T3D Medium format macro needs 2 more stops?

[john bercovitz <bercov@xxxxxxxxxxx>]

> I don't understand the thing John said about medium format needing 2
> more stops for macro.
>
> I understand that the film is larger, but isn't it still just as
> sensitive to light?

Yes, I was saying if you use film which is just as sensitive to 
light in the medium format, then you would need two more stops 
of light from your strobes.  The reason is that the light has to 
cover four times as much area (MF is larger than 35 mm format).  
You can also look at it with effective f/stops on the camera:

If the entrance pupil diameter is 1.4 mm as in the previous 
example, and the distances from the lens to the film is 40 mm 
for the small format and 160 mm for the large format, then the
f/numbers are 40/1.4 = f/28 for the small format and 160/1.4 = 
f/114 for the large format.  If it had been medium format, it
would have been 80/1.4 = f/57.  As you can see, the lens focal 
length is changing to give the same coverage but the entrance 
pupil has to stay the same to give the same physical resolution 
and the same DOF.

Now comparing the f/numbers for 35 mm and medium format, we see
f/28 and f/57  These f/numbers are two stops apart.  So we need 
four times as much light (each stop is a doubling).

Zat make any sense?

John B


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