Originally Posted by
Eric Woodbury
It's all very confusing. Thing to remember is longer lenses reduce apparent DOF and there is no winning.
Same f/# and same lens gives same DOF. Lens doesn't know how big your image is. However, if you compare "normal" lens for different formats, this will change the circumstances.
DOF approximately equals
2 * (D ^ 2) * N * c /(f^2),
where D is subject distance, N is f/#, c is chosen circle of confusion diameter, and f is focal length. [I know, I'm not fond of the math either.] This shows what changes DOF. If you hold the focal length constant, the f/# constant, the confusion constant, and the distance to subject constant, then DOF doesn't change. If you switch from 4x5 to 8x10 AND twice the lens, then you can see that there is a change. Change just the lens by twice and DOF changes by 4X since focal length (on the bottom) is squared. Chances are you'd change your distance twice, too, and it is squared on top, cancelling the focal change. However, with a longer lens, there is "compression", that means a given distance looks shorter, but measured it's the same.
(Usually with 8x10, it is assumed that the enlargement is less, too, and that the circle of confusion need not be so small.)
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