Amateur Photographer & Cinematographer (1933)

December 27th, 1933 Ihi AMATEUR PHOTOGRAPHER p 6 CINEMATOGRAPHER o Supplementary Lenses Qj for Cameras and Enlargers A very useful accessory for the amateur photographer to possess is a set of sup¬ plementary lenses to be used in conjunction with the lens on his camera or enlarger to increase the scope of both. The following article tells how to select and employ these lenses to the best advantage. AT this time of year the owner of . /A a single-extension or fixed^ ^ focus camera sometimes wishes to photograph small articles, perhaps coins, or flowers, etc., but gives up the idea because his camera will not focus closer than six feet or more. Let him set to work and see what results can be obtained with the aid of a supplementary lens. Again, the addition of a magnify¬ ing lens to the lens on an enlarger allows a much greater degree of enlargement for a given distance between negative and paper, a valuable consideration where space is limited. It can only be applied, however, to horizontal enlargers, and those of the semi-automatic vertical type where the distances can be varied independently. In the latter type it is especially valuable. When a magnifying or positive lens is placed in front of a camera lens, the focal length of the combina¬ tion is less than that of the original, so that the infinity setting on the camera is now equivalent to a good extension with a shorter focus lens. Ordinary spectacle lenses are quite suitable, and it is possible to buy the circular type used in horn¬ rimmed spectacles for as little as . 6d. , and in a variety of focal lengths. The 40-mm. (i|-in.) size is larger than the lens on the average camera, but this is an advantage optically as only the centre of the added lens will then be used. An effective cap can be made of two pieces of 3/i6ths-in. wood, cutting in one piece a i-in. diameter hole and in the other a hole to just ht on the camera lens. The lens can be laid in a recess countersunk in one piece and the two then glued together. With the camera set at infinity, the distance measured from the added lens to the object to be photo¬ graphed is equal to the focal length of the added lens, whatever the focal length of the camera lens. The power of spectacle lenses is usually stated in diopters, which is simply the ratio between one metre and the focal length. The focal length in inches is obtained by dividing 39.4 by the number of diopters. Thus a lens of 3 diopters has a focal length of 13 inches, approximately. It is an advantage to calculate the size of image to be obtained. With a roll-film camera it is in¬ convenient to focus on an improvised screen, particularly if photographing several objects in succession, but the image must be brought within the limits of the film. At short distances no view-finder is accurate, and the best method is to “ centre ” the camera by actual measurement. Set it square with the object and the centre of the lens exactly opposite what is required to be in the centre of the film. The size of image is obtained by multiplying the size of the object by the ratio of the distances, lens to image and lens to object. Call the height of the image I, height of the object 0, focal length of camera lens Fc, and focal length of added lens Fa. Then, with the camera lens set at infinity, this comes to T ~ Fc Fa Keep all measurements in inches or centimetres. An example will make this clear. We wish to photograph a vase 5 in. high, using a 3^x2^ film camera with a 4j-in. lens. A convenient reduction would be to half size, that is, image 2\ in. high. Using the above formula we get 4.25 2.5 =5 X 4^ra From this the focal length of the added lens must be 8^ in., and if we are going to use a spectacle lens it will be 4.6 diopters, or the nearest we can get, probably 4.5. If we wish we can now obtain several lenses to give various reduc¬ tions. Thus lenses of 4.5, 2.25 and 1.5 diopters will reduce our object to half, quarter and one-sixth full size respectively. The distance from the object to the added lens would be 8f , 17^ and 26^ in. in these cases. A point to bear in mind is that the depth of focus is very small at these short distances, and a small stop, say f/22 or f/32, must be used if the nearest and farthest points of even a small object are to be in approximate focus. The ex¬ posure may be calculated from the actual camera stop used as the lens is set at infinity. When using an auxiliary lens on an enlarger it is necessary to know the focal length of the combined lenses, because it is useless having this less than the shortest distance it is possible to bring the lens to the negative. The approximate focal length of two combined positive lenses is found by dividing the product of the individual focal lengths by their sum. Thus if the enlarger lens is 5 in., and the added lens 15 in., the resultant focal length is 3| in., and to use this combination it must be possible to bring the enlarger lens to this distance from the negative. Using the two lenses in this ex¬ ample, it is possible to obtain a magnification of nine times with the lens only 37! in. from the paper. Its advantage is obvious when trying to get the most out of a small portion of a negative. Remember that the focal length of a combined lens is from a point, very difficult to determine, within the lens, and allowance should be made for this when noting the closest distance it can be brought to the negative. 579 9