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Archive 2013 · Central AF point size
  
 
ciprian.trofin
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p.1 #1 · p.1 #1 · Central AF point size


How much area does the central AF point cover on a typical xxxD camera ? I know the actual area is larger than the square in viewfinder, but how much larger ? 2-3 times ?

My problem: I'm doing some testing using 400D + 50mm f/1.8 @ f/2, on static target, 10m away, clowdy day, ISO 100, 1/800s, and the in-focus rate is 1/4 or worse.

I know the nifty is unreliable, but even so...



Mar 21, 2013 at 09:29 AM
MintMar
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p.1 #2 · p.1 #2 · Central AF point size


For 400D and fast lens, the central point is wider in the horizontal, as additional sensor baselines kick in (they work with lens f/2.8 and brighter). In the vertical, the AF area has the same size, no matter what lens you put on the camera.

Also, I hate to say it, but Nifty is very reliable, unfortunately only on reliable AF systems like 1D Mk2. It's a big difference how the same lens performs on my 1D Mk2 N compared to 40D and 400D I have, and 30D and 350D I had.



Mar 21, 2013 at 11:25 AM
ciprian.trofin
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p.1 #3 · p.1 #3 · Central AF point size


Thank you. Wider in the horizontal means "sensitive to vertical lines" ?


Mar 21, 2013 at 12:10 PM
MintMar
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p.1 #4 · p.1 #4 · Central AF point size


Yep.


Mar 21, 2013 at 12:19 PM
 

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ciprian.trofin
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p.1 #5 · p.1 #5 · Central AF point size


Thanks again.

I found something interesting here:

Canon EOS Digital Rebel XTi (400D), Rebel XSi (450D), Rebel T1i (500D), Rebel T2i (550D), Rebel T3i (600D), 20D, 30D
The center point is high-precision only when the aperture is at least f/2.8. If below f/2.8 but at least f/5.6, all points function as either horizontal or vertical sensitive only, except for the center point, which functions as standard-precision cross-type. Below f/5.6, autofocus does not function.

Canon EOS 6D
The center point is high-precision cross-type only when the aperture is at least f/2.8; the other points are either horizontal or vertical sensitive (not both). If below f/2.8 but at least f/5.6, the center point remains cross-type but is normal precision only; all other points continue to function normally. Below f/5.6, autofocus does not function.

Kinda' same thing...



Mar 21, 2013 at 12:36 PM
MintMar
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p.1 #6 · p.1 #6 · Central AF point size


Yes, true, but there's more variables behind the actual AF sensor which on paper may look the same. You can have different AF calculation algorithms, stronger CPUs to make them... My 1D2N's central sensor is very similar to above sensor's on paper, maybe even worse, because it loses one baseline from the cross with slow lens. Yet, in reality, it performs world apart from all the above systems in optimal light. 6D would beat it easily in the real dark.


Mar 21, 2013 at 03:19 PM
Alan321
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p.1 #7 · p.1 #7 · Central AF point size


Just in case you overlooked the obvious, the AF sensors effectively cover one or two lines on the image. Because of this they don't really cover much "area". Furthermore, some of the cross-type AF sensors are more like a T than a +. i.e. they do not all cross in the middle of the two sensor lines.

Personally I think it is unreasonable and unforgivable that Canon displays boxes in the viewfinder instead of whatever the real shapes of the AF sensors are. Even if they cannot get the location perfect they ought to get the shape and size right so that we know what we are dealing with.

- Alan



Mar 25, 2013 at 05:29 PM
ciprian.trofin
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p.1 #8 · p.1 #8 · Central AF point size


Well, it's not so obvious for me :-)

Can you point me to some web resources, to read more about this ?



Mar 25, 2013 at 08:20 PM





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