Autofocus problem with my Canon EOS40D

Fr0dders

Fr0dders

Fr0dders

Caporegime
Joined
18 Oct 2002
Posts
33,438
Location
West Yorks
Hi all

Lately the autofocus on the kit lens on my 40D has been a bit erratic. I've got a 50mm F1.8 that appears to be ok but I'm not 100 % sure.

I've no idea what it is or how to fix it. It's hard to describe so I've taken a video


I can see what appears to be a few specs of dust inside the lens stuck to the inside of the glass. I had thought this was causing it, but googling seems to suggest this is normal for variable zoom lenses and unlikely to cause this.

This is on one shot or servo. It appears to "hunt" on one shot, not sure what to focus on, before it decides on something to focus on which is usually wrong
Any ideas
 
I would reset the camera to factory defaults before doing anything else.

Check the contacts are clean on the lens and the camera.

Stick the camera on a tripod put something with good contrast in front of it at a reasonable distance and see if it focus hunts.

If it is only doing it with the one lens, then you can pretty much rule out the body.

It may well be that the focus ring encoder inside the lens may have become dirty or worn.

You can see what the encoder looks like at this post on another forum - not for the faint hearted though. It might be better to send it for repair (dependant on value of lens v cost of repair).
 
I've not done anything yet so I'll give that a try.

I've been so convinced its something on the glass I've cleaned it loads, but not anywhere else.
 
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