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Author Topic: Canon 50mm f1.2L lens focus shift  (Read 1319 times)

Hjalmarg1

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Canon 50mm f1.2L lens focus shift
« on: January 06, 2014, 06:30:04 AM »
Hi,
anyone have experience focus shift and/or back/front focus with the Canon 50mm f1.2L lens? I am still learning to shoot with this and I think I have steady hands, however, sometimes (using single focus point) I experience front focus. I already applied AF microadjustment to -1 and looks to have improve but still difficult to nail.
Any though?
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Canon 50mm f1.2L lens focus shift
« on: January 06, 2014, 06:30:04 AM »

neuroanatomist

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Re: Canon 50mm f1.2L lens focus shift
« Reply #1 on: January 06, 2014, 06:46:40 AM »
I'll paste my previous comments from another thread:

The 50mm f/1.2L is something of a special case.  That lens suffers from a particularly bad focus shift (many lenses have some focus shift, especially, fast primes, but usually not enough to notice).  Focus shift is when the focal plane of the lens changes when you change the aperture of the lens.  All lenses focus with the aperture wide open, then stop down the aperture to your chosen setting as the shot is taken.  In the case of the 50mm f/1.2L, if you select an aperture narrower than f/1.2, down to about f/4, focus shift means that the lens will actually focus on a point that's behind your chosen focal plane.  At f/4 or a little narrower, the DoF is deep enough to mask the effect, because by then your chosen focal plane is within the DoF.  At very close subject distances, the DoF is shallower, and the effect of focus shift is exaggerated (you may need f/5.6 or even f/8 to get a deep enough DoF to mask the shift).

So, what can you do?  Here are some options:

  • Stop down to f/4 or narrower.  But, I'm going to assume that shooting at f/8 is not the ideal solution...
  • Shoot at f/1.2.  There's no shift if you don't stop down.
  • Use Live View.  Granted, that's not a good option with typical 50/1.2 subjects.
  • Manual focus with the DoF Preview button pressed.  You'd almost certainly need a high-precision focusing screen the Eg-S instead of the stock screen.
  • More complex AFMA.  You could perform an AFMA at f/2, and you'd get different results than wide open - that adjustment would compensate for the focus shift at f/2, but not be applicable at f/1.6 or f/2.8, for example.  So, you'd need to have a list of AFMA values, and change the setting to match the aperture you want to shoot at.  Might work if you can pick an aperture for a shoot and stay there.  (Side note here: the 1D X can store two AFMA settings for a zoom lens, one for the wide end and one for the long end; in theory, Canon could allow multiple, aperture-dependent AFMA settings for the 50L, which would certainly help with this issue.)
  • Intentionally front focus.  Use an AF point over a feature that's a little bit in front of what you really want to focus on.
  • Tweak on the fly.  The 50L has full-time manual focus, so you can use AF get you close, then turn the MF ring slightly to bring the focal plane forward a little.  You'd likely want to be using back-button AF for that, and it would take a fair bit of practice to get it working reliably.
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Hjalmarg1

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Re: Canon 50mm f1.2L lens focus shift
« Reply #2 on: January 06, 2014, 08:00:00 AM »
I'll paste my previous comments from another thread:

The 50mm f/1.2L is something of a special case.  That lens suffers from a particularly bad focus shift (many lenses have some focus shift, especially, fast primes, but usually not enough to notice).  Focus shift is when the focal plane of the lens changes when you change the aperture of the lens.  All lenses focus with the aperture wide open, then stop down the aperture to your chosen setting as the shot is taken.  In the case of the 50mm f/1.2L, if you select an aperture narrower than f/1.2, down to about f/4, focus shift means that the lens will actually focus on a point that's behind your chosen focal plane.  At f/4 or a little narrower, the DoF is deep enough to mask the effect, because by then your chosen focal plane is within the DoF.  At very close subject distances, the DoF is shallower, and the effect of focus shift is exaggerated (you may need f/5.6 or even f/8 to get a deep enough DoF to mask the shift).

So, what can you do?  Here are some options:

  • Stop down to f/4 or narrower.  But, I'm going to assume that shooting at f/8 is not the ideal solution...
  • Shoot at f/1.2.  There's no shift if you don't stop down.
  • Use Live View.  Granted, that's not a good option with typical 50/1.2 subjects.
  • Manual focus with the DoF Preview button pressed.  You'd almost certainly need a high-precision focusing screen the Eg-S instead of the stock screen.
  • More complex AFMA.  You could perform an AFMA at f/2, and you'd get different results than wide open - that adjustment would compensate for the focus shift at f/2, but not be applicable at f/1.6 or f/2.8, for example.  So, you'd need to have a list of AFMA values, and change the setting to match the aperture you want to shoot at.  Might work if you can pick an aperture for a shoot and stay there.  (Side note here: the 1D X can store two AFMA settings for a zoom lens, one for the wide end and one for the long end; in theory, Canon could allow multiple, aperture-dependent AFMA settings for the 50L, which would certainly help with this issue.)
  • Intentionally front focus.  Use an AF point over a feature that's a little bit in front of what you really want to focus on.
  • Tweak on the fly.  The 50L has full-time manual focus, so you can use AF get you close, then turn the MF ring slightly to bring the focal plane forward a little.  You'd likely want to be using back-button AF for that, and it would take a fair bit of practice to get it working reliably.
Many thanks! However, it looks that the solution isn't easy. I'll take your advise and use it while learning how to use properly focus. I bought this to shoot at largest possible aperture
Body: 5DIII. Prime Lenses: 15mm f2.8, 100mm f2.8L IS, 35mm f2 IS, Extender EF 2X III.
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Ruined

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Re: Canon 50mm f1.2L lens focus shift
« Reply #3 on: January 06, 2014, 07:16:23 PM »
At f/1.2 your focus should be dead on. If not, you need to use AFMA to tune it.  Mine was dead on out of the box.

However, f/1.4-f/4 do experience some focus shift, but this is really only noticeable with objects closer than 1.5m.  This is one of the casualties of a f/1.0-f/1.2 50mm with excellent bokeh.  Even the $10k Leica Noctilux who many faint over has the same thing.

So, in short, AFMA your lens so f/1.2 is dead on, then just use caution stopping down if you are close to your target.

Hjalmarg1

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Re: Canon 50mm f1.2L lens focus shift
« Reply #4 on: January 06, 2014, 11:09:44 PM »
At f/1.2 your focus should be dead on. If not, you need to use AFMA to tune it.  Mine was dead on out of the box.

However, f/1.4-f/4 do experience some focus shift, but this is really only noticeable with objects closer than 1.5m.  This is one of the casualties of a f/1.0-f/1.2 50mm with excellent bokeh.  Even the $10k Leica Noctilux who many faint over has the same thing.

So, in short, AFMA your lens so f/1.2 is dead on, then just use caution stopping down if you are close to your target.

Thanks! I did some AFMA to -1 and at f1.2 the AF improved a lot at short distance.
I already have the lens mounted on my 7D to force the learning process with this lens. I want this lens for half-body, head shots and some close protraits so, it my disctance to subjects may vary from 3m to 1m.
Body: 5DIII. Prime Lenses: 15mm f2.8, 100mm f2.8L IS, 35mm f2 IS, Extender EF 2X III.
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Re: Canon 50mm f1.2L lens focus shift
« Reply #5 on: January 07, 2014, 01:49:07 PM »
As long as you use it at f/1.2, it will not focus shift, but lenses all tend to front focus when close, so you need to do AFMA at 1M, 2M, and 3M and see how much difference there is.  You may have to compromise if the AFMA values vary too much.

AmbientLight

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Re: Canon 50mm f1.2L lens focus shift
« Reply #6 on: January 14, 2014, 10:29:48 AM »
One of the problems with the 7D + 50mm f1.2 setup is that this camera's AF is not very well suited for this lens. When I first purchased the 50mm f1.2 it took me a significant time to learn how to properly focus this lens. I eventually learned the technique, but as soon as I switched to 1D-X and 5D Mark III, I could lay that technique to rest as with those cameras the lens focuses perfectly. This is a technical difference you cannot really make up for.

With the 1D-X acquiring focus also appears to be a bit more snappy, because it provides more power to the lens, which has an effect with large aperture lenses (the 85mm f1.2 in particular).

Regarding technique you can try two things:
1) Allow the camera more time to achieve focus. Don't shoot or avoid fast-moving, erratic moving subjects.
2) Experiment with manual focus. You might be surprised.

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Re: Canon 50mm f1.2L lens focus shift
« Reply #6 on: January 14, 2014, 10:29:48 AM »

RLPhoto

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Re: Canon 50mm f1.2L lens focus shift
« Reply #7 on: January 14, 2014, 10:38:20 AM »
No issues if shot wide open. The shift gets pretty bad if at the Min. Focusing distance @ F/2-F/4.

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Re: Canon 50mm f1.2L lens focus shift
« Reply #7 on: January 14, 2014, 10:38:20 AM »