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Author Topic: Autofocus Query  (Read 1074 times)

rj79in

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Autofocus Query
« on: April 01, 2012, 10:46:23 AM »
I'm just wondering as to what is the aperture at which Canon DSLRs do the AF (not considering LiveView). I've read somewhere that the AF is done using a f/5.6 aperture but am unable to confirm.

I've tried searching the net but am unable to dig deep enough to find an answer. Anyone have any information on this?

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Autofocus Query
« on: April 01, 2012, 10:46:23 AM »

neuroanatomist

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Re: Autofocus Query
« Reply #1 on: April 01, 2012, 10:59:58 AM »
All Canon SLRs and dSLRs require a lens max aperture of f/5.6 or faster, with the exception of the 1-series dSLRs prior to the 1D X and the film EOS-1V and -3, which can AF using the center point only with an f/8 max aperture.  That means any Canon EF/EF-S lens will AF on any EOS body, but only some lens + teleconverter combos will AF.

Note that the AF is actually performed with the lens wide open, at the lens' max aperture. The f/5.6 spec is just the minimum requirement.
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rj79in

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Re: Autofocus Query
« Reply #2 on: April 01, 2012, 11:08:42 AM »
Note that the AF is actually performed with the lens wide open, at the lens' max aperture. The f/5.6 spec is just the minimum requirement.

So basically the lenses will focus wide open regardless of the aperture actually selected?

neuroanatomist

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Re: Autofocus Query
« Reply #3 on: April 01, 2012, 11:16:29 AM »
Correct - AF is performed wide open, and the iris diaphragm is stopped down to the selected aperture just before the shutter is opened.
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rj79in

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Re: Autofocus Query
« Reply #4 on: April 01, 2012, 11:19:09 AM »
Correct - AF is performed wide open, and the iris diaphragm is stopped down to the selected aperture just before the shutter is opened.

Next question (sorry!) ... how to address focus shift when it happens on my fast lenses, esp. the 50mm f/1.2 L?

neuroanatomist

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Re: Autofocus Query
« Reply #5 on: April 01, 2012, 11:54:32 AM »
Next question (sorry!) ... how to address focus shift when it happens on my fast lenses, esp. the 50mm f/1.2 L?

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 (Eg-S for the 5DII, for example) 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.

Hope that helps...
« Last Edit: April 01, 2012, 11:56:16 AM by neuroanatomist »
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rj79in

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Re: Autofocus Query
« Reply #6 on: April 01, 2012, 12:25:56 PM »
Thanks for this ... its back to testing for the 1.2 ... again  :'(

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Re: Autofocus Query
« Reply #6 on: April 01, 2012, 12:25:56 PM »