Problems with autofocus in snow

[Anders Nielsen]

I am unable to get a bird in sharp focus when there is snow on the ground (not even in the frame). I haven't heard of this before but perhaps someone else has? The problem only occurs when there is snow. Can anybody tell me what's wrong or how I can get the focus right?

I use a Canon EOS 400D (XTi) and a EF 400mm f/5.6 L.

Thanks in advance
Anders Nielsen

[Doug West]

Are you using the center af point only? If not, you might want
to try that for focusing on the bird.

The reasoning is, if you're using all of the focus points lets say,
the camera might be having a hard time picking a focus point
because of the contrast (or lack of). All its seeing is the snow,
so it doesn't know what to look for.

Doug

[Anders Nielsen]

I am using the centerfocus point only. I can't even see the snow when looking through the viewfinder - the snow is on the ground and the bird is perched which is why I don't get it. The contrast on the birds aren't bad at all and I have worked in worse contrast-situations without having any problems - it is only when there is snow.

[Doug West]

I'm sorry Anders. I don't know why, but I thought you
were talking about problems with the af locking onto
the bird.

Doug

[Anders Nielsen]

Thanks for giving it a try. I have not been specific then - the focus will lock but it looks like there is some front/backfocus. In all other conditions the focus is spot on and I have tested it indoors for front/backfocus and there was no problem at all - it is only when I'm outside and there is snow on the ground. I have used the different AF-modes and the result is allways the same.

[JH Tugs]

Random thought, but is there any possibility that it is temperature related in some way?

[Anders Nielsen]

Thanks a lot for the suggestions and help.

Peter: The AF will lock but it is allways wrong no matter how many times I refocus.

JH: I can't say that it isn't temperature related it for sure but I don't think so - I have used the camera in temperatures below zero before without any problems. As said - I only have problems when there is snow.

I guess the best explanation is low contrast on the subject.