Depth of Field with Lenses on the GXR M Mount

Started Jul 5, 2013 | Discussions thread
Godfrey Forum Pro • Posts: 29,520
REVISION - Depth of Field with Lenses on the GXR M Mount

M Dennis Fern wrote:

My apologies if this subject is a repeat of an older one, but I was wondering how the crop factor on the GXR M Mount affects the depth of field scales on lenses used with that mount. I understand that the aperture is not affected by the crop factor but that depth of field is. Can anyone explain this in a clear and cogent way?

As I said up-thread, I intended to replicate Reg Ister's tests with the 21mm and do it also with a longer focal length lens, because 21mm is still a wide angle on APS-C and judging in-out of focus is much harder with a wide angle than a tele. I used a 50mm lens.

As a result, I'm revising my recommendation on using the DoF scales.

The results didn't line up with what I told M Dennis Fern, so I'll correct myself.

There is indeed the approximately 1 stop difference in DoF, but it's in the other direction as Reg Ister said, although not for the convoluted reasons that he construed. The reason is quite simple.

Given a reference output, reference format, specific focal length and DoF calculation expressed by the lens' scale, if you use the same lens on a smaller (crop) format, you are effectively increasing the lens magnification (that is, a 50mm lens acts like a 75mm lens on the GXR-M's APS-C sensor relative to the 35mm full-frame format). The increase in magnification nets less DoF relative to the reference output, and the factor is about 1.3-1.5 stops ... One stop is close enough given the resolution of the DoF scale markings.

My sign error was a result of thinking in the equivalent focal length, that is, on APS-C a 35mm lens nets the FoV of a 50mm lens but with the DoF growing one stop faster.

So: Given a particular lens, if you use the DoF scale marking for a focus zone, set the aperture to the next smaller lens opening (for example, use f/8 markings and set f/11 instead of f/8). Conversely, if you set the focus scale according to a set of DoF markings, use the next larger lens opening to achieve that zone (for example, set the lens for the zone indicated by the f/8 markings, set the lens to f/11).

This aligns properly with the standard DoF calculations provided by DOFMaster and with my observations with the 50mm lens.

My apologies for the flap and confusion.


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