PHOTOGRAPHY IN MEDICINE There are three principal applications of photography in medical work, namely : (a) Radiography; (b) Photomicrography, and (c) Record photography, i.e., the photographing of patients speci mens, apparatus, etc.
The last named section is a particular branch of technical photography and often the results obtained are unsatisfactory or even useless. This is usually because the camera is regarded as an automatic recording apparatus which can only give one impression of a subject. Hence when it is found that the original is repro duced in an unfamiliar form, with essential details unaccented and irrelevant ones exaggerated, it is naturally thought that the process is inefficient. The truth is that the camera will not discriminate between those details which are required and those that are not; but it is possible to accentuate the former by lighting, etc.
Neglecting for a moment the question of colour, a photograph reproduces in a series of tones the light reflected from the cor responding areas of the subject. If the exposure has been suffi cient and the development adjusted to the printing paper this representation is correct over a considerable range. The con ditions specified are not by any means always fulfilled and much of the failure of photographic recording is due to faulty technique at this stage. The range of tones is limited by the white of the printing paper itself and the black of the deepest deposit, which is about 20 to I and, if the intensities of the original subject cover a greater range, accurate reproduction is impossible in a print.
The compressing of an extended scale of tones into a shorter range is not very apparent but, on the other hand, there is a typical photographic distortion of the perspective which is often very noticeable. The perspective as reproduced by the lens of a camera is always mathematically correct but may yet appear unnatural to the eye. The explanation lies in the fact that to see the subject as the lens saw it, it is essential to look at the print from a distance equal to that which separated lens and plate when the exposure was made. The photograph which shows abrupt per
spective was taken with a lens of short focal length and, if the print is moved up to that distance from the eye, the perspective will be seen to have become normal in appearance.
To avoid this distortion, it is clearly of importance to use a long focus lens. It is impossible to specify the exact length as that depends on the size of plate used. Such a lens is large and expen sive and requires a camera with a long extension and plenty of room in the studio. An alternative method is to use a shorter focus lens on a small plate and make an enlarged print provided the loss in detail is not too great.
Another reason for the failure at times of the photograph lies in the difference between the eye and the plate in their response to light of different colours. An untreated plate is sensitive to the blue and violet end of the spectrum. By treating the plate with certain dyes, it has been found possible to extend the sensitivity first to the green and yellow (orthochromatic plates) and finally to the whole spectrum (panchromatic plates). The latter still retain their excessive sensitivity to the blue end of the spectrum and, in order to obtain a true rendering of the relative intensities of different colours, they must be used with their appropriate light filter. This consists of a piece of dyed gelatine usually yellow in colour placed over the lens.
In some cases, e.g., faint reddish markings on the skin, a cor rect rendering would be unsatisfactory and here it is necessary to exaggerate the degree of contrast. An ordinary untreated plate, to which red is the same as black is to the eye, will give a more valuable if less correct photograph. Another case where increased contrast is desirable is when the print is to be reproduced by some half-tone process in a publication. The tones of the photograph are broken up into dots and the consequent reduction in contrast implies a somewhat harsher print.