November 21, 2014, 07:29:36 PM

Author Topic: How does max lens aperture affect low light autofocus?  (Read 5616 times)

bdunbar79

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Re: How does max lens aperture affect low light autofocus?
« Reply #15 on: June 25, 2013, 02:13:11 PM »
Well it's not always about AF speed.  It's about accuracy too.  My 70-200 focuses super fast, but if it can't find the object due to low light, it doesn't even focus.  In general yes, the wider the max aperture, the easier it is to auto focus.  In general.

Which 70-200 variant do you have?  I have a 70-200 2.8 II and its AF accuracy is pretty good in low light.

Hmm, I'm sorry.  What part of my post made you infer that I was saying it didn't perform well in low light?
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Re: How does max lens aperture affect low light autofocus?
« Reply #15 on: June 25, 2013, 02:13:11 PM »

drmikeinpdx

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Re: How does max lens aperture affect low light autofocus?
« Reply #16 on: June 25, 2013, 09:28:21 PM »
More reliable source:
http://www.the-digital-picture.com/photography-tips/Canon-EOS-DSLR-Autofocus-Explained.aspx

I knew I could do better than wikipedia ;-)

Thanks, that's a great article!  I now have a better understanding of how the system works. :)

I think I'll still want to do some low light focus tests with my available lenses.  The author emphasizes the importance of using lenses with a max aperture of 2.8 or wider, but that's not the whole story.  I'll put on my geek hat and head to my studio!  Oh, and I'll include the 40 pancake in the tests.
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fugu82

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Re: How does max lens aperture affect low light autofocus?
« Reply #17 on: June 25, 2013, 11:06:10 PM »
I have done a fair amount of street shooting at night with the 24-105 on my 5DM3, and it focuses fine. I'm sure my 50 1.4 would be better, but it's less versatile and doesn't have IS. Usually there's a lot more light than you would think on the streets. But then, I'm an old lady, so I don't creep down a bunch of dark alleys too often.

mifho

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Re: How does max lens aperture affect low light autofocus?
« Reply #18 on: June 26, 2013, 12:01:21 AM »
The author emphasizes the importance of using lenses with a max aperture of 2.8 or wider, but that's not the whole story.

The last firmware update for the mkIII supposedly adds center point cross-type AF for apertures as low as f8.  I haven't updated mine yet as i hear it bricks third party batteries.
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fugu82

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Re: How does max lens aperture affect low light autofocus?
« Reply #19 on: June 26, 2013, 01:05:48 AM »
Mifho, the update hasn't bricked my Wasabi Power. It's even showing all the data for it. Don't know about other brands, tho.

bdunbar79

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Re: How does max lens aperture affect low light autofocus?
« Reply #20 on: June 26, 2013, 11:14:17 AM »
The author emphasizes the importance of using lenses with a max aperture of 2.8 or wider, but that's not the whole story.

The last firmware update for the mkIII supposedly adds center point cross-type AF for apertures as low as f8.  I haven't updated mine yet as i hear it bricks third party batteries.

That really doesn't matter as far as focusing in low light.  If the light isn't there, doesn't matter what type of AF sensor because it's not going to work.
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vblackbear

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Re: How does max lens aperture affect low light autofocus?
« Reply #21 on: June 26, 2013, 01:09:17 PM »
Well it's not always about AF speed.  It's about accuracy too.  My 70-200 focuses super fast, but if it can't find the object due to low light, it doesn't even focus.  In general yes, the wider the max aperture, the easier it is to auto focus.  In general.

Which 70-200 variant do you have?  I have a 70-200 2.8 II and its AF accuracy is pretty good in low light.

Hmm, I'm sorry.  What part of my post made you infer that I was saying it didn't perform well in low light?


wait... are you joking here?

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Re: How does max lens aperture affect low light autofocus?
« Reply #21 on: June 26, 2013, 01:09:17 PM »

bdunbar79

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Re: How does max lens aperture affect low light autofocus?
« Reply #22 on: June 26, 2013, 01:24:32 PM »
Well it's not always about AF speed.  It's about accuracy too.  My 70-200 focuses super fast, but if it can't find the object due to low light, it doesn't even focus.  In general yes, the wider the max aperture, the easier it is to auto focus.  In general.

Which 70-200 variant do you have?  I have a 70-200 2.8 II and its AF accuracy is pretty good in low light.

Hmm, I'm sorry.  What part of my post made you infer that I was saying it didn't perform well in low light?


wait... are you joking here?

You're comment is idiotic.  So just b/c sometimes a lens can't focus in really low light, it performs poorly in low light?  Repeat 3rd grade please.
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drmikeinpdx

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Quick and dirty low light focus test
« Reply #23 on: June 26, 2013, 01:27:04 PM »
OK, I took my 5D3 and four lenses into my studio and reduced the ambient light to the point that the lenses were just barely able to focus.  I varied the light a little bit to sort of bracket the point at which each lens would begin to focus. 

I aimed each lens at my usual focus grid target placed flat on the floor. The grid is essentially a ~ 3x5 foot piece of white graph paper with 1 inch squares.  It is actually a cardboard layout mat for sewing projects. The focus point consisted of a 12 inch length of black tape across the center of the grid so it was a fairly high contrast target.  I stood about ten feet from the focus point and aimed down at about a 30 degree angle.  This is the setup I use for MFA, although I normally let the camera have a lot more light to focus.

The actual exposure was made with a strobe to eliminate camera shake.

My results:


24-105  F/4:  needed the most light to focus, but focused accurately when focus lock was achieved.  The target appeared dim in the viewfinder and it was sometimes hard to find the black tape focus target.

Tamron 28-75 F/2.8:  needed moderate light to focus, accuracy was acceptable, but not as precise as the Canon lenses.

Canon 40mm F2/8:  needed moderate light to focus, but very good accuracy

Canon 85mm F/1.8:  needed the least amount of light to focus and had extremely good accuracy.

As always, your mileage will certainly vary!
« Last Edit: June 26, 2013, 01:29:13 PM by drmikeinpdx »
Current bodies:  5D3, 7D, 550D, S100
Favorite lenses: 135 f/2.0 L, 85 f/1.8 200 f/2.8 L, 50 f1.4 Sigma, 40mm pancake, 24-105 L.
blog:   http://www.BeyondBoudoirPhoto.com

fugu82

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Re: How does max lens aperture affect low light autofocus?
« Reply #24 on: June 26, 2013, 02:11:32 PM »
drmikeinpdx, good scientific approach to this issue.

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Re: How does max lens aperture affect low light autofocus?
« Reply #24 on: June 26, 2013, 02:11:32 PM »