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Canon 50mm f/1.2L and focus shift



A common question that comes up in various fora is whether the Canon 50mm f/1.2L exhibits focus shift and if so at what apertures and at what focus distances. I did my own testing to come up with an answer.

What is focus shift?

Focus shift is the phenomena where the focal appears to shift (usually backwards) when the lens is stopped down. The real world impact of this is that since your camera auto focuses with the lens at its widest aperture, the desired element that is to be in focus suddenly isn't when the picture is taken (at a smaller aperture). This phenomena is found mostly in fast primes (faster than f/2). I'm no optical engineer but am told that focus shift occurs due to uncorrected (or under corrected spherical aberration). So why not just correct the spherical aberration? I'm told that correcting SA is usually done at the expense of bokeh, one of those tradeoffs in optical design. Whether one believes or understands this reasoning isn't important. What is important is to note that focus shift is inherent to the optical design of a lens and is not something that varies from copy to copy of a lens. Repeat after me, focus shift is inherent to the optical design. Period.

Also be careful to not confuse focus shift with focus error. Focus Error is when the focus point is placed at some point other than the intended point. If you are using auto focus, this can happen for a variety of reasons. I won't go into all the reasons, save for the most obvious with the 50 f/1.2L, focus error caused by spherical aberration. Since your auto focus works with the lens wide open, the haloing caused by spherical aberration may confuse your camera resulting in focus distance not set as intended. This is a seperate issue from focus shift.

Tests

To test focus shift, I used a LensAlign Pro. I manually focused wide open, trying as much as possible to get the plane of focus precisely at 0. I tested 4 different focus distances, ~ 2 feet (which is close to the minimum focus distance of the lens), ~3.2 feet (a common distance for a tight portrait with this lens), 6 feet and 10 feet (starting to get close to infinity for this lens).

Images were converted in Lightroom 3.2, sharpening and clarity were cranked up and the image converted to black & white to better accentuate the region in focus.

Focus distance of 2 feet

Mouse-over apertures to compare
1.2 1.4 2.0 2.8 4 5.6 8 11
One can see a bit of focus shift here, however its not severe enough to throw the intended point of focus out of focus. So the intended focus target will still be sharp at this distance, but things behind that target will be sharper than things in front.

Focus distance of 3 feet

Mouse-over apertures to compare
1.4 2.0 2.8 4 5.6 8 11
Here is where we see some more pronounced focus shift. However again, the intended point of focus still remains within the depth of field and at all apertures should be acceptably sharp, though even a very small amount of back focus (either because of the subject shifting, the camera shifting or misfocus) can easily render the intended focus point unsharp.

Focus distance of 6 feet

Mouse-over apertures to compare
1.2 1.4 2.0 2.8 4 5.6 8 11
Once again at 6 feet, we see more pronounced focus shift. Its difficult to see, but it does appear that at f/2 and f/2.8 the intended point of focus is just on the border of being sharp. At smaller apertures, the depth of field is wide enough to compensate. Once again, any small error in focus can easily render the intended focus point unsharp.

Focus distance of 10 feet

Mouse-over apertures to compare
1.4 2.0 2.8 4 5.6 8 11
At this distance, focus shift isn't that much of an issue. The depth of field is wide enough to compensate for the focus shift.

Conclusions

I have no agenda in performing these tests, only in providing information. Focus shift with the Canon 50mm f/1.2L is real. One has to learn to deal with it if one is to get great images from this lens (one is within their rights to simply say no and not use this lens at all, but then you do miss out on other things it does exceptionally well). The focus shift is inherent to the optical design and so one shouldn't expect copy to copy variation. There are ways to work around focus shift depending on your subject:
  1. Shoot wide open - Its obvious, but yes you could shoot wide open. I realize the 'haze' you see with this lens wide open is not desirable to some depending on the subject but this is a valid alternative.
  2. Focus stopped down - If you are shooting at a reasonably large aperture (like say f/2 or f/2.8) focus stopped down. Hold down the DOF preview button on your camera when focusing. As a bonus the 'haze' from the spherical aberration also gets diminished as you stop down. The disadvantage is that it can be awkward.
  3. Compensate - If you manually focus the lens, pull the focus plane a little in front of the desired point of focus. If you are auto focusing, you can manually adjust the focus to pull it a little closer once AF has locked on. You could also dial in some correction (on bodies equipped with MA). The advantage is that with a lot of practice, one will get a natural feel for where to place the point of focus to get the plane in the desired location when stopped down over time. The disadvantage is that it can be very hard to do, especially if you are getting paid to shoot. The MA route is filled with peril because the focus shift is different at different focus distances and apertures and with MA you can only set a single value for a lens.
Finally, in addition to focus shift, watch out for focus error. If the focus plane ends up being just a little in front of your intended area of focus, that combined with focus shift can render the final exposed immage useless.


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All text and images (c) 2000-2017 Aravind Krishnaswamy. All rights reserved.