Maximize Image Quality with Shot Discipline, Part 2: Depth of Field and Diffraction, Focus Stacking (December 2018)
Related: depth of field, diffraction, digital sensor, focus stacking, medium format, MTF and Micro Contrast, shot discipline
Parts 1 of this series introduced the idea of shot discipline, and covered covered several of the key areas in making a technically optimal image. This issue discusses depth of field and diffraction and touches on focus stacking.
The tradeoff between depth of field and diffraction is a tug-of-war that must be dealt with for optimal results on any camera: diffraction increases depth of field, the “acceptable” sharpness near to far. But within a few stops, its effects quickly degrades image contrast for both coarse and fine details, with loss of fine detail following soon after. Shot discipline requires that these effects be understood and mitigated when appropriate.
If it were possible to stop-down for depth of field with no repercussions, then photography would be a lot simpler. As a rule for medium format, diffraction is a subtle factor at f/8 and the decline accelerates from there. For medium format with current sensor sizes and pixel densities: stopping down to f/11 is undesirable when f/8 will do, f/11 being the last desirable aperture. At f/16 and beyond, Bad Things happen to image quality. The cutoff depends on pixel size, which is implied by sensor size and megapixels. A 100-megapixel sensor is 1.4X times (one stop) more impacted by diffraction (on a per-pixel basis) than a 50MP sensor, for the same size sensor, because its pixels are 1.4X smaller linearly.
Below, the size of the Airy Disc is depicted with stopping down. It is the size relative to the pixel size that matters most; as depicted the relative sizes are roughly appropriate for common medium format cameras, like the Hasselblad X1D and Fujifilm GFX-50S and Hasselblad H6D-100C. It should be obvious that f/16 is not a great choice and that f/22 and f/32 are an unmitigated disaster for overall and micro contrast as well as detail.
Technically: When rays of light pass through an opening (e.g., a lens diaphragm), they form a wavefront interference pattern know as the Airy disc (Huygens-Fresnel principle). The Airy pattern with all of its rings has an identical shape for any perfect lens with a round aperture. What changes with stopping down is the size of the Airy pattern, scaling linearly with the f-stop (1.4X for each stop). This assumes a perfect aberration-free lens. As the aperture becomes smaller, the scattering effect increases; this causes a drop in contrast. At the same time, the Airy disc steadily becomes larger, with its diameter placing a fixed limit on resolution at any given aperture. The Airy disc is much more complicated that a simple spot of light, but it doesn’t matter for purposes of understanding its effects or depicting its size.
Below are actual image crops shot from f/5.6 through f/32. Cameras with higher sensor pixel densities will look worse earlier, and cameras with lower sensor pixels densities will look better a little longer. A simple rule of thumb applies to all: prefer f/8 over f/11, and avoid f/16 unless absolutely necessary.
Depth of field
Depth of field is the subjective “acceptable” sharpness near to far. For my work, I use a standard of “critical sharpness”, meaning the zone of sharpness that fully exploits the sensor resolution or very close to it. This zone is quite narrow even at f/11. The zone is relative to distance: it may be 10 meters to infinity, or it may be 1.01 to 1.03 meters (both at f/11)—it has to do with magnification and distance.
With medium format and its larger sensor sizes, the equivalent aperture versus 35mm for the same depth of field may be f/10 or f/11, as compared to f/8 for 35mm, which means that adequate depth of field is more of a challenge given the impacts of diffraction.
The confounding factor is that real lenses do not comply with theoretical depth of field tables; such tables assume perfect lenses. Especially with wide angle lenses, the outer zones develop far less depth of field than more central areas (with both medium format and 35mm cameras). For that reason, I deem depth of field tables delusional for real photography—not just for that reason but because the visual impact of an image depends on precise placement of the zone of focus as well as the visual impact that results from overall contrast and micro contrast.
How then to read the diffraction vs depth of field riddle? One answer is meticulous shot discipline in choosing the exact point of focus, taking into account focus shift (if present) and field curvature (most lenses). A lens or lens adapter with tilt capability is a partial answer but often unsuitable and bringing further downsides. The only viable general solution is focus stacking, and it too has its limitations (subject movement, multiple shots required, post-processing work).
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See the in-depth coverage of focus stacking at diglloyd.com in Making Sharp Images.
While shooting the 100-megapixel Hasselblad H6D-100C in November 2018, I quickly learned two things: the Hasselblad HCD 50mm f/3.5 II delivers highly consistent high-grade performance across the field, but develops less depth of field less than one might hope for. Using f/11 does not provide adequate depth of field for even a mildly demanding landscape shot. I quickly determined that most images had to be shot with focus stacking.
Focus stacking is a whole subject in itself, but in brief it involves the following:
- Shooting two or more images at different focus distances.
- “Stacking” the images with software in the computer.
- Retouching the resulting “stacked” image when necessary (e.g., for subject motion).
Below, this 8-frame focus stack has corner-to-corner sharpness (reproduction quality/size might interfere with seeing all the benefits). Using a single frame at f/11, only a small portion of the log can be sharp but with 8 frames focus-stacked there are a real 100 megapixels of eye-popping detail.
Below, this 7-frame focus stack achieves the impossible: full sharpness near to far. A single frame at f/11 cannot even sharpen the boulder fully.
Critical sharpness on medium format is a challenge worth mastering, which starts with understanding the tradeoffs between depth of field and diffraction from stopping down. That necessarily leads to focus stacking as a critical skill that must be mastered for the full benefits of medium format.
Next month we’ll continue with handheld shooting technique, and gotchas of shutter/mirror vibration and avoiding pitfalls of different shooting modes.
Lloyd’s photography blog is found at diglloyd.com; it covers many brands, lenses, cameras including diglloyd Medium Format. To get the most out of any format requires perfect execution; see diglloyd Making Sharp Images. By subscription. Other areas Lloyd covers are cycling at WindInMyFace.com and computers at MacPerformanceGuide.com.