Monday 12 May 2008

Autofocus systems can be fooled

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The idea of having to shoot a wedding without autofocus would not be an appealing prospect. Autofocus allows you to respond to situations incredibly quickly and is stunningly accurate. Modern cameras and lenses are designed specifically for autofocus - the throw on lenses (the distance the focal plane moves in response to adjustments to the focusing ring) is not suitable for manual focus. Autofocus is more accurate than my eyesight, even though I've replaced the Ee-A focusing screen in my Canon EOS 5D with the Ee-S variant (only suitable for lenses faster than f/2.8 due to reduced light reaching the viewfinder the Ee-S delivers - to quote Canon - "a steeper parabola of focus to make the image pop in and out of focus more vividly"). Autofocusing systems do, however, occasionally struggle. In the photograph above (wedding photography in Sandwich, Kent) the camera has focused on the wall behind the subjects, despite the fact that I focused upon the line of contrast between the bride's face and her hair, and the camera confirmed locking focus on this point (I only ever have one autofocusing point active on the camera - for the above shot the central one). I've got away with it in this case as shooting at f/8.0 has given me sufficient depth-of-field (DOF) [this photo has been significantly cropped, I was standing much further away than it appears giving a greater DOF which extends 1/3 in front of the focal point, 2/3 behind]. I've looked through a lot of other wedding photographer's books and online photos recently and noticed that this is a common occurence. The solution? If there is a tempting target for your autofocus system sitting just behind your subjects, focus on something else at the same distance - in the case above I'd be tempted by the border between the white and blue dresses. I'd be interested to hear your thoughts.


Matt Davis said...

Yeah, back focusing when you don't want it! Tends to be more evident when you have a small subject relative to the scene. Take a 70-200 shooting a subject slap bang in the center who fills say only 5-10% of the frame and it's VERY unlikely the focus will pull on them. It's skip that despite sowing as on them. and hit the background. The solution - who knows! f11 probably!!! LOL.

AF Sensors on my 5D are also poor in low light, especially anything other than center.

Nice blog of your thoughts & findings. Bookmarked by Matt Davis photography now.

Harrythephot said...

I always remove focus from the shutter release and have it on a custom button. That way I can focus once and take multiple shots without refocussing.

Thomas A said...

Hi David,

Please explain your comment in relation to the 5DMk2 - you have a different focus button?

Thank you

David said...

Hi Thomas,

Check out the custom functions on the camera.