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Excerpt from American Cinematographer...
A highly sophisticated, easy-to-use calculator for coping with the depth of field problems of modern zoom lenses, without a computer
According to the depth of field tables published by Cooke TTH, the comparative depth of field for 25mm lenses focussed on 5' and set at T/4 is 37" 8'43A" for a Series III fixed-focal-length lens, and 4'4" - 6¼" for a Varotol Zoom lens. A depth of field of 4'93A", compared with 1'8".
Similar figures for other focal lengths, focussed at 5' at T4 are:
50mm 4'6" - 5'7V2" (13Vz") compared to 4'91/2" - 5'2V2" (5") and
100mm 4'1OV2" - 5'11V2" (3") compared to 4?1V2" - 5'03/4" (11A").
These are enormous differences and are the manufacturers' own figures, computed by measuring actual lens performance. It seems that the new zoom lenses have a depth of field of approximately one-third of that which we were previously used to, which underlines the necessity for a new method of calculating focus splits.
As all cinematographers know, there is no definite point at which "in focus" begins and "out of focus" ends, making it all the more difficult to assess accurately just what will, or will not, be acceptable on the screen. This is why the Samcine Mk Il Calculator can be such an important accessory to all those concerned with critical focus calculations. The fact that the new calculator has separate scales for fixed-focal-length and zoom lenses and a choice of three circles of confusion makes it the only possible way to accurately calculate depth of field and focus splits, short of working them all out individually on a sophisticated electronic calculator, or working to the nearest focal length and focus distances printed in the manufacturers' depth of field tables, where such tables exist.
The reason that the depth of field of the new high-definition zoom lenses differs so greatly from fixed-focallength lenses is because (a) depth of field must be calculated from the front nodal point of the lens and cameramen measure from the film plane, and, (b) modern lenses and film stocks require circles of confusion to be half that which was previously acceptable.
Previous calculators and most depth of field tables allowed 2" in their calculators for the difference between the front nodal point of the lens and the film plane. With zoom lenses this distance may be 10" or more. Hence, when calculating depth of field for a zoom lens, it is possible that the calculatable distance is 8" less than heretofore.