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Light-Filters Lens Accessories Supplementary Lenses

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LENS ACCESSORIES SUPPLEMENTARY LENSES, LIGHT-FILTERS, PRISMS AND MIRRORS, POLARIZERS, LENS HOODS, SKY SHADES 118. Supplementary Lenses (Magnifiers).' Cameras with fixed focus will produce a sharply defined image of the object only if it is at a sufficient distance from the camera, the picture being necessarily on a small scale. It has long been the custom to remedy this defect by mounting additional positive supplementary lenses or magnifiers in front of the lens. The camera " seeing " only at a distance, like a long-sighted person, it was natural to correct it by the same means.

It must be emphasized in the first place that such and also the negative mentary lenses referred to later, must be cor rectly centred with the objective. This excludes the use of all universal spring mounts, which can be adapted to lens tubes or hoods of different diameters. The supplementary lens should be mounted in a small tube, which is either screwed into the inner thread of the lens hood or on to the outer mounting of the objective, or it can be simply slipped tightly over the barrel of the lens.

For example, suppose that it is desired to photograph a subject 1-5o metres (5 ft.) from a camera which is focussed on infinity or which gives sharp definition of objects at an infinite distance. It will then be sufficient to place a converging lens of 1-5o metres (5 ft.) focal length in front of the objective, that is, a glass of 0-67 diopters for a long-sighted person. The subject will then be situated in the focal plane of such a supplementary lens. Rays of light coming from any point on the object to the magnifier are then transmitted from the latter and arrive on the objective as a beam of almost parallel light, i.e. in the form of rays issuing from a very distant It will thus be seen that if a set of supplementary lenses with properly chosen focal lengths is available, a fixed focus camera can be used to photograph objects at all distances which are less than the minimum distance fixed by the limits of the camera.

With a camera which has a focussing adjust ment but is of too short extension to allow of a very near object being photographed, the use of suitably chosen supplementary lenses will further extend its scope. If, for example, the camera cannot be focussed on a shorter distance than 2 metres (6i ft.) and it is desired to photo graph an object placed at a distance of 1-5o metres (5 ft.), the camera should be set at 2 metres and on the front of the objective is placed a magnifying lens of power equal to the differ ence of the proximity ( di I) of the object and of the point on which the camera is focussed, i.e. in this case: r 0-67 — 0-50 = o-17 diopters.

r -5o 2 This corresponds with a focal length of = 6 metres (i9 ft.). o-i7 With subsequent adjustment of the focus on a convenient distance, it is possible to use any magnifier whose focal length comes within the limit thus calculated and that corresponding to the use of the camera focussed on infinity.

Due regard must, however, be paid to the fact that the supplementary lenses usually employed are uncorrected, and their use with an objective introduces aberrations into the resulting image (chromatic, spherical, astig matic, etc.), which are more pronounced the stronger the additional lens. For this reason, if there is a choice between two supplementary lenses of different focal lengths, it is advisable to choose the one of the greater focal length, and to focus the camera accordingly.

Another fact must be taken into account, viz. that the focal length of the combination is smaller than that of the objective itself, so that the latter will have a larger effective aperture than that indicated by the marking on the diaphragm. For example, suppose that the objective has a focal length of 15 cm. (6 in.), and that a magnifier of imetre (3 ft.) focal length is being used (6-7 and i diopters respectively), the resulting focal length will then be = 13 cm.

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