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Archive 2012 · f2.8 and Canon 5d Mark III Autofocus
  
 
cmor
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p.1 #1 · p.1 #1 · f2.8 and Canon 5d Mark III Autofocus


I don't understand the intricacies of the 5d AF, but know it has 5 dual diagonal sensors at f/2.8 :

61 AF points (41 cross-type at f/4, 5 dual-diagonal at f/2.8)

So do i need an f2.8 lens to take advantage of these? How much would I lose when using something slower?



Mar 05, 2012 at 12:28 AM
Photon
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p.1 #2 · p.1 #2 · f2.8 and Canon 5d Mark III Autofocus


I think you would lose the dual diagonal function with a lens slower than f/2.8, but maybe those sensors would still function with sensitivity to only one direction? That's essentially what happens with the cross-type sensors when you use a lens below their limit.


Mar 05, 2012 at 01:12 AM
SeverianTL
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p.1 #3 · p.1 #3 · f2.8 and Canon 5d Mark III Autofocus


The X-cross sensors won't function at all with lenses slower than f/2.8. The marginal rays that could reach those f/2.8 sensors don't exist in the slower lenses. But those central five points will still act as +cross-type f/5.6 sensors.

http://learn.usa.canon.com/resources/articles/2011/1dx_af_precision_crosstype_article.shtml
http://www.the-digital-picture.com/Photography-Tips/Canon-EOS-DSLR-Autofocus-Explained.aspx
http://graphics.stanford.edu/courses/cs178/applets/autofocusPD.html
http://books.google.com/books?id=cuzYl4hx-B8C&pg=PA208&dq=phase+detection+autofocus&hl=en&sa=X&ei=vRZUT8-WM4rW0QHVw9TJDQ&ved=0CDIQ6AEwAA



Mar 05, 2012 at 01:31 AM
Don Clary
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p.1 #4 · p.1 #4 · f2.8 and Canon 5d Mark III Autofocus


This is from memory:

The central group of 21 cross sensors includes the 5 dual cross sensors effective at f2.8. The remaining 16 are cross sensors effective at f5.6.

There are two outer groups of 20 sensors total (10 on each side) that are cross sensors effective at f4.

There are 20 remaining linear sensors for a total of 61. There are a lot of cross sensors here!!!!



Mar 05, 2012 at 01:34 AM
SeverianTL
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p.1 #5 · p.1 #5 · f2.8 and Canon 5d Mark III Autofocus


From the first link in my previous post (on page three of the link):

Rudy Winston wrote:
Twenty-one Cross-type AF points, with any lens f/5.6 or faster

The central 21 AF points in the EOS-1D X
(and 5D3) provide cross-type coverage, with separate horizontal and vertical line sensors, regardless of the lens in use (as long as the lens or lens plus tele extender combination, if applicable has a maximum aperture of f/5.6 or faster). This is especially useful to pros and serious enthusiasts who prefer to use central AF points. And, it extends beyond the center-most AF point, as indicated on the adjacent diagram. Any and all of these points provide cross-type AF coverage, even with lenses such as the EF 100-400mm f/4.5-5.6L IS, or the EF 800mm f/5.6L IS.

The central five AF points: cross-type and
...Show more

...

But, users should also be aware that there are some special attributes of the central five AF points that is, the center-most point, and the two AF points directly above and below it. This is completely in addition to the cross-type central AF points with f/5.6 lenses just mentioned above.

This vertical row of points also has a completely separate set of high-precision AF line sensors. They combine true high-precision (the highest ever in an EOS AF system), along with a diagonal cross-type layout of the AF points. If you could see them, they'd appear more like a letter "X" than
...Show more



Mar 05, 2012 at 02:19 AM
 

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Pixel Perfect
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p.1 #6 · p.1 #6 · f2.8 and Canon 5d Mark III Autofocus


A note for those that aren't aware of what the high precision means in this context.

With high precision AF sensors, what this means is that the depth of focus (not to be confused with depth-of-field) is reduced, when using fast (f/2.8+) lenes. When you AF there is an acceptable range (tolerance) over which the AF will say it's achieved focus. his depth is small, but it means a series of otherwise identical shots all using AF, would all have very slightly different focus, but each shot would like the same. The tolerance in high precision mode is much smaller than in normal mode and for current Canon cameras, the tolerance has been reduced by 67%. So for very fast glass where depth of field is very small, and you want high accuracy, these high precision sensors are important. Canon is implying the tolerance may have been reduced even further with the 1D X/5D III, so AF with very fast glass may be finally very accurate.



Mar 05, 2012 at 02:52 AM
wickerprints
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p.1 #7 · p.1 #7 · f2.8 and Canon 5d Mark III Autofocus


All I want my camera to be able to do is AF accurately near a rule-of-thirds spot in the frame without recomposing, for my 35L and 85L wide open. If the 1DX/5D3 AF system can't do that, I don't know why I'd spend the money to buy such a system.


Mar 05, 2012 at 03:02 AM
bortoni
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p.1 #8 · p.1 #8 · f2.8 and Canon 5d Mark III Autofocus


wickerprints wrote:
All I want my camera to be able to do is AF accurately near a rule-of-thirds spot in the frame without recomposing, for my 35L and 85L wide open. If the 1DX/5D3 AF system can't do that, I don't know why I'd spend the money to buy such a system.


I shoot Nikon but AMEN to that! They need to figure out how to extend cross-type sensors further out in the frame.



Mar 05, 2012 at 03:14 AM
arbitrage
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p.1 #9 · p.1 #9 · f2.8 and Canon 5d Mark III Autofocus


Have a look at digitalpicture's 1D X preview and they overlay the 3x3 grid over the AF layout. The f/4 sensitive x-type points are very close to the magical points you are looking for.
http://www.the-digital-picture.com/Reviews/Canon-EOS-1D-X-Digital-SLR-Camera-Review.aspx



Mar 05, 2012 at 03:31 AM





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