T3D Re: Med Format needs more light?

[john bercovitz <bercov@xxxxxxxxxxx>]

BobH writes:
> So it is NOT true that medium format needs more exposure if you
> use f.stops and correct of extension..i.e. f.stops take care of 
> the format differences and is why they are used.

That's correct; if you use the same f/stops, you use the same 
illumination to get the same film exposure.  However, if you want
the same depth of field, you don't use the same f/stop.  The larger
the format, the more you stop down.  If you want to use the same 
illumination stopped down, you have to go to faster film or if it's
a timed exposure, a longer shutter speed.

Here I am talking about the stereo situation where f/stop is chosen
to give the same resolution and DOF from the observer's point of view 
regardless of format size.  For other than stereo needs, other criteria 
may well apply.

This is extremely simple to try and then you don't need all the 
chatter from me or anyone else: Grab a 35 mm SLR, set the aperture,
shoot a scene.  Now grab a medium or large format camera and put a 
lens on it that has the same coverage as the 35 SLR's lens does.
Set it to the same f/stop, take a picture of the same scene from
the same point, and you will find you have much less DOF.  The 
only way to get the same DOF is to set the linear apertures the
same, not the relative apertures.

John B


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