, polarized lenses are available in a variety of colors.
Select your polarized lenses carefully, as lens color does have an impact on what you see.
Melanin lenses are believed to help protect against macular degeneration.
Melanin lenses are made with a synthetic form of this pigment that never loses its potency.
Polarized lenses can hinder your ability to read an LCD screen.
Polarized lenses can create an effect called cross-hatching that makes car windows (made of tempered glass) appear to have a pattern of lines, splotches, or large dots in the glass.
Funkier types could wear colored lenses in unusual shapes.
Polarized lenses cut down on glare and create contrast so that you're able to react more quickly.
Optical shops often have some you can try on to really compare polarized and non-polarized lenses.
Tennis players benefit from teal lenses; skiers from vermillion.
There are clear lenses just for that purpose.
The best part is, you can change your lenses.
So if you're a man or woman of many athletic talents, you won't have to have a new pair for every sport; just stock up on lenses of many colors.
The way you get that is by using orange, yellow, or vermillion lenses.
If you're trying to see through a haze or low light, opt for the orange or yellow lenses.
While gray lenses won't give you more contrast between your target and its background, they are the best for shooting in bright sunlight.
Even better, take it up a notch by choosing polarized lenses.
For all activities, aside from regular daily wear, polycarbonate lenses are your best bet.
For daily wear, plastic lenses should suffice, unless you've only got one good eye.
Long faces need lenses with height whereas shorter faces can benefit from the elongating effect of shorter frames.
For driving, you may want to consider polarized lenses that are dark gray or amber.
Call around for specials that offer a discount on the second pair of glasses and simply add a tint to your second pair of prescription lenses.
Check Target or small boutiques for colored lenses, varying shapes, and a wide array of frame prints, metals, and more.
Shop for polycarbonate lenses for everything from motorcycle riding to target practice, and on to golf.
Of course your first impulse may be to visit an optical shop if you're in need of prescription lenses.
There are so many variations on camera types and lenses that a single article could not possibly cover them.
The main issue for eyeglass wearers is that beautiful eyes sometimes get hidden behind the lenses and frames.
Also, what are the best colors of contact lenses should I use?
One that is optically correct has the benefit of simulating the effect of a prescription glasses or contact lenses.
The optical lenses are available in a variety of magnification levels.
For those who rely on prescription glasses or contact lenses, this is indeed a miracle!
Optically perfect lenses are thick, distortion-free and offer magnification at a high strength.
If you choose to be extra bold, add special effect contact lenses to maximize the wicked look.
I have sensitive eyes with contact lenses that cling onto every little flake and irritant, so an eye shadow that knows what it's doing is important to me.
Invest in dramatic contact lenses, fangs and prosthetics to look authentically frightening.
As a result, the green-eyed Kristen Stewart had to wear brown contact lenses.
When getting ready to apply the product, be sure that you remove your contact lenses, if you wear them, and clean your face from all oil and makeup.
Most DSLR cameras have detachable lenses and generally take better pictures compared to point and shoots.
The series features professional quality lenses and video capability.
If the camera is a pro or semi-pro camera with exchangeable lenses, reviewers must ask themselves about the lens mount.
Dpreview is a popular digital camera site that rates all types of digital cameras (from point and shoot to pro) and accessories such as flashes and lenses.
Learn how to select the right digital camera and lenses for your photography projects, get the most from your batteries, and find ideas for experimental photos.
In addition to being able to handle a wide range of situations, digital SLR cameras can accommodate hundreds of different lenses and flash options.
This is impossible to achieve with compact cameras which feature fixed lenses.
In addition, take time to test out multiple lenses, different lighting equipment and other photo accoutrements that will help you improve your shots.
In addition to securing a high-end DSLR camera, you should also consider investing in a tripod, lighting gear and a variety of different lenses.
This camera allows for interchangeable lenses, so you can refine your equipment to meet the type of shooting you are going to be doing.
The first was that more serious photographers were getting bored and wanted to have more control over their images and lenses again.
However, in time more lenses will become available and the prices will fall.
Another way to express your creativity is to employ photo equipment such as wide-angle or fish-eye lenses.
This features gives the photographer complete creative control and access to high-quality lenses for more precise, sharper images.
These cameras often have a fixed, single lens, but may have attachable extra lenses to extend or change the angle of view.
It is fast, responsive, has 24 megapixels and uses lenses from Nikon's F-mount lens collection.
Since larger, heavier lenses require more stability than shorter lenses, you should also pack a tripod before heading out to photograph the moon.
With so many lenses, f-stops and ISO combinations to choose from, it may take a while to find the magic formula that works for your individual shooting situation.
In effort to illustrate this confidence, the company offers services and support for its cameras, lenses and other photo accessories.
In addition, if you own a SLR, repairing it will help you preserve the investment you made in lenses or accessories.
Digital SLR's offer a multitude of features, including attachable lenses, instant shooting, and expandable store.
For serious photographers, the ability to switch lenses, use external flash units and manipulate manual settings makes it imperative to own a digital SLR with all the bells and whistles.
Compared to previous Rebel lenses, the T2i lens has increased sharpness and decreased chromatic aberration.
Although it sounds like techno-jargon, its usage is literal in the sense that devices like the Canon Rebel XT 8 MP digital SLR camera use a totally electronic form of communication between the lenses and the camera itself.
Like the EOS Rebel T21, the EOS Rebel XT is compatible with the entire line of EF-mount lenses, even across other brands.
Like other Rebels, it's easy to use and takes great photographs, but if you long to turn your great images into true works of art, consider adding a few specialty lenses to your gear stash.
However, if you are an advanced photographer working with a SLR camera with interchangeable lenses, then you could opt to add a specialized macro lens and manipulate light sources to snap super shots.
In most cases, you'll need to have your own gear for each job including cameras, waterproof housings, lenses, diving gear, light sources and any specialized equipment needed for underwater photo shoots.
The lenses are available in different tints, which are appropriate for different types of work environments.
Another helpful feature of this style of eyewear is the lenses wrap around the face.
These lenses are a good choice for outdoor work environments where reducing the level of glare is an important consideration.
Choosing blue lenses on safety goggles means that the level of glare is reduced.
Recommended for outdoor use, this tint choice means that the level of bright light penetrating the lenses is reduced.
If the lenses are mirrored, this feature cuts the amount of light passing through them.
Light blue lenses on safety goggles are the right choice for jobs where the work space is being lit by fluorescent or halogen lighting.
When safety goggles are needed for outside use, pick up a pair with amber lenses.
Not all safety goggles with tinted lenses provide UV protection, and consumers who are planning to use them outside should ensure that the style they are considering has this important feature before making a buying decision.
The lenses should be cleaned daily using a solution recommended by the manufacturer.
Not only can scratches on the safety goggles interfere with the wearer's vision, but they can weaken the lenses.
Prescription safety lenses are thicker than non-prescription ones, and some people consider the added thickness to be extra protection.
Prescription goggles have stronger lenses and stronger frames than eyewear that isn't earmarked for safety.
The most popular prescription safety goggles have lenses made from polycarbonate.
An anti-reflective coating added to the lenses can affect the impact resistance of any type of protective lens, necessitating additional testing of the lens after the coating is applied.
Ask your optician or optometrist to confirm that the lenses have been tested for impact resistance.
High velocity lenses don't break, chip or crack when hit with a quarter-inch diameter steel ball moving at a speed of 150 feet per second.
If your trade or hobby is outdoors at least part of the time, consider getting tinted prescription lenses with anti-glare protection.
They prescribe corrective lenses and offer other vision-related treatments.
Their license limits them to making lenses.
Sadly, you won't be able to pop out the plain lenses in order to have your prescription lenses installed.
All have smoke colored lenses and rhinestones.
Gray frames with gray lenses give this pair a monochrome look any retro girl will love.
You can have your prescription lenses tinted and inserted.
If you're fair with light hair, opt for lighter colors like the gray frame/gray lenses pair listed above.
Another advantage of these lenses is that you are able to wear them for an extended amount of time.
In fact, it is possible for you to wear these lenses for up to six consecutive nights, and then replace with a fresh pair.
If you would like a greater depth of information about Acuvue contact lenses, your best bet is to visit the official website.
This is a great benefit because it might help you decide, almost instantaneously, if this is the right pair of contact lenses for you.
All in all, these contact lenses might be a great choice for you if you suffer from presbyopia but don't like either the way reading glasses look on you, or are not interested in the limited variety they come in.
In short, Acuvue contact lenses provide you with yet another viable option from which you can choose.
You can get black lenses that are shaped like regular contact lenses or purchase sclera lenses in black.
As with most things online, you won't find yourself limited to certain times during the year when you want to shop for black lenses.
You will need a prescription from your eye care provider to order, even if you do not need corrective power in your lenses.
There are a few types of black contact lenses.
If you are familiar with clear or tinted contact lenses designed for everyday wear, these are similar in coverage but are an opaque black.
You can purchase custom made all black contact lenses or you can just buy them pre-made whether you need a prescription or not.
Please note that it can sometimes take up to two months to get handmade, custom contact lenses and plan accordingly.
You may want to speak with your eye care provider about the water content and material of a specific lens before placing your order, especially if you have dry eyes or often experience discomfort when wearing contact lenses.
Color VUE Lens has the black dolly lenses that will make your eyes look wide open.
Vision Direct has both the Black Out version and the sclera version of black contact lenses.
Coastal Contacts and Sanders Contacts have Black Out lenses, too.
Caring for your black lenses is not much different from caring for your regular contact lenses.
Always wash your hands before taking the lenses out or putting them in to cut down on the risk for eye infection.
If you experience any unexplained discomfort while wearing your lenses or after their removal, seek the help of an eye care professional.
Wearing black contact lenses can be fun!
Just be sure to do your research ahead of time so that when your lenses arrive, you can wear them comfortably.
You can also get deals on photochromic lenses.
Any extras you wish to add, such as tints, lighter-weight lenses, and protective coatings will cost extra.
America's Best has an eye care club that saves you money on exams and contact lenses if you're interested.
This may be especially helpful for those who wear glasses sometimes but also enjoy wearing contact lenses.
That gets you five free eye exams, glaucoma screenings, and dilations over the next three years, 10 percent off on eyeglasses, and discounted contact lenses for the life of your membership.
Most Anarchy glasses have polarized lenses, which are designed to reduce glare.
Scratch resistant coatings, gradient lenses, and mirror or flash coatings are available in a variety of styles.
Impact resistant polycarbonate lenses and 100 percent UV protection are also important features of Anarchy glasses.
In this pair, you'll get a sleek plastic tortoiseshell frame and brown polycarbonate lenses for $46.50.