320. Influence of Type of Subject on Time of Exposure. We have stated (§ 316) that, in prin ciple, the minimum time of exposure is that which produces a useful image of the deepest shadow and in consequence assures a satis factory rendering of the details in the darkest tones.' This rule is nothing but a re-statement of the old adage, " Expose for the shadows and let the high-lights take care of themselves." There are, however, a large number of cases where the exposure may be considerably less than this minimum, as for example in all open landscapes with entire lack of objects in the foreground.' The shadows in this type of subject only appear on the image as small patches, and it would therefore be absurd to attempt to bring out their detail.' The presence of such details could, in certain cases, spoil the effect intended by the artist. This would certainly be the case in a photograph taken purposely against the light, in which it would be unnatural to record shadow details which could not be detected by the eye under such conditions of glare. Another such case is that of snow scenes, in which it is often much more important to exaggerate a little the play of the light on the surface of the snow than to bring out the details in the shadows, which are usually of interest only for their mass effect. In the same way, when making cloud photo graphs for the purpose of meteorological studies, any landscape foreground may quite well be ignored ; it will be sufficiently represented as a bare expanse without detail.
321. Influence of Relative Displacements of the Subject and of the Camera. If, during the exposure, a movement of the subject, of the camera, or of both at the same time takes place, the image will no longer be rigorously sharp, and the unsharpness produced will be greater according to the velocity of the image in the plane of the sensitive surface, and also according to the length of exposure. In order to obtain an image which appears no point in it should be displaced beyond a certain distance, and it is only logical to allow the same latitude in this displacement as has already been ad mitted in the case of sharpness of reproduction of points situated in front and behind the plane of sharp focus ( 76). This, according to circum stances, is about 1/250th in., or about 1/2,000th of the focal length.
If the velocity of the image in the plane of the sensitive plate is v in. per second, then if we adopt as the limit of sharpness 1/250th in. or F/2,000 (the focal length F being expressed in inches), the time of exposure I should be such that the product vi is at the most equal to 1/25oth in. or to F/2,000 in.
It is easily seen that the velocity of the image on the ground glass screen becomes smaller, firstly as the scale on which the subject is represented is reduced, and secondly, as the angle between the direction of its movement and the optical axis of the lens is reduced.' Based on these facts, it is easy to construct a table showing, for the most usual cases, the maximum time of exposure which may be given. 2 Except in cases of absolute necessity, nothing should be attempted if the maximum exposure indicated by this method falls considerably below the minimum time of exposure worked out on the basis of the sensitiveness of a plate or film, or is less than the shortest exposure which the shutter in use is capable of giving.'
In the table given on p. 215, the maximum times of exposure are roughly calculated, allow ing a latitude in sharpness of iI25oth in., and sup posing that the displacement is at right angles to the optical axis of the lens. To convert these times to the basis of any other standard of sharpness it is only necessary to multiply them by the ratio of the new standard of sharpness to the one in use. They may be doubled, for example, if a sharpness of 1425th in. be con sidered satisfactory, while, on the other hand, they should be halved if a sharpness of only r/sooth in. is allowed. It should be noted that if the relative sharpness is calculated from 1/2,000th of the focal length, the times given in the table below should be multipied lw the num erical factor F/8, the focal length F being ex pressed in inches.
With a lens of from 4 in. to 6 in. focal length (as used in the majority of hand cameras), the maximum time of exposure, expressed in hundredths of a second, which will give a sharpness of approximately 1/250th in., may be calculated in the following manner (E. Pitois, 1921).
Divide the distance of subject from the camera iii yards, by the speed of the object in miles per hour; multiply this quotient by o-2 if the dis placement is at right angles to the axis, by o-4 if the displacement is oblique, or leave it as it is if the displacement is parallel to the axis.
322. Instantaneous and Time Exposures. From the point of view of carrying out the operations, there is no real distinction between instantaneous and time exposures ; it is probably more correct to say that there is no such thing as an instantaneous exposure. Every negative is more or less exposed ; a negative which has been exposed for a long time may still have had insufficient exposure, while a negative exposed only for a few thousandths of a second may be over-exposed.
A misunderstanding, probably due to the wonderful promises made by certain dealers, has led many novices t9 believe that, once possessed of an instantaneous (snapshot) camera they have only to press a button to obtain excellent photographs with certainty in about 1/25th second, whatever the time or place,' even though it he in the depths of a tunnel, or on a moonless night ! 323. Practical Determination of the Time of Exposure. The professional, e.g. the studio portraitist, who is always working in the same place on subjects which differ very little one from another (photometrically speaking), is very soon able to acquire the practical experi ence which enables him, almost instinctively, to estimate the time of exposure with an accuracy which nearly always brings it within the limits of normal exposure. This is more particularly the case in portraiture, where the restricted range of contrasts increases the latitude of exposure.