SECTION II.
Directions for the use of Photographic Lenses.
Those who are comparatively unacquainted with the science of optics require simple rules to guide them in the choice of a photographic lens, and in the proper mode of using it.
Two kinds of Achromatic lenses are sold, the Portrait lens and the View lens; the former of which is constructed to admit a large volume of light, for the purpose of copying living objects, etc.
A convenient-sized Camera for small portraits is "the half-plate" with a lens of about 2¼ inches diameter, and giving a tolerably flat field on a surface of 5 inches by 4. Much however in this respect will depend upon the quality of the glass and also upon its focal length; a short focus lens taking a picture more quickly, but giving a smaller image, and a field which is misty towards the edge. There is also a great tendency to distortion of the image in portrait lenses of large aperture and short focus, such as those employed for operating in a dull light.
The "whole plate" portrait lens may be expected to cover 6½ by 4¾ inches, and has a diameter of about 3¼ inches. It will take larger pictures than the last, but not necessarily in a shorter time; since, although the aperture for admitting the light is larger, the focal length is proportionately greater and the light less condensed.
The "quarter-plate" portrait lens of 1¼ inch diameter is useful for stereoscopic subjects and small portraits; which are usually more sharply defined when taken with a small lens.
The distance at which the Camera is to be placed from the sitter in taking a portrait, will depend upon the focal length of the lens. The effect of bringing the Camera nearer is to add to the size of the image, but at the same time to increase the chance of distortion; hence with every lens of full aperture, there is a practical limit to the size of picture which can be taken.
When it is required to obtain a large image with a small lens, a stop with a central aperture (which may be readily made of a piece of circular cardboard blackened with Indian ink) must be placed in front of the lens. This will diminish the amount of light, but will render the picture more distinct towards the edge, and bring a variety of objects at different distances into focus at the same time. With a stop attached, the lens may also be brought nearer to the object without distorting.