To be thoroughly successful in photography, it is of the utmost importance that the operator should be perfectly familiar with the lens he is working with. A lens should be examined and tested for any serious defects, as well as for the determination of its capabilities. Small air-bubbles, spots, or scratches on the glass, are of but little consequence, although scratches should be filled up with black varnish to prevent diffusion of light; It is important, however, that the glass should be highly polished, in order to secure brilliancy in the picture and freedom from irregular reflection.
Focus a distant object on the ground glass. Now focus another object so near that the image is as large as nature. The difference between the 2 positions is the focal length of the lens. To find the optical centre of a combination of lenses, Abney recommends the following plan. Place 2 rods (10 yd. apart) at a spot exactly 50 yd. from the front of the camera, which must be perfectly level, and in a line with one of the rods, and at right angles to the line joining the two rods. Now focus the nearest rod accurately on to a pencil line drawn in the centre of the ground glass. Then the equivalent focal length =
Distance of ground glass from nearest rod X apparent distance apart of rods on ground glass.
Actual distance of rods apart + then* distance apart on glass.
The equivalent focal length measured from the ground glass along the axis of the lens will give the optical centre.
Rule a piece of paper, with both horizontal and vertical lines, and focus it on to the ground glass, so that the image falls near the margin. Either one or the other system of lines will be indistinct. Closely connected with this defect is spherical aber-ration, which causes general blurring of the whole image, and curvature of the field, which causes marginal blurring when the centre is in focus, and vice versa. These defects, due to the spherical forms of lenses, are more or less present always, but may be diminished by using smaller stops.
Depth of focus depends on the size of the angle made by the rays, and therefore increases as the aperture diminishes, and also as the distance of the object increases. When used for pictures 1/20 natural size, the depth of focus is 400 times greater than when used for pictures the size of nature.
Take the equivalent focal length and measure the greatest breadth of the picture, or of the circle of light, on the ground glass. Transfer the latter length on to a sheet of paper, and from the middle of the line so obtained erect a perpendicular equal to the focal length. Join the ends of the base with the summit of the perpendicular, and the angle so formed is the angle of view.
This is any rectangle having for its diagonal the diameter of the circle of light on the ground glass. For near objects, a smaller size will be sharply covered than for distant objects.
To determine whether the optical and chemical foci coincide, a focimeter is necessary. This may be roughly constructed by pasting strips of thick paper, so as to form a series of steps. Now number each step, and focus accurately on to the central one. On development, if any other number is more distinct than the central one, a chemical focus is present.
This is a circular patch of light appearing on the ground glass,' in a line with the axis of the lens. It is owing to the image of the diaphragm coinciding with the focal distance of the lens, and may be remedied by altering the position of the diaphragm.
If f F are the focal lengths of 2 lenses, and a A their apertures, their illumination varies as f2/a2 : F2/A2
Thus it is important to estimate correctly the effective aperture of a lens. This may be done accurately thus: - focus any distant object on to the ground glass, which is then removed and an opaque board, with a small hole in the centre, is substituted. A lamp is now put behind this hole, and a piece of transparent paper is placed over the front of the lens. The diameter of the bright spot on the paper is the effective aperture for that particular diaphragm. The time of exposure varies inversely as the fractions f2/a2, F2/A2 but it must be remembered that the nature of the glass will also influence the quantity of light admitted by a lens. (J. V. Elsden.)