I need input on a focusing issue with my Canon 40d and certain lens.

[Tallyther]

I'm beginning to see something that I don't recall having been a problem in the past. Maybe I've just never noticed it but if I hadn't its more pronounced now. I want to know if anyone else has experienced this and what canmera / lens you were using. My problem is occurring on my Canon 40d with both my 300 mm f4 stabilizing lens and my 70-200 mm f2.8 stabilizing lens. I notice it more when I use the 1.4 extender though.

Here's the scenario. When I focus using auto function I almost always use the spot focus setting. When I put the spot on the object of interest and press the button to prefocus the viewfinder center focus spot moves off to the side about an 1/8th of the view finder width and tracts back to the spot when it locks on. Its annoying! I've noticed my photos seem to be more out of fucus slightly but I thought that was the fact I was moving too much for the stabilization function.

Anybody else noticed the view window center shifting any during focusing and maybe know what is wrong? Thanks for your assistance!

[Bob Atkins]

The image does shift when the IS kicks on. I don't think the shift is related to focusing, it's just the IS system itself initializing and IS starts up when you half depress the shutter, so if you are prefocusing, the IS starts up at the same time

Any time IS is operating the image will slowly drift. That's why it needs to be turned off when making long exposures with the lens on a tripod.

[Tallyther]

Thank you so much Bob. It is only recently I have been using longer lens and f32 long exposures.  So that's why its only recently been apparent to me.  Do cameras that have IS in the body have the same problem as those within the lens?.

[KeithB]

In Body would have the same issues.  You can always turn it off for your long exposures.  One problem with In-body stablilization is that they probably don't have a dedicated button to turn it off!