Simple uncorrected lenses such as spectacle lenses or "monocles," suffer from the defects that have already been described, and are valued on this account by some workers because they give blurred or "soft" images. With a small enough diaphragm they will give good definition, and generally it may be stated that reducing the aperture lessens the effect of any fault that a lens may possess. To get the best definition that a simple lens will give, the plate must be brought nearer the lens after focussing by about one-fiftieth of the focal length of the lens, so that it may be brought from the best focus of visual light into the best focus of the photographically active light. If the object photographed is nearer to the lens than about one hundred times its focal length, the amount of movement after focussing must be increased. If four focal lengths distant, the correction is nearly one-thirtieth of the focal length, at three focal lengths distant, nearly one-twentieth, and at two focal lengths, about a thirteenth.
Pinholes give an image that for all practical purposes may be said to be equally blurred or "soft" over the whole plate. Much has been written about pinholes and their use, but it is not definitely known yet whether the exposure should be longer or shorter than the exposure required when a lens is used, allowing, of course, for the smallness of the aperture. The following short table and exposure rules from the writer's "Science and Practice of Photography," will probably prove useful:—
| Pinholes—diameters | 1/16 | 1/22 | 1/32 | 1/45 | 1/64 | ||
| 64 | 32 | 16 | 8 | 4 |
All the above figures are in inches. Whatever pinhole and at whatever distance, estimate the exposure for a lens at f/16, f/22, f/32, f/45, or f/64, as the case may be, and multiply it by the square of the number of inches that the plate is distant from the pinhole. But if the distance is as given above for any hole, it is sufficient to expose for as many minutes as the plate is inches distant from the hole, for a subject that would require one second with an aperture of f/16.
Chapman Jones.
Portraiture.
