Sony A900 - Alpha 24.6MP DSLR Camera Manual - Page 18
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FINE FOCUS
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RACK & PINION FOCUSER
Focusing a DSLR is far more
critical than focusing an SLR
because the DSLR sensor is
flatter than the depth-of-
focus, whereas 35mm film is
bowed towards the pressure
plate by several hundredths of
an inch (by approximately
twice the film thickness). The
only way to critically focus a
DSLR is by making small
incremental adjustments of
the focuser's fine focus knob.
A high quality rack & pinion or
Crayford focuser, with a 10:1
fine focus knob is the
minimum requirement.
Feather Touch® 3545 3".5 Dual Speed Focuser