SUGGESTIONS FOR CHOOSING A LENS.
THE branch of optics having reference to the construction of Achromatic Lenses for the camera is almost a new division of science, and may be considered a fair field for much valuable inquiry in reference to the possibility of giving absolute perfection to the photographic lens. At present, the best that are constructed fall short of theoretical perfection.
The subject is a difficult one, and requires much study even to master its most simple application. The photographer, however, does not so much want to know how his lens is constructed as to know how to choose a good one, and to be made acquainted with the qualities his lens should possess. There are three qualities which should always be sought for in choosing a lens: they are freedom from colour in the refracted image, or achromaticity; perfect flatness of field, and distinctness of image over the whole length of the focussing glass, or correction of spherical aberration; and a perfect coincidence of the chemical and visual foci. The latter quality will be found to accompany a perfect adjustment of the two first qualities.
To ascertain that his lens is achromatic, or capable of giving a colourless image, the operator should examine the sides of the picture as refracted by it on to the ground glass screen; when, if the lens is not achromatic, a blue colour will border one side of every object, and an orange colour the other side. The focussing glass will also tell him if the lens possesses flatness of field, by examining on it the image of any straight line placed parallel with the camera; if this line throughout its whole length on the focussing glass is found to be sharp and distinct, the lens may be considered perfect in this quality.
To ascertain with precision that the lens possesses the third quality, a picture should be made with it on a prepared collodion plate, taking care that the surface of the collodion film coincides in its distance from the lens with the ground side of the focussing glass.
To obtain this distance with certainty, the prepared plate must be put, to obtain the impression, in the place of the focussing glass itself; and if, after the image has been developed, no perceptible difference can be detected in the distinctness of the image as seen on the ground glass, and that developed on the sensitive plate, it may be considered that the chemical and visual foci are coincident.
The above three qualities are most important, and when the lens fails in either of them it is a source of annoyance and uncertainty to the operator, for no amount of skill can make up for deficiencies of this kind. For general use, and to keep between the two extremes, I should recommend a double combination of 81/2 in. or 91/2 in. focal length, of such arrangement as will allow of the front meniscus lens being used alone for landscapes and long distance.
The double combination should also be capable of taking landscapes as well as portraits, by an arrangement for putting within the tube diaphragms, when required for the former purpose. In taking landscapes and bright objects, very much of the cleanness and distinctness of the picture will depend upon the aperture of the stop not being too large; it will be better rather to retard the action of the light than, by admitting too much, to produce indistinctness and confusion of image.
A double combination of lens 21/2 in. in diameter and 81/2 in. or 9 in. focal length will generally give a portrait 5 in. by 4 in., and with a diaphragm of 1/2in. diameter placed in the middle of tube, a landscape 7 in. by 5 1/2 in.; the front achromatic lens, about 15 in. focal length, when placed in the back of the tube, may give a landscape 10 in. by 8 1/2 in.
These are very general measurements, and not to be relied on as exact proportions, but are merely given as a guide; the double combination lens, without any stop to cut off light, can be used only for portrait, as the distinctness of image given by it is very limited in regard to different distances; in fact, a double lens with a large aperture can only give those objects distinct which are situated in one plane. The aperture for the admission of light will vary with the distance of the object; near objects will bear a larger aperture than distant objects. For street views and architectural subjects, a double combination is preferable to a single achromatic lens; as the former, if well constructed, will give upright lines to the margin of the picture, which a single achromatic lens of the same focal length cannot give.
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