Why do 6X7 lenses have a shallow depth of field even stopped down all the way? Chris mentioned it with the 165/2.8. Are they all like that? I could understand it if it was at close focus with tubes WO, but for a landscape shooting at infinity? Please enlighten me. Steve Larson
The simple answer is that 6x7 lenses don't have a difference in DOF compared to 35mm lenses, but you are using the lenses differently to accommodate the larger format.
DOF is a function of magnification and aperture size. You really learn that with macro work where the DOF at any given magnification (say 1:1) is the same regardless of the focal length used to take the shot.
With the larger negative on a 6 x 7 you need more magnification (a longer lens) to frame up a shot the same way as you do with 35mm. Let's say you are shooting a landscape with the lens set to infinity. You get it nicely framed up on a 35mm with a 50mm lens. Deciding to shoot that scene on your 6x7, you find that you need a 110mm lens (or so) to frame it up the same way. That's because the lens is projecting the image onto a larger piece of film, so you need more magnification. That increase in magnification results in a decrease in DOF.
The reverse hold true when shooting with small formats - e.g. digicams with their really tiny sensors. A frame filling shot that would be 1:1 on 35mm is more like 1:4 on the digital, and so the DOF is seemingly larger. But again, the magnification is the driver.
If my explanation is not clear, play around with the math. The basic equation is:
d = 2fc*(m+1)/m^2
where d=dof, f = f stop, c = circle of confusion size, and m = magnification.
- MCC
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Mark Cassino
Kalamazoo, MI
[EMAIL PROTECTED]
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http://www.markcassino.com
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