More DOF management, CoC, MTF50, image plane & object field

Started Jun 6, 2016 | Discussions thread
JimKasson
JimKasson OP Forum Pro • Posts: 11,663
Portrait DOF simulations

I ran a simulation today at 2m focusing distance, which might be considered a portrait distance for a 55 mm lens.

The horizontal axis of the graph is subject distance in meters. The vertical axis is MTF50 measured in cycles per picture height. The simulated sensor is 42 MP, 14 bit, full frame Bayer CFA with no AA filter, like the sensor in the Sony a7RII. Fill factor is 100%. I turned off all sources of noise (photon, read, PRNU) — they don’t affect slanted edge measurements much anyway, since the technique is intended to calibrate out noise. Diffraction is computed at 450, 550, and 650 nm for the respective blue, green, and red raw color planes. The CFA is Adobe RGB, and the illuminant is D65.

I’m using a lens blur model that I originally created a couple of years ago to approximate the on-axis behavior of the Zeiss Otus 55/1.4. Now that I have improved my focusing accuracy with a motorized rail and my target with a razor blade edge, I now realize that my Otus model is actually somewhat worse than the lens itself, particularly at wide apertures. Nevertheless, it can serve as a stand-in for very good, if not great, 55mm lenses.

One of the striking things is how little DOF there is for critical work, even stopped down to f/5.6 or f/8, which many people  -- but not me -- think of as having too much DOF for portraits.

I would welcome other people's DOF strategies based on these curves. Is the end of acceptable DOF a function of the maximum resolution at that aperture, or is it absolute in some sense?

Here are the same results in the object field:

The curves are not strikingly different, though they do tail off slightly on the distant side. It has been said that you make different decisions about where to focus with object field orientation, even with small ratios between near and far. Anyone want to comment on that based on these curves?

Jim

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