BOLEX H16 SBM Instruction Manual page 13

Focusing
The reflex viewfinder of the H 16 SB and SBM cameras shows you the picture as it
will appear on the film, with its sharp and blurred areas faithfully reproduced.
It is thus simple to focus accurately and, at the same time, to estimate the depth-
of-field.
The depth-of-field is the area within which the picture is in focus. It varies
according to the focal length of the lens, the diaphragm opening and the filming
distance. The longer the focal length, the wider the diaphragm opening or the
shorter the filming distance, the shallower the depth-of-field will be. Focusing
must therefore be especially accurate.
Conversely, the shorter the focal length, the smaller the diaphragm opening or
the longer the filming distance, the wider the depth-of-field will be-and the
greater the margin for focusing inaccuracy.
On most lenses, a depth-of-field scale indicates the distances within which the
picture will be in focus. Lenses are also supplied with a depth-of-field chart.
Distances are calculated from the film plane, marked <p.
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