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The Barnet Book of Photography A Collection of Practical Articles

Much has been written about pinholes and their use


All

good lenses work to focus, but some of the cheaper ones do not. To test this, any series of small objects arranged side by side, but at distances varying by intervals of say two inches from the camera, is photographed after carefully focussing on the middle one. If any other than the middle one is the best defined, the lens is at fault. But in this, as in all similar tests, it must be remembered that ordinary dry plates are not quite flat, and the error of the plate may make an appreciable difference.

The use of lenses comprises the whole art of working with the camera, it is therefore not our province to say much about it. But so far as lenses themselves are concerned it may be remarked that, if a lens has a round field, it may be advantageous to tip up the lens with regard to the plate when only a part of the plate is being used, as for example sometimes in taking a landscape. But in using the modern flat field lenses special care should be taken to keep the lens and plate exactly true to each other, the plate exactly at right angles to the lens axis. The image and plate must coincide or definition will suffer. If the image is rounded and the plate flat, then in any case the result is only a compromise, but to take full advantage of the larger apertures when the field is flat, much more care than has been usual must be devoted to this matter.

_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 | +----------------------------+------+------+------+------+------+ | Distance from plate for | | | | | | | sharpest image ... | 64 | 32 | 16 | 8 | 4 | +----------------------------+------+------+------+------+------+


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