Focus Shift and Spherical Aberration
Focus shift is covered in detail in Making Sharp Images.
Focus shift is a little-discussed optical phenomenon which can lead to blurred images and erroneous conclusions about lens sharpness. It is of particular concern when shooting a “fast” f/1.2 or f/1.4 lens stopped down to the f/2 - f/4 range. While slower lenses (eg f/2) can be affected, this is uncommon. Shooting distance (close vs far) is a factor, and the amou≠nt of focus shift can varies with the optical design.
Focus accuracy is a related issue — see diglloyd article Focus Accuracy.
Observe the crop below — precise focus was achieved at f/1.2 on the eye at right using Live View. However, the image was taken at f/2.8. We expect f/2.8 to give us very sharp results, and it does, but not on the eye at right where we focused! A mistake like this is not only frustrating, but could be expensive professionally (eg re-shoot and/or appear incompetent).
In the example above, overcoming the focus shift to achieve desired sharpness requires f/8!
Stopping down an f/1.2 lens to f/8 to achieve a crisp image is an ironic proposition. For shooting fast lenses (f1.2, f/1.4), you absolutely must understand the issue of focus shift, how your particular lens behaves, and what to do about it.
For a thorough discussion of focus shift, including analysis of seven different 50mm lenses and five different 85mm lenses, subscribe to diglloyd’s Advanced Photography (DAP). More research on focus shift is planned for other focal lengths. See DAP articles on Focus Shift, and the Table of Contents.
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.
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.
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.