SECTION III.. The Photographic Camera.

The Photographic Camera.

The Photographic Camera is in its essential nature an extremely simple instrument. It consists merely of a dark chamber, having an aperture in front in which a Lens is inserted. The accompanying figure shows the simplest form of Camera.

The body is represented as consisting of two portions which slide within each other; but the same object of lengthening or shortening the focal distance may be attained by making the Lens itself movable. A luminous image of any object placed in front of the Camera is formed by means of the Lens, and received upon a surface of ground glass at the back part of the instrument. When the Camera is required for use, the object is focussed upon the ground glass, which is then removed, and a slide containing the sensitive layer inserted in its place.

The luminous image, as formed upon the ground glass, is termed the "Field" of the Camera; it is spoken of as being flat or curved, sharp or indistinct, etc. These and other peculiarities which depend upon the construction of the Lens will now be explained.

Chromatic Aberration of Lenses.--The outside of a biconvex lens is strictly comparable with the sharp edge of a prism, and therefore necessarily produces decomposition in the white light which passes through it.

The action of a prism in separating white light into its constituent rays may be simply explained;--all the coloured rays are refrangible, but not to the same extent. The indigo and violet are more so than the yellow and red, and consequently they are separated from them, and occupy a higher position in the Spectrum. (See the diagram at p. 47.)

A little reflection will show that in consequence of this unequal refrangibility of the coloured rays, white light must invariably be decomposed on entering any dense medium. This is indeed the case; but if the surfaces of the medium are parallel to each other the effect is not seen, because the rays recombine on their emergence, being bent to the same extent in the opposite direction. Hence light is transmitted colourless through an ordinary pane of glass, but yields the tints of the Spectrum in its passage through a prism or a lens, where the two surfaces are inclined to each other at an acute angle.

Chromatic aberration is corrected by combining two lenses cut from varieties of glass which differ in their power of separating the coloured rays. These are the dense flint-glass containing Oxide of Lead, and the light crown-glass. Of the two lenses, the one is biconvex, and the other biconcave; so that when fitted together they produce a compound Achromatic lens of a meniscus form, thus:--

The first Lens in this figure is the flint- and the second the crown-glass. Of the two the biconvex is the most powerful, so as to overcome the other, and produce a total of refraction to the required extent. Each of the Lenses produces a spectrum of a different length; and the effect of passing the rays through both, is, by overlapping the coloured spaces, to unite the complementary tints, and to form again white light.

Spherical Aberration of Lenses.--The field of a Camera is not often equally sharp and distinct at every part. If the centre be rendered clear and well defined, the outside is misty; whilst, by slightly altering the position of the ground glass, so as to define the outside portion sharply, the centre is thrown out of focus. Opticians express this by saying that there is a want of proper flatness of field; two causes may be mentioned as concurring to produce it.

The first is "spherical aberration," by which is meant the property possessed by Lenses which are segments of spheres, of refracting rays of light unequally at different parts of their surfaces. The following diagram shows this:--

Observe that the dotted lines which fall upon the circumference of the Lens are brought to a focus at a point nearer to the Lens than those passing through the centre; in other words, the outside of the Lens refracts light the most powerfully. This causes a degree of confusion and indistinctness in the image, from various rays crossing, and interfering with, each other.

Spherical aberration may be avoided by increasing the convexity of the centre part of the Lens, so as to add to its refracting power at that particular point. The surface is then no longer a segment of a sphere, but of an ellipse, and refracts light more equally. The difficulty of grinding Lenses to an elliptical form however is so great, that the spherical Lens is still used, the aberration being corrected in other ways.

A second cause interfering with the distinctness of the outer portions of the image in the Camera is the obliquity of some rays proceeding from the object; in consequence of which the image has a curved form, with the concavity inwards, as may be seen by referring to the figure given at page 53. The following diagram is meant to explain curvature of the image.

The centre line running at right angles to the general direction of the Lens is the axis; an imaginary line, on which the Lens may be said to rotate as a wheel turns on its axle. The lines A A represent rays of light falling parallel to the axis; and the dotted lines, others which have an oblique direction; B and C show the points at which the two foci are formed. Observe that these points, although equidistant from the centre of the Lens, do not fall in the same vertical plane, and therefore they cannot both be received distinct upon the ground glass of the Camera, which would occupy the position of the perpendicular double line in the diagram. Hence it is that with most lenses, when the centre of the field has been focussed, the glass must be shifted forwards a little to define the outside sharply.

The Use of Stops in Lenses.--Curvature of the image and indistinctness of outline from spherical aberration are both remedied to a great extent by fixing in front of the Lens a diaphragm having a small central aperture. The diagram gives a sectional view of a Lens with a "stop" attached; the exact position it should occupy with reference to the Lens is a point of importance, and influences the flatness of the field.

By using a diaphragm the quantity of light admitted into the Camera is diminished in proportion to the size of the aperture. The image is therefore less brilliant, and a longer exposure of the sensitive plate is required. In other respects however the result is improved; the spherical aberration is lessened by cutting off the outside of the Lens, and a portion of the oblique rays being intercepted, the focus of the remainder is lengthened out, and the image is rendered flatter, and improved in distinctness. Hence also, when a small stop is affixed to a Lens, a variety of objects, situated at different distances, are all in focus at once; whereas, with the full aperture of the Lens, objects near at hand cannot be rendered distinct upon the ground glass at the same time with distant objects, or vice versâ.

The Double or Portrait Combination of Achromatic Lenses.--The brightness of illumination of an image formed by a Lens is in proportion to the diameter of the Lens, that is, to the size of the aperture by which the Light is admitted. The clearness or distinctness of outline however is independent of this, being improved by using a stop, which lessens the diameter.

The Portrait combination of Lenses is constructed to ensure rapidity of action by admitting a large volume of light. The following diagram gives a sectional view.

In this combination the front Lens is an Achromatic plano-convex, with, the convex side turned toward the object; and the second, which takes up the rays and refracts them further, is a compound Biconvex Lens; there are therefore in all four distinct glasses concerned in forming the image, which may appear at first to be an unnecessarily complex arrangement. It is found however that a good result cannot be secured by using a single Lens, when a "stop" is inadmissible. By combining two glasses of different curves, the aberrations of one correct those of the other to a great extent, and the field is both flatter and more distinct than in the case of an Achromatic Meniscus employed without a diaphragm.

The manufacture of Portrait Lenses is a point of great difficulty, the glasses requiring to be ground with extreme care, in order to avoid distortion of the image: hence the most rapid Portrait Lenses, having large aperture and short focus, are often useless unless purchased of a good maker.

The Variation between the Visual and Actinic Foci in Lenses.--The same causes which produce chromatic aberration in a Lens, tend also to separate the chemical from the visual focus.

The violet and indigo rays are more strongly bent in than the yellow, and still more than the red; consequently the focus for each of those colours is at a different point. The following diagram shows this.

V represents the focus of the violet ray, Y of the yellow, and E of the red.

Hence, as the chemical action corresponds more to the violet, the most marked actinic effect would be produced at V. The luminous portion of the spectrum however is the yellow, consequently the visual focus is at Y.

Photographers have long recognized this point; and therefore, with ordinary Lenses, not corrected for colour, rules are laid down as to the exact distance which the sensitive plate should be shifted away from the visual focus in order to obtain the greatest amount of distinctness of outline in the image impressed by chemical action.

These rules do not apply to the Achromatic Lenses recently described. The coloured rays being in that case bent together again and reunited, the two foci also nearly correspond. By a little further correction to a point higher in the Spectrum, they are made to do so perfectly.