EF600/4L IS unreliable focus

[Frank Kolwicz]

I've had this 600/4 since summer and it had been giving me critically sharp images, then I broke it in half, it was repaired and again worked fine. Today I had a longish session with the birds and I can't get anything that sharp anymore. For the last month, at times it would sometimes be sharp, sometimes not, and, since I was working in low light, I blamed myself.

Today's session was partly in full sun with shutter speeds of 1/1000 sec. or faster and none of the images is right on the money - anywhere in the image, including areas in front and in back of the subjects. I did this with two cameras: 5dII and 7d and both have the same soft look.

The lens, with or without 1.4x converter, is used on a window mount, IS is on, AF point selection set to spot (5dII is on center spot). Both cameras have been microfocus adjusted, the 5dII when it was working fine, the 7d as best I can under the circumstances. (I'm doing this to evaluate the 7d.)

I have images from this lens with the 1.4x that are spectacularly sharp, but not today and I shot more than 300 frames, total.

Is there anything I can check that may be causing this problem? Is there anyone in the US or Canada who can do a serious repair and/or adjustment without toeing the Canon line about what "meets specs" and refusing to divulge details?

[Bob Atkins]

I'd do some testing that you can reproduce, i.e. shooting a specific target (e.g a resolution test chart of some sort) under specific conditions (e.g. full sunlight).

I'd put the lens on a sturdy tripod, shoot with IS on and IS off, shoot at various apertures, with and without TCs and I'd shoot both with AF and with manual focus using magnified live view. If I had two camera bodies, I'd do the same tests with each body.

Then I'd analyze the results and see if I could tell what set of conditions were responsible for lowered performance and if that lowering of performance appeared to be random or systematic Ii.e. if AF is failing is always failing or if it's failing occasionally and randomly.

There's also a big difference between AF being unreliable, and the inability to ever get a sharp image under any conditions.

I'd also look at the images closely to make sure they were symmetric, i.e. the four corners are all of equal sharpness. If they aren't it suggests a misalignment of the optics.

There's probably nothing you can do yourself. The "... then I broke it in half, it was repaired and again worked fine..." makes me nervous. It's possible that something done in that repair is a problem.

I don't know of any independent repair shop that has the capabilities that Canon does for testing, repairing and aligning long Canon telephoto lenses. There are places like phototech.com in New York but I have no idea how good they are or what equipment they use for lens alignment and testing. I don't know of anyone who specializes in Canon telphotos. KEH.com also have a repair service, but again I've never used them.

If I had a problem, I'd document it as I describe above and send the lens along with the documentation to Canon. They can be difficult to get information from, but it is possible if you persist. They do tend to fix it and and just say "we fixed it". I guess as long as they actually have fixed it, that's all you need to know (though more information on exactly what they fixed would be nice to have).

[Frank Kolwicz]

I replaced that original 7d with another used one and everything is working fine.