# Focal length Sentence Examples

They are placed at a distance apart less than the

**focal length**of a, so that the wires of the micrometer, which must be distinctly seen, are beyond b.Foucault, who employed a scale of equal bright and dark alternate parts; it was found to be proportional to the aperture and independent of the

**focal length**.If we suppose the diameter of the lens to be given (2R), and its

**focal length**f gradually to increase, the original differences of phase at the image of an infinitely distant luminous point diminish without limit.Throughout the operation of increasing the

**focal length**, the resolving power of the instrument, which depends only upon the aperture, remains unchanged; and we thus arrive at the rather startling conclusion that a telescope of any degree of resolving power might be constructed without an object-glass, if only there were no limit to the admissible**focal length**.The distance f i, which the actual

**focal length**must exceed, is given by d (f1 2 R2) x; so that f1 = 2 R2/X (1) Thus, if X = p j, R= i ?, we find f1= 800 inches.AdvertisementAs the minimum

**focal length**increases with the square of the aperture, a quite impracticable distance would be required to rival the resolving power of a modern telescope.Calculation shows that, if the aperture be s in., an achromatic lens has no sensible advantage if the

**focal length**be greater than about II in.If we suppose the

**focal length**to be 66 ft., a single lens is practically perfect up to an aperture of 1 .When parallel rays fall directly upon a spherical mirror the longitudinal aberration is only about one-eighth as great as for the most favourably shaped single lens of equal

**focal length**and aperture.A rotation of this amount should therefore be easily visible, but the limits of resolving power are being approached; and the conclusion is independent of the

**focal length**of the mirror, and of the employment of a telescope, provided of course that the reflected image is seen in focus, and that the full width of the mirror is utilized.AdvertisementOwing principally to differences in the length of the inch in various countries this method had great inconveniences, and now the unit is the refractive power of a lens whose

**focal length**is one metre.A lens of twice its strength has a refractive power of 2 D, and a

**focal length**of half a metre, and so on.The width of each of the portions aghc and acfe cut away from the lens was made slightly greater than the

**focal length**of lens X tangent of sun's greatest diameter.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.On the other hand it is not necessary to reset the telescope after each reversal of the segments.4 When Bessel ordered the Konigsberg heliometer, he was anxious to have the segments made to move in cylindrical slides, of which the radius should be equal to the

**focal length**of the object-glass.AdvertisementStruve also points out that by attaching a fine scale to the focusing slide of the eye-piece, and knowing the coefficient of expansion of the metal tube, the means would be provided for determining the absolute change of the

**focal length**of the object-glass at any time by the simple process of focusing on a double star.The amount of separation is very small, and depends on the thickness of the glass, the index of refraction and the

**focal length**of the telescope.The instrument has a

**focal length**of 54 ft.The collimator of a spectroscope should be detached, or moved so as to admit of the introduction of an auxiliary slit at a distance from the collimator lens equal to its

**focal length**.The sharpness of image in Kepler's telescope is very inferior to that of the Galilean instrument, so that when a high magnifying power is required it becomes essential to increase the

**focal length**.AdvertisementJames Bradley, on 27th December 1722, actually measured the diameter of Venus with a telescope whose objectglass had a

**focal length**of 2124 ft.The magnifying power obviously depends on the proportion of the

**focal length**of the object-lens to that of the eye-lens, that is, magnifying power where F is the**focal length**of the object-lens and e that of the eye-lens.But while an achromatic combination of o 60 and 0.102 alone will yield an objective whose

**focal length**is only 1.28 times the**focal length**of the negative or extra dense flint lens, the triple combination will be found to yield an objective whose**focal length**is 73 times as great as the**focal length**of the negative light flint lens.Hence impossibly deep curvatures would be required for such a triple objective of any normal

**focal length**.The magnifying power of the telescope is = Ff /ex, where F and f are respectively the focal lengths of the large and the small mirror, e the

**focal length**of the eye-piece, and x the distance between the principal foci of the two mirrors (=Ff in the diagram) when the instrument is in adjustment for viewing distant objects.AdvertisementEvery time, therefore, that a speculum is repolished, the future quality of the instrument is at stake; its

**focal length**will probably be altered, and thus the value of the constants of the micrometer also have to be redetermined.In this case the image is formed without secondary magnification and the

**focal length**is 25 ft.In this case the equivalent

**focal length**is 150 ft.For example, it is possible, with one thick lens in air, to achromatize the position of a focal plane of the magnitude of the

**focal length**.For infinitely distant objects the radius of the chromatic disk of confusion is proportional to the linear aperture, and independent of the

**focal length**(vide supra," Monochromatic Aberration of the Axis Point "); and since this disk becomes the less harmful with an increasing image of a given object, or with increasing**focal length**, it follows that the deterioration of the image is proportional to the ratio of the aperture to the**focal length**, i.e.In the neighbourhood of 550 pu the tangent to the curve is parallel to the axis of wave-lengths; and the

**focal length**varies least over a fairly large range of colour, therefore in this neighbourhood the colour union is at its best.This is effected by the power of accommodation of the eye, which can so alter the

**focal length**of its crystalline lens that images of objects at different distances can be produced rapidly and distinctly one after another upon the retina.The eye is strained in bringing its

**focal length**to the smallest possible amount, and when this strain is long continued it may cause pain.Since H' P = F 0, = y, from the

**focal length**of the simple microscope, the visual angle w' is given by tan w'/y=I/f'=V, (I) in which f', = H' F', is the image-side**focal length**(see Lens).Triplets are employed when the

**focal length**of the simple microscope was less than in.Let O01=y, O'01' =y', the focal distance of the image F I 'O' =A, and the image-side

**focal length**f l ', then the magnification M =y /y=o/,/1' (3) The distance A is called the " optical tube length."In immersion systems the object-side

**focal length**is greater than the imageside**focal length**.The image viewed through the eyepiece appears then to the observer under the angle w", and as with the single microscope tan w" = I /f 2 ' (4) where f' 2 is the image-side

**focal length**of the eyepiece.The lens nearer the eye, which has about the same

**focal length**as the collective lens, is distant from it by about its**focal length**.By the magnification of the objective is meant the ratio of the distance of distinct vision to the

**focal length**of the objective.The distance of the concave mirror from the stage plate is about equal to its

**focal length**.By a correct choice of the

**focal length**of the illuminating lens in relation to the**focal length**of the mirror, it is possible to choose the size of the image of the source of light so that the whole object-field is uniformly lighted.The size of these details in the image depends only on the magnification of the objective, M and can by appropriate choice of the

**focal length**of the objective be brought to the right value.It There are many methods for determining the

**focal length**of the objective.The same method can be used to determine the

**focal length**of the eyepiece.The

**focal length**of an objective can be more simply determined by placing an objective micrometer on the stage and reproducing on a screen some yards away by the objective which is to be examined.If the size of the image of a known interval of the objective micrometer is determined by an ordinary scale, and the distance of the image from the focal plane of the objective belonging to it is measured, then the

**focal length**can be calculated from the ratio y/y'=fl', in which y is the size of the object, y' that of the image, and xi' the distance of the image from the focal plane belonging to it.A convex lens has a

**focal length**of 150 mm.The larger blocking filter allows you to extend the

**focal length**without vignetting but does not enhance detail or lower the bandpass.A 100mm diameter, 500mm

**focal length**refracting telescope was used, equipped with a solar filter.He inserted a slip of metal, of variable breadth, at the focus of the telescope, and observed at what part it exactly covered the object under examination; knowing the

**focal length**of the telescope and the width of the slip at the point observed, he thence deduced the apparent angular breadth of the object.The

**focal length**of the objective and the distance between the optical centre of the lens and the webs are so arranged that images of the divisions are formed in the plane of the webs, and the pitch of the screw is such that one division of the scale corresponds with some whole number of revolutions of the screw.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).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).If your subject needs time to warm up, then start the session shooting with a longer

**focal length**.