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FORUMS Canon Cameras, Lenses & Accessories Canon EOS Digital Cameras 
Thread started 30 Apr 2010 (Friday) 19:50
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Is the 7D AF biased toward high-contrast background elements?

 
trale
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Apr 30, 2010 19:50 |  #1

That's been my observation with my first several outings on the 7D. I've observed this with both my 50mm 1.4 and Tamron 17-50mm 2.8. Both these lenses have already gone through MA, and I found that my 50mm F/1.4 is back-focusing and requires a correction of -15 MA. While the Tamron front-focuses in controlled conditions and requires a correction of +5 MA.

So basically, I have both a backfocus and a frontfocus lens (tested and corrected under controlled conditions using Spot AF - the tiniest AF sensor option available on the 7D). However, in real world shooting, I don't use Spot AF because it can too easily land onto a small un-detailed area. I use "Point AF" in real life, which gives a bigger sensor area (slightly bigger than the area of the bounding AF rectangle). With Point AF or the bigger AF-expansion, I find that both lenses have an affinity to focus on high-contrast elements behind my subject (for example, outlines of windows, doors, posters, etc). This happens quite often when the bounding AF rectangle of my selected AF point encompasses outlines of both of subject and any background element, especially if the BG element is contrasty.

My fix for this is move my camera so that the bounding AF rectangle is fully inside my subject so it doesn't "see" any background element at all.

This is not too big of a deal, not something I'm going send the 7D to Canon over, but I'm wondering if anyone else experience something like this.




  
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NaKiD ­ EyE
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Apr 30, 2010 20:09 |  #2

the live view in "contrast-based"




  
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AJSJones
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May 01, 2010 16:03 |  #3

trale wrote in post #10098566 (external link)
My fix for this is move my camera so that the bounding AF rectangle is fully inside my subject so it doesn't "see" any background element at all.

This is not too big of a deal, not something I'm going send the 7D to Canon over, but I'm wondering if anyone else experience something like this.

I think you have discovered what it means to "put the AF point over the part of the image you want to be in focus" :D:D:D (don't put it over the background if you don't want the background to be in focus :( )


The spot AF is the smallest AF area and therefore is the most critical to be exactly on the part you want to be in focus. The others are usually quite a bit bigger than the marked square in the viewfinder, so if there's a contrasty item that seems to be just outside the AF area, it will probably be selected as the item the camera thinks you want to AF on, especially if the subject is low contrast. You have to learn how to work with the camera and its AF system.


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Is the 7D AF biased toward high-contrast background elements?
FORUMS Canon Cameras, Lenses & Accessories Canon EOS Digital Cameras 
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