Thanks Chuck, that was a very helpful way of expressing things. And much
more useful than my prejudicial view that on a digital camera, *everything*
is in focus. A view which I can now dump. Partially.
--
Piers
-----Original Message-----
From: olympus-owner@xxxxxxxxxx [mailto:olympus-owner@xxxxxxxxxx] On Behalf
Of Chuck Norcutt
Sent: 20 December 2004 14:05
To: Olympus mail list
Subject: [OM] Depth of field - the aha! factor
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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