Aha! I think I finally understand the depth of field relationships
between lenses of "equivalent" focal lengths on different form factors.
The ratio between the vertical size of a 35mm film frame and the Minolta
A1's sensor is 24mm/6.6mm = 3.64. Using the free DoF calculator I
mentioned earlier at the Digital Light & Color site
<http://www.dl-c.com/Temp/> I noticed that, applying the ratio above to
focal length, f-stop and resolution yields the same DoF.
For example, with a 35mm using a 50mm lens at f/16 and assuming 30
lines/mm resolution, you can use the A1 to take an equivalent picture
with respect to final print size and DoF if the A1's focal length is
50mm/3.64 = 13.75, aperture is 16/3.64 = 4.4 and resolution is 30*3.64 =
109 lines/mm. (I'm rounding some numbers for the illustration so don't
beat me up on precision here)
The key finding here for my own understanding is that you can *maybe*
produce the same depth of field using the smaller format if you've got
enough aperture in the small lens. The key is that, to get the same DoF
you must divide the aperture of the larger format by the ratio between
it and the smaller format. At f/16 on the 35mm camera with 50mm lens
the A1 can replicate the DoF by shooting at 13.75mm at f/4.4. However,
if we open the 50mm lens to f/8 we'd have to open the A1's lens to
f/2.2. Unfortunately, the A1 would already be out of gas as its max
aperture is 2.8. And opening the 50mm to 1.8... well, it's hopeless to
try to match it with a 13.75mm f/0.5 lens.
The same analysis will apply to an E-1 but there the ratio is a more
easily obtainable 2:1 on focal length and aperture. However, you still
can't replicate the DoF (and bokeh?) of the 85mm f/2 when it's wide
open... at least until there's a 42.5mm f/1 digital Zuiko. :-)
Chuck Norcutt
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