This is a continuation of a report on new ways to look at depth of field. The series starts here:
A reasonable, if somewhat relaxed, portrait distance for a 55mm lens is 2 meters. I have done some actual DOF testing with the 60mm f/4 Coastal Optical macro lens as a byproduct of looking for focus shift and longitudinal chromatic aberration (LoCA).
Here’s what one of those curves looked like at 1:10 magnification, which is much closer than 2 meters.
Distance is the horizontal axis, with the left hand size having the subject farther from the camera than the right hand side (The camera moves closer to the subject after each exposure). There were 101 exposures in the series, and thus the total travel was 120mm. The vertical axis is MTF50, measured in cycles per picture height, assuming the entire sensor is used.
I asked my simulator to plot a similar set of curves for a 55mm lens at 2 meters distance.
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.
You’ll notice that there isn’t any focus shift. That’s because I didn’t program any into my model for this lens. You’ll also notice 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.
This Is a good opportunity to think about whether the end of acceptable DOF a function of the maximum resolution at that aperture, or is it absolute in some sense? I welcome anyone’s point of view on that subject. This is not a tradeoff usually associated with DOF calculations.