Ze.Dong wrote in post #12320269
99% of prime lens being soft is due to focus shift
What is focus shift?
Focus shift is a displacement of the sharp plane of focus when the lens is focused wide open, but the image is made with the lens stopped down.
Quite literally, the optimal plane of focus moves, depending on aperture! With every lens I’ve tested to date, the focus moves farther away. For example, if focus at f/1.4 is centered at 1.00 meters, then by f/2.8 it might now be centered at 1.02 meters. That apparently small difference means sparkling-sharp eyes versus not-quite-there eyes—it matters, especially with high-resolution digital cameras.
Focus shift is caused by spherical aberration (see vanwalree.com for an excellent technical discussion). Instead of a sharply-focused point of light a spherically aberrated lens produces a point of light with a “halo”. This is visually confusing when focusing by eye (because of the lowered contrast) making it difficult to find optimal focus. It also is confusing to autofocus systems. In spite of these issues, accurate focus can generally be obtained—but it’s no longer accurate when the lens is stopped down.
Mitigating focus shift
There are multiple approaches to dealing with focus shift. One solution is to avoid f/1.2 and f/1.4 lenses entirely; shoot an f/1.8 or f/2 lens instead! But assuming an f/1.2 or f/1.4 lens exhibits focus shift, here are some options, none of which are entirely satisfactory.
Stop down — unfortunately, the entire zone of focus is shifted, and some lenses require f/5.6 or even f/8 to regain “lost ground”. If your zone of sharpness needs are precise, stopping down isn’t much of a solution. However, given the random variability of focus error, stopping down “kills two birds with one stone”, improving the odds. But didn’t you get that “fast” lens to shoot at wider apertures?
Compensate — deliberately focus slightly in front of the desired point (with most lenses). With practice this is feasible, but it’s a skill that takes time to acquire.
Shoot wide open —what you see is what you get.
Focus at the shooting aperture — focus and shoot at f/1.4, focus and shoot at f/2, focus at f/2.8 to shoot at f/2.8, f/4, etc (by f/2.8 spherical aberration is all but eliminated).
Focusing at the shooting aperture is the only option for optimal results, but not always feasible. Stopped down lenses are not compatible with autofocus, so this means manual focus using the depth of field preview lever (or a lens with a manual diaphragm control eg Zeiss ZF on Canon EOS).
Live View (at the shooting aperture) is the most accurate choice of all, but requires a static subject and a tripod. Beware of Live View that automatically stops down the lens under bright conditions, and then exposing at a wider aperture than the focusing aperture. Canon EOS does this with Live View mode.
Autofocus compensation — some cameras now offer a lens-specific focus compensation adjustment. If you know you’re going to shoot (for example), the Canon EF 50mm f/1.2L at f/2, you can “dial in” a correction factor to front-focus slightly, correcting the focus for f/2. This feature is available on the Canon 5D Mark II. Nikon also offers the feature on the D3 and D700. Of course, correcting for f/2 will throw off other apertures.
For the full report on focus shift, subscribe to DAP (see Table of Contents).
Photographers utilizing the f/2 - f/4 aperture range with f/1.2 and f/1.4 lenses at close range (eg portraits) must take into account the focus shift characteristics of the lens. The shift is enough to throw eyes out of focus, or at least to render them with less than crisp detail.
It would be no surprise to find that many of the complaints read online about lenses being “soft” are due not just to focus error, but focus shift, particularly when assessed in the f/2 - f/4 range.
Some cameras today offer a fine-tuning autofocus adjustment on a lens-specific basis. It would be a modest step for Canon and Nikon to allow this functionality for the aperture range of the lens, allowing the user to dial in focus compensation for f/2, f2.8, f/4, etc; the camera could interpolate intermediate partial apertures.