This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Fig. II flatness of its field, the area of critical definition (see paragraph No. 602), evenness or unevenness of illumination, and last but not least, the relative aperture of the lens. The extreme limit of covering power is determined by the area of the field. The largest plate a lens can under any circumstances cover, is the largest one which can be placed within its image circle; that is, a plate the diagonal of which is equal to the diameter of the image circle. If in Fig. 10 the circle represents the field of a lens, it is evident that the largest plate fully covered would be one, the corners of which would come out to the edge of the field. The plate could vary in shape, but no matter what the shape, the diagonal of the plate would be the diameter of the field.
Curvature. How curvature or flatness of field affects the covering power, can be understood from Fig. 11. The greater the curvature, the smaller is the area which is sufficiently flat to coincide with our plate (p). The three curves, aa, bb, cc respectively, may be taken to represent the comparative curvature of three types of lenses, the portrait lens, the rapid rectilinear and the anastigmat lens. The superiority of the anastigmat over the two older types is too apparent to require any further explanation.
Critical Definition. The area of critical definition is an important factor. The larger the area over which the lens gives critical sharpness, everything else being equal, the greater is its covering power. This area can be extended by reducing the aperture of the lens, thus excluding the more or less imperfectly corrected marginal rays, and having the image formed by the more central and more perfectly corrected rays. The area of best definition being in the central portion of the field, the most advantageous position of the plate is as shown in Fig. 12, the center of the plate coinciding with the center of the field.
Relative Position Of Plate And Field. The position of the plate relative to the field, therefore, plays an important part. Raising the lens and consequently raising the center of the field above the center of the plate as in Fig. 13, the upper portion of the plate would be within the area of critical definition but the lower portion, especially the corners, would be outside of it. More or less curvature of the field would also be perceptible. Lowering the lens or moving it to one side, the effect would be similar at the top or at one end of the plate. Remedy: reduced aperture resulting in extended area of critical definition and increased depth of focus. If the plate is not at right angles to the axis of the lens but inclined, as (pp) in Fig. 14, (f representing the field of the lens), we are working under still greater disadvantage. The remedy here also is reduced aperture. Fig. 14 shows the position of the plate when the camera is tilted upwards, as is often necessary when we photograph a tall building at close range. If the lens is also raised, we add the disadvantage illustrated by Fig. 13.