T3D Re: medium format macro stereo

[john bercovitz <bercov@xxxxxxxxxxx>]

I think Sam hit on the head when he said that the problem he's had 
with macro medium format is DOF and exposure.  The one is traded off 
for the other.

As we can see from my self-incriminating numerical example yesterday, 
the situation is that you have to use the same diameter hole in the 
front of the lens (entrance pupil) to get the DOF you need regardless 
of format size used, yet you have to cover a much larger film area 
when you go to a larger format.  It's as simple as that and I have to 
say this is one of the simpler results I've seen in these sorts of 
derivations.  The result is that you have to use enormous strobes for 
doing macro in MF and for LF macro, you'll have to dispense with the 
strobes and go to tactical nukes.

(Simple?) conclusion:  In macro work, whether you consider geometrical
or physical optics or both, it is only the diameter of the hole in the
iris as seen from the front of the lens that matters.  That hole's 
diameter tells you how much DOF you will have.  The total film area to
be exposed tells you how much light has to pass through that hole.  The
MF vs 35 mm macro example is that when you go up to MF, you have doubled 
the size of the format which means you have four times as much film area 
to expose so you need two stops (4X) more light to expose the film.  In
LF macro, you will have a 4X larger format meaning you need 16 times as
much light (4 stops more light).

John B


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