It has also been proposed (Lehmann, 1878; Boissonas, 190o, etc.) to use with large lenses a diaphragm with two apertures (Fig. 43) to obtain a single image in which the doubling of certain outlines would suggest some idea of relief.
The painter or draughtsman, observing his model with two eyes, synthesizes the two views. It thus seems logical for the portrait photo grapher to use a lens of which the useful diameter is at least equal to, preferably greater than, the mean separation of the eyes (G. Cromer, 1921), without, however, falling into an exaggeration which, viewed at a short distance, would spoil the image. A lens of small diameter gives a view as seen by a one-eyed person.
In scientific photography, in which a mathe matically correct perspective is required, the use of lenses of large diameter should, on the contrary, be avoided.
59. Defects of Workmanship and of Material. Photographic lenses made by opticians of repute are always carefully examined before leaving the workshops, and run hardly any risk of showing any faulty material or bad workmanship, but these faults are sometimes met with in lenses which carry no maker's name or bear a more or less fancy name.
Defects of material comprise non-homogeneity of the glass and imperfect annealing.
Want of homogeneity is not usually evident except in lenses of large diameter ; it can be recognized by forming on a ground glass screen the image of a point source of light (e.g. the image of the sun in a well-polished metal ball or small silvered bulb) close to the axis of the lens. If now the screen is moved out of focus until a circle of light of about in. diameter is obtained, any defect will be visible as stria or dark zones.
Bad annealing, which gives rise to double refraction of the rays of _light (leading to a doubling of the image) can only be seen by examination in polarized light in an optical laboratory provided with the proper equipment.
Excessive pressure on the glass in its mount may also lead to double refraction.' Beginners have a tendency to consider the bubbles seen in every anastigmat as a defect.
These bubbles, enclosed in the glass in the course of the second melting (after the first melting has been broken up and faulty pieces rejected) cannot be removed except by com pletely liquefying the glass, which would have the effect of separating the constituents in the order of their density and thus cause a defect much more serious than the bubbles, of which the sole effect is to diffuse about one-thousandth part of the light—an absolutely negligible amount.
The form of faulty workmanship most fre quently met with in lenses of poor quality is bad centring (§ 40). This can easily be tested by looking at the images of a point or small source of light reflected at the various air-glass surfaces. These ought to lie exactly on a straight line.
Another defect which is likely to arise when the lens has been remounted in a different mount from that supplied by the maker is incorrect separation of the different components, or incorrect placing of the diaphragm, an error of less than o•oo4 in. having a fatal effect on the sharpness of the image, especially in lenses of short focal length. As a general rule, any modi fication of the mount (particularly those with a between-lens shutter) should be made, wherever possible, by the lens maker, who, more than one, is interested for the sake of the reputation of his name or trade-mark in preserving the original perfection of the instrument. If it is impossible to do this, it would be as well to make a thorough test of the lens before and after any modification (§ u6).