Maximize Image Quality with Shot Discipline, Part 1: Introduction, Focusing (November 2018)
Related: depth of field, digital sensor, dynamic range, Eye AF, focus shift, focusing, gear, Live View, medium format, MTF and Micro Contrast, shot discipline, visual impact, zone of focus
Anyone moving to medium format surely wants the best possible results, since medium format is a great expense and more of a hassle than smaller formats. Why bother with the bother if not the best results?
In this and subsequent articles, I’ll discuss technique for making a technically optimal image, lumped together with the umbrella term “shot discipline”, a term I credit to Ming Thein and which he tangentially touched upon in his essay in the debut issue. Shot discipline includes composition and lighting, but this series will cover only the technical aspects. The related term “shot execution” refers to the full process of the capture. Shot discipline is consistently doing all the things that ought to be done for shot execution.
The three key elements of image quality are: dynamic range (subsumes noise), color gamut / color fidelity / tonal gradation, and sharpness (more than just resolving power). Those qualities all have a maximum potential within any given camera system. Shot discipline is a systems approach in how to bump up against the limits in order to extract the full potential of the system as often as possible. Those three aspects of image quality fall roughly into three buckets: exposure, mechanical/physical, settings and shooting choices (an implicit 4th aspect is the maximum potential image quality of the camera system).
After a decade of photography in the field often under difficult conditions, I’ve made just about every execution error that can be made and vowed never to make those mistakes again. That is, I made a commitment to shot discipline by adopting best practices. Shot discipline is thus about developing best-practices and making them habitual.
As to medium format: “big sensor” along with “small” execution mistakes is a self-defeating approach. Developing shot discipline is a systemic approach of steadily perfecting all aspects of shot execution so that the hit rate for optimal or near optimal image quality is high and stays that way. Once those habits develop, they subside into the background, taking little mental effort, and what is left are the creative/artistic decisions.
Optimal results are more challenging than ever
That’s not quite true: improved autofocus and dynamic range and so on keep improving. But what I’m talking about here is getting truly optimal results on high resolution cameras that approach that full image quality potential of the system. That is not so easy.
Assuming a 44 X 33mm sensor and a high-grade lens: the optical quality is unlikely to improve after f/6.3 or so. However, field curvature and focus shift and focus errors and depth of field requirements all mean that on medium format, the use of f/8 is a good way to hedge your bet. While the new crop of medium format mirrorless autofocus is usually very accurate and thus wider apertures can be used with a good degree of confidence, optical behaviors can undermine the best autofocus system. Maximal sharpness where desired is often a challenge.
It’s about to get harder: just having gotten 50 megapixel medium format systems at reasonable prices sorted out, the market is now moving to 100 megapixels on the same size sensor. The photosites are thus 1.4X smaller linearly (half the area per pixel). Which means that the Airy disc (resolved dot) quickly becomes larger than a pixel when stopping down. If larger than a pixel, then the sensor can no longer resolve at its potential. Thus focus accuracy and the placement of the zone of focus have to be even incredibly precise, and lens and camera tolerances have to be held to extremely strict value. So-called “good samples” of lenses are going to be hard to come by at 100 megapixels since the slightest deviations will stand out with optimal shot execution.
The irony is that for 100MP is not really twice 50MP in practice, because in terms of per-pixels sharpness, 1.4X less depth of field is available at the same aperture. Thus more stopping down is needed for depth of field (in terms of maximizing per-pixel detail), but at the same time diffraction degrades the image sooner since the pixels are smaller. Pick your poison. Diffraction and depth of field will be covered in a future article but are key concepts for anyone moving to 100 megapixels.
Medium format dovetails with coming technology
While the value of old or iconic images has little or nothing to do with sharpness, it’s exciting to me to see an entire image at full resolution—immersive if done right. So I shoot today for tomorrow’s display devices, namely 8K displays which are already here and also happen to have incredible dynamic range and color gamut—all the things that with sharpness make medium format worth the bother. Hence shot discipline is an investment for today and tomorrow and the year 2025. My friend Ming Thein argues that for some jobs, relatively low resolution is just fine, but at the same time he has is UltraPrint efforts. I agree—both viewpoints are valid—but for my part, I assume that someday we will have 200 megapixel displays.
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This series over four months starts with focusing because nothing else matters with an out of focus image.
Consider a slightly out of focus eye, meaning the iris of the eye. To get 50 megapixels of full sharpness on the iris of the eye means focus within a millimeter or so. Try getting a sharp iris of the eye at f/2.8 reliably with a 50 megapixel camera without Eye AF at a wide aperture—very hard since the photographer or subject need only move a few millimeters. A blurry iris with sharp nose or sharp ears is a throw-away. This is why Eye AF is a 'killer' feature for portraiture (such as on Sony mirrorless cameras). Stopping down hides small errors, but a lot less than one might hope for, particularly at closer range.
A similar principle applies to any type of photo for what I call “visual impact”. There are multiple factors to visual impact, but the zone of sharp focus is a key ingredient. By “focus” I mean both what is in focus and what is out of focus as well as the “zone of focus”—the near to far range that looks sharp even under scrutiny. The placement of that zone of focus can impact some images strongly, hence the need for accurate focus, even shooting at f/11.
Camera autofocus is getting better and better, but in years of testing I have found that it is sometimes optimal, frequently almost optimal, and sometimes way off. 100-megapixel cameras will be unforgiving of tiny errors when the goal is resolving to the sensor. It’s not clear to me that the mechanical aspects of lenses are even up to the task. With a camera + lens combination, shot discipline is assessing focusing accuracy with every camera body and every lens you use and then applying that knowledge when shooting.
The crop of new mirrorless medium format cameras are far better than medium format DSLRs in focusing accuracy and I expect further gains. I am pleased to alter the prior statement to say: mostly near optimal or better, and rarely way off. There are exceptions: for example I encountered a focusing error early on with the Hasselblad X1D—now fixed in firmware some time ago now. That error required about 2 stops to overcome—I consider that a large error and yet is was only slightly out of focus. I anticipate Eye AF in medium format at some point; once experienced, it is a must-have feature for the portrait photographer.
Below this pair of actual-pixels crops was made on a medium format DSLR. I saw this type of error frequently with that particular camera (and with more than one camera body and multiple lenses). Shot discipline with that camera meant (a) not assuming accuracy and thus checking for it, and (b) resorting to manual focus with a focusing screen optimized for manual focus, foregoing autofocus. Fortunately the days of this kind of ridiculous 4-stop error are ending with the arrival of medium format mirrorless.
That autofocus even in magnified Live View can fail is shown here in this split image crop at f/2. The top portion was autofocus, the bottom portion is manual focus. Not until f/8 did the detail catch up. In my book that’s huge error. Always validate the performance of new gear. Which I did by shooting this as one of the very first tests to check focusing. Since I could repeat this error at will, I knew I had better focus manually. In this case, I was told that it likely a firmware issue with a pre-production lens, but such things can and have happened to me before with production gear.
Magnified Live View
The newer medium format cameras for the most part have Live View. Magnified Live View (zooming in to 50% or 100%) takes time, and that’s where the discipline comes in: pressing an AF button is so quick and easy! But it is poor shot discipline if time allows for verifying optimal focus, especially if a wider aperture is used, where a small error is really a big one.
When available/feasible , use magnified Live View at full aperture (assuming no focus shift) to manually focus and/or use camera autofocus on that tiny area (when available). Cross check manually if the image isn’t crispy crunchy sharp using autofocus as per the example above.
Check across the width of the frame also: because of field curvature, there may be a slightly tweaked focus that is significantly better across the frame in overall sharpness but slightly less good at the point of focus. Stopped down a little, a very substantial difference in total sharpness can accrue without sacrificing peak sharpness on the desired spot.
I prefer using an EVF as it is higher resolution with no stray light, but here is shown the rear LCD with the full image, and in magnified Live View, in exact focus, and just slightly off. I use a Zacuto loupe on cameras not having an EVF.
Shown below are two methods of focusing: Pinpoint AF and 100% magnified (AF or manual). Even Pinpoint AF on the camera with the best AF I have yet used still makes mistakes, which I have proven by example in my publications. Use the best shot discipline: magnified Live View focusing whenever feasible.
Optimize focus for key elements
Excluding relatively flat/planar subjects (similar distance) and relative to the resolving power of the sensor, depth of field is shallow even at f/11. Maximize the visual impact by optimizing sharpness for the most important elements, which means considering the depth (distance range) over which the image is very sharp, and where that sharpness drops off, and how it affects the image.
Biasing focus at or very near the most important elements is the idea. It makes no sense to shoot a stunning scene of high mountain peaks slightly blurry so that some unimportant foreground element is fully sharp—emphasize the key element(s) by making them sharpest. There is an additional benefit to this approach in some cases: what is de-emphasized by some amount of blur makes the image more powerful by juxtaposition. Surroundings/context are often very important, but that purpose is usually served well by having the context be recognizable—sharpness is not required.
In the example below, I could not obtain the depth of field I wanted at f/11, but I felt that an out of focus boulder or corners or immediate foreground, being prominent, would not be acceptable. So I focused about half-way through the boulder so that the most prominent areas (roughly the bottom half of the frame) would be tack sharp—and they were, but that left a visibly soft background at f/11. So as shown below, I “cheated”: it is a 2-frame focus stack sharp from right in front of the camera to the far peaks—but focus stacking is a topic in its own right.
The example below illustrates a success at f/11 by focusing appropriately, with a twist. I knew that I wanted the entire image sharp but I absolutely wanted the trees and peaks to be tack-sharp. I also knew from prior evaluation of the lens that there would be a modest amount of forward focus shift ). Accordingly, I focused on the tall dead tree near center, hoping that f/11 plus the focus shift would land the zone of sharp focus just right. It was a calculated decision with potential for error, but it worked perfectly. Had I focused on the closer stumps, the distant elements would have gone just a bit soft. Reproduction size here cannot show it, but the leading ice is not fully sharp—that was the tradeoff I was willing to make, but the sharpness loss is low enough that it is not obvious at reduced size.
Know your lens: beware of focus shift
With some lenses, focusing wide open and then shooting stopped down displaces the zone of sharp focus significantly rearward in central areas (typical) or forward (typical around the periphery), or both! This is called focus shift and it has to do with central versus peripheral rays. Many optical designs introduce some aberrations that lead to focus shift as part of an overall optimization.
Focus shift is a type of focusing error that varies by aperture and as such it modifies the visual impact by displacing the zone of focus, as discussed previously.
Focus shift can be tough to deal with because it varies by aperture (mostly disappearing two stops down). . Compensate accordingly by understanding the lens behavior, or, with Live View, focus one or two stops down (assuming the image is to be made stopped down), which eliminates most of the shift.
Example—I would have preferred an outdoor example, but showing focus shift in this presentation format is difficult, so I’ve chosen to use boring crops that make it fairly obvious—at least I hope it comes through in reproduction.
Focus wide open at f/3.2 was centered on the “110” mark. Stopping down to f/5.6 shifts the enter of the zone of sharp focus forward (closer to the camera) to the “100” mark, maybe even the “95” mark. At f/5.6 depth of field has increased and this partially compensates in that “110” remains sharp. It also masks the shift. But the zone of focus has been displaced significantly. This can lead to disappointing results with some subjects. In my experience, most photographers just don’t realize that this is happening. Since it also varies by aperture, things like Fine Focus Adjust can be optimized for one aperture only. A few cameras compensate for focus shift, but that only works with autofocus.
Focus accuracy is the single most important aspect of technical image quality. Seemingly small tweaks can deliver oversize gains in total sharpness as well as delivering a higher hit rate at wider apertures, where the margin for error is very small.
The shot discipline is knowing exactly how gear performs (focus accuracy, focus shift), thinking carefully about what really matters and optimizing focus for that, and in taking the time (when feasible) to verify that all is just right.
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.