Lens Sharpness, and Why Optical Focus Shift is the #1 Suckiest Lens Compromise
Related: build quality, coin-operated product review, fantasy MTF, field curvature, MTF, optical focus shift, shot discipline
For a landscape photographer or similar, artistic vision aside, it’s all about sharpness.
As the coin-operated youtubers will attest, any lens they can get their hands on is freaking ridiculously sharp. Now please “like” and click that link.
Let’s look at the various aspects of sharpness. You thought it was simple, didn’t you? Not.
Inherent sharpness
This is defined by how many clicks the review gets, or how many “likes” that tiny Instagram image the model picture gets. Just kidding.
The peak detail a lens resolves at a specified contrast and resolution, eg 50% MTF at 40 line pairs/mm is a standard reference, downgraded by most companies to computed fantasy MTF at 30 lp/mm. A very low bar for today’s high-resolution cameras. You really need 60% MTF at 40 lp/mm for a lens to look snappy. IMO.
Designed sharpness
Inherent sharpness is subject to intentional design tradeoffs. For example, the new Sony FE 24-70mm f/2.8 GM is outrageously sharp in the central 1/2 of the frame or so. It’s a very fine lens. But its field curvature is problematic for landscape—a strongly curved zone of focus can be dealt with in the 12-24mm range as with its sibling, but in the 24-70mm range it’s a bridge too far.
Build sharpness
Inherent sharpness includes build quality (adherence to optical design specifications) vs optical design potential. Build sharpness always falls short of potential. And potential sharpness is generally false advertising, as most any computed fantasy MTF chart will prove vs a real lens you can buy, even if funhouse distortion does not hugely degrade the sharpness of the final corrected image.
Aperture sharpness
Sharpness varies by aperture and across the frame, at first due to inherent sharpness (limits imposed by optical aberrations), but also field curvature which focuses at different distance in different parts of the frame so much with some lenses that f/8 is required to overcome it but that only smooths out a rough ride. And with stopping down sharpness inexorably degraded by diffraction, with f/8 already degrading things noticeably vs f/5.6 on high-res sensors.
Execution sharpness
What distance to focus at for the 3D scene at hand, the focus shift and field curvature behaviors, the f-stop in use, etc? Shot discipline includes that but also having no camera movement, using pixel shift when possible, and using focus stacking for overcoming otherwise impossible depth of field needs.
Predictable sharpness
Does the lens center its zone of sharpness where focused, or does it undermine you by landing focus somewhere else? As the lens is stopped-down, optical focus shift can substantially move sharpest zone of focus forward or back and/or both (in different parts of the frame).
If you cannot focus and expose and know that you will have optimal sharpness where focused, you are guaranteed inferior results much of the time. Over 15 years of objectively evaluating lenses, I can state with confidence that few photographers deal with this issue (or at all), let alone mitigate it. But you must deal with it, or you will consistently get sub-optimal results.
The often nasty sidekick to focus shift is often field curvature, which focuses at different distance in different parts of the frame. Worse, it can increase and accentuate with stopping down, along with focus shift. The Sony FE 12-24mm f/2.8 GM is a great example of increasingly pronounced field curvature by f/8. The key is understanding it so as to mitigate it by appropriate focus. And it’s not like there are superior alternatives, if any—in spite of its behaviors, it is all but unbeatable if you use it properly.
