SECTION II.. _directions for the Use of Photographic Lenses._

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 2-1/4 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 6-1/2 by 4-3/4 inches, and has a diameter of about 3-1/4 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 1-1/4 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.

With regard to this subject of the distortion often produced by lenses, observe particularly, that with the portrait combination of full aperture, and especially when the powers of the glass are rather strained by its being advanced too near to the sitter,--all objects near to the lens will be magnified, and those more removed will appear diminished; hence, as the position of the sitter is never quite vertical, the Camera must be inclined a little downwards, or the hands and feet will be enlarged, the figure in fact becoming pyramidal with the base below; whereas on the other hand, if the inclination of the Camera be too great, the head and forehead will be enlarged, and the figure becomes a pyramid with the base above.

When groups are taken, arrange the objects as near as possible equidistant from the lens, and use a stop if practicable. Long-focus lenses are the best for this purpose, allowing the Photograph to be taken further off, and giving a greater variety of objects in focus at the same time.

Portrait lenses may often be advantageously substituted for View lenses in copying objects of still life which are badly lighted. The aperture of the lens being large, a Negative can be obtained with an amount of light which would not suffice if a small stop were used. On the other hand, if the light be unusually bright, the lens of full aperture is always the most likely, from its extent of reflecting surface, to produce a misty and indistinct image. Hence the object should be well backed up with some neutral colour, or, if that cannot be done, a pasteboard funnel, projecting about a foot and a half, may be fastened in front of the lens, in order to exclude rays of light not immediately concerned in the formation of the image. If the lens were turned towards distant objects brightly illuminated, and a portion of sky included, there would probably be diffused light, and consequent fogging of the plate on the application of the developer. This effect will also invariably follow if the sun's rays be allowed to fall directly upon the glass.

Directions for finding the Plane at which the Sharpest Image can be obtained.--Non-Achromatic Lenses are understood by all to require correction for the chemical focus; but it is usually said of the compound glasses, that their two foci correspond. The amateur is recommended, in order to avoid disappointment, to test the accuracy of this statement, and also to see that his Camera is constructed with care. To do this, proceed as follows:--

First ascertain that the prepared sensitive plate falls precisely in the plane occupied by the ground glass. Suspend a newspaper or a small engraving at the distance of about three feet from the Camera, and focus the letters occupying the centre of the field; then insert the slide, with a square of ground glass substituted for the ordinary plate (the rough surface of the glass looking inwards), and observe if the letters are still distinct. In place of the ground glass, a transparent plate with a square of silver-paper which has been oiled or wetted, may be used, but the former is preferable.

If the result of this trial seems to show that the Camera is good, proceed to test the correctness of the Lens.--

Take a Positive Photograph with the full aperture of the portrait Lens, the central letters of the newspaper being carefully focussed as before. Then examine at what part of the plate the greatest amount of distinctness of outline is to be found. It will sometimes happen, that whereas the exact centre was focussed visually, the letters on a spot midway between the centre and edge are the sharpest in the Photograph. In that case the chemical focus is longer than the other, and by a distance equivalent to, but in the opposite direction of, the space which the ground glass has to be moved, in order to define those particular letters sharply to the eye.

When the chemical focus is the shorter of the two, the letters in the Photograph are indistinct at every portion of the plate; the experiment must therefore be repeated, the lens being shifted an eighth of an inch or less. Indeed it will be proper to take many Photographs at minute variations of focal distance before the capabilities of the lens will be fully shown.

The object of finding the point at which the sharpest image is obtained will also be assisted by placing several small figures in different planes and focussing those in the centre. This being done, if the more distant figures come out distinctly in the Photograph, the chemical focus is longer than the Visual, or vice versâ when the nearest ones are most sharply defined.

The Single Achromatic Lens.--A useful lens for landscape Photography is one of about 3 inches diameter and 15 inches focal length, which may be expected to cover a field of 10 inches by 8. With the lens, stops are supplied of various diameters, the largest of which will be useful in dull weather; the smaller when the field is required to be rendered sharp to the very edge.

The stop is arranged at a certain distance in front of the lens, and must not be moved. If it were brought close up to the glass, the field would not be so flat; the effect being then the same as that of a stop placed in front of a Portrait Lens, viz. simply to cut off the outside portion of the glass.

See this subject explained in 'Photographic Journal,' vol. ii. p. 133.

In taking Photographs of architectural and other subjects with vertical outlines, it is very important to have the Camera placed perfectly horizontal; since, if it be inclined either upwards or downwards, the perpendiculars will be destroyed and the object will appear of a pyramidal form, falling inwards or outwards, as before shown. It is convenient to rule the ground focussing glass with a number of parallel lines in both directions, which enables the operator at once to see that the position of the instrument is correct.