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Autofocus problems with all grey bird [Townsend's Solitaire]
http://birding.bc.ca/community/viewtopic.php?f=10&t=15671
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Author:  BirderBert [ Apr 13 7:07 pm ]
Post subject:  Autofocus problems with all grey bird [Townsend's Solitaire]

ImageTownsends Solitaire 2s by BirderBert, on Flickr

I found a lifer Townsend's Solitaire at Hope Airport a couple of days ago and merrily blasted away at the bird, which was only 20-25 ft away. I am used to getting really sharp pics with my Olympus E-M1 and 300/4 lens and was quite surprised when ALL of the pics missed the critical focus on the eye. The camera uses both Phase Detect [PDAF] and Contrast Detect [CDAF] systems but Olympus does not allow us to select and instead lets the camera decide! I'm wondering if the solid grey of the bird fooled the AF or did I just screw up? Any theories?

Author:  MDB8 [ Apr 13 10:16 pm ]
Post subject:  Re: Autofocus problems with all grey bird [Townsend's Solita

The attached image still looks sharp to me.

I can offer a couple of comments. Firstly, from experience, when I encounter a Lifer, I usually get excited and consequent camera shake often occurs. However, this effect quickly wears off after I take a few shots.

My camera body is set up so that the shutter will not release unless the camera belileves it is properly focused.

Secondly, I notice you use a 1.4 tele-extender. These accessories shorten the depth of fiield in an image. Hence if your lens is not perfectly calibrated, focus errors will be exaggerated. I use a tool called "SpyderLensCal" to check the autofocus accuracy of a lens/lens plus converter. Tele-extenders also lessen the contrast of an image. However, if your camera model is not full-frame, such contrast degradation will be minimized.

I don't think the colour of an object will fool the autofocus. Different colours refract at different angles when passing through a lens, but all lenses are built to counteract this optical effect. I have found brightness to impact autofocus, even when using spot focusing. However, your subject seems to be neither too bright nor too dark. Gray-scale quality, so to speak.

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