EF 28-135 focusing issues?

Hello, wondering if anyone has seen an issue similar to this. My EF 28-135 has shown some difficulty focusing on my 5D (original). I've only owned the lens for a few months and didn't notice this issue earlier, but seems to becoming more frequent. Then lens basically will occasionally not focus on a point that seems to have plenty of contrast - no other lens would even hesitate to focus on the scenes I'm describing: outdoors, plenty of light and contrast at the focus point, only using center point focus, etc. If I recompose slightly, or change the focal length slightly, it will usually focus then I can shoot. Seems to happen throughout the zoom range, but probably more common at the long end. I haven't tested it rigorously yet. It hasn't rendered the lens useless, but it is getting a bit annoying.

I've never seen this on any other lens on my 5D. I haven't tried the lens on my 40D yet to see if I can get it to do the same thing there - that would let me see if it's a lens problem or a problem specific to the 5D/lens combo.

Anyway, any advice is welcome. Thanks!

If it's an intermittent issue it could be an issue with the contacts between the lens and the camera body, it happened to me once a while ago with my old 28-135 and still does occasionally with the 24-70mm.

Try giving the contacts a clean with a soft cloth to remove any grime or corrosion and see if that helps.

Ben

One other item to check is whether the lens can rotate slightly when in the locked position on the body. I once bought a new lens that apparently had a slightly wide slot where the locking pin fits. I figured out that it would work flawlessly when I kept the lens fully rotated in one direction, but not the other. Exchanged it for another and have since had no problems.

If you have rubbed the contact surfaces with your fingers it can transfer some oil from your skin onto them. Having oily skin myself I have learned to be very careful when handling lenses. If the dry cloth doesn't work, try moistening it with a very small amount of alcohol to lift anything on them. Do not ever try using an eraser on them. Some sites still recommend this old practice, but it is a bad thing to do. It can wear away the gold plating which will make the problem much worse, to the point that the lens can become virtually unusable.

Thanks guys - I'll give the contacts a good cleaning and see if that helps.

One other thing that occurred to me is this is the slowest lens I own - by a long shot. At the long end, it's a f/5.6 lens - the rest of my lenses are primes at f/1.8 or faster. Does the 5D have a tough time focusing with slow lenses? If so, this is good information - I'm in the market for a telephoto lens. I'll stay away from the consumer 70-300 zooms if f/5.6 bothers my camera!

The other thought I had was the filter on the lens interfering with the focus, but that's not it - took the filter off any same issue appears.