Olympus OM 10 - Manual (Page 39)

< How to check the depth of field >
The photograph is focused at a distance of
about 4m (13 ft).
In this case, if the aperture is set at F 4, the
range which appears clearly defined will be
approximately from 3m (10 ft) to about
4.5m (15ft).
If an F16 is used, the range will be approxi-
mately between about 2m (6.6 ft) and 10m
(33 ft).
When you want to check inside the viewfinder
the correct focusing range, it simply requires
pressing this preview button.
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