FOCUSSING OF THE IMAGE AND THE POSITION OF THE SUBJECT ON THE PLATE 307. Focussing. In all cameras fitted with a ground glass screen, the focussing is done, as far as possible, by visual examination. The image formed by the lens on the ground is examined under a black cloth, 2 while both the distance of the lens from the screen and the aperture are varied until the image possesses the definition most suitable for the work in If the structure of the ground glass is very coarse, it is often of advantage to reduce the graininess by smearing a thin uniform layer of vaseline or glycerine over the surface of the screen. The transparency of the ground glass is, at the same time, greatly improved, an advantage which is particularly noticeable when photographing dark interiors. It is then neces sary, however, when examining the edges of the field, to view the screen slantwise, i.e. in the direction of the rays from the lens, and not directly as in the normal manner.
If the ground glass is not too coarse, the sharpness can be judged much more easily with the assistance of a focussing magnifier (magni fication of from 2 to 4 times), which is held against the screen in a sliding adjustment. This permits of setting the lens so as to give the best possible definition of the ground surface, or of any engraved or pencilled marks on its that can be obtained.
In principle, focussing should be done with the same aperture as that to be used for taking the photograph, since, with lenses possessing spherical aberration, a variation in the size of the aperture causes a displacement of the position of sharp focus.
When using a camera of the type in which the moving part is fixed in position after focus sing, by means of a screw concentric with the one controlling the rack-extension, it is always advisable, having secured the position, to make sure that the focus has not been accidentally altered.
To facilitate the accommodation of the eyes during the focussing of a dimly-lighted subject, the well-illuminated parts should be examined first, afterwards coming to the darkest areas by a gradual transition through the parts of inter mediate illumination (§ 246, footnote).
If the subject is too dark to permit the examination of the image, good definition may be successfully obtained by focussing on the flame of a candle or lamp' placed in line with the chief parts of the subject.
Hand cameras without ground glass screens are focussed directly by a coupled telemeter ( 174) or, after the distance of the subject from the lens has been either judged or measured, by means of a distance scale (§ 88) or by the use of supplementary lenses (§ 118).
In all cases it is necessary to bear in mind the rules which have already been given dealing with the depth of field and the best distribution of sharpness between two given points (§§ 76 to 85).
When copying fiat originals or prints by means of a suitably equipped apparatus ( i5o to 152), sharp focus and size of image are obtained by bringing the various movable adjustments opposite the scale divisions corresponding with the required scale of reproduction, finally veri fying that no error has been made during these operations by an examination of the ground glass screen.
308. Fine Focussing. When it is necessary to obtain very precise focussing (scientific work, or tests for making a focussing scale), it is well to replace the ordinary method of judging the point of maximum sharpness by a coincidence method known as parallax focussing. For this purpose the ground glass screen should be rendered transparent in one or two conveniently placed areas (centre and edges of field), either by protection during the grinding or by covering the desired areas with pieces of very thin glass (microscope cover slips) cemented with canada balsam. A cross should have been scratched on the unground part of the screen, or marked out with a hard pencil on the area to be covered.