Re: [OM] CZ Jena Tessar

[Chuck Norcutt]

Preset was "the thing" before auto diaphragms predominated.  With an 
auto diaphragm you may have the lens set at f/8 but the lens stays wide 
open while you're metering and focusing and only closes down at the 
moment of exposure.  A preset diaphragm allows you to preset the 
aperture (say f/8) with a mechanical limit but then manually fully open 
the lens for focusing.  The preset aperture allows you to turn the 
aperture ring without looking at the ring and turn it until it stops at 
your preset aperture.  It's just a convenience which allows you to 
preset the shooting aperture but then allows you to do focusing wide 
open and metering (probably at the preset aperture) and control the 
aperture ring without having to move your eye from the viewfinder.  Just 
rotate it without looking until it stop either wide open or at your 
preset aperture if turned in the opposite direction.

Even after auto diaphragms became common long telephotos often had 
preset apertures rather than auto.  In the days of mechanically actuated 
diaphragms (ala OM vs electronically in AF lenses) a very long lens 
might have a very long actuation rod reaching to the diaphragm.  The 
large mass to be moved could make it hard on the camera unless there was 
real kick in the diaphragm activation lever (ala OM) :-)

Chuck Norcutt


On 12/5/2010 9:44 AM, AS wrote:
> How does preset make a difference?
-- 
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