70-200mm f/4 IS focus issue

[scotch196]

Wonder if anyone can throw some light on this? I have a brand new 70-200 f/4 IS lens that's very highly rated for sharpness across all apertures, but am getting strange results from basic tests I've done.

I taped a page of newsprint with colorful grocery ads to a board about 2ft x 2ft and stuck a note of my local currency in the center for the fine detail. Put camera on a tripod about 20 ft from board, IS off, using mirror lock and remote release, shot normal jpegs using auto focus.

Tried this on three different bodies (40D, 450D, 1D MkII) and in each case the images get less sharp, specially in the center, as I stop down. At f/8, the center is distinctly fuzzy although the corners aren't too bad.

On the 40D, did the same test with my 20-yr-old magic drainpipe and got very different results. At f/4 images are similar (maybe marginally sharper on the old lens), but at 5.6 and 8 the drainpipe is miles ahead. Also did this with a 70-300 IS lens set at 200mm and again, this beats the new lens hands down.

I also did a test with books standing on a table at different distances from the lens and it appears that as I stop down, the 70-200 is focusing further and further back.

I've read some articles about focus shift, but it seems this is usually on very fast lenses. I haven't tested the lens in the field, particularly on distant subjects, but from what I've seen so far there appears to be a definite problem.

As mentioned, it's out the box and under guarantee - but nevertheless a hassle to return it as bought over the internet.

Before doing anything drastic, just wondering if I'm being alarmist. Any suggestions, comments, or sage advice greatly appreciated.

Thanks,
Scotch

[Bob Atkins]

Most lenses should sharpen up when stopped down a stop or two especially at the corners of the frame and, to a lesser extent, in the center. If you stop down excessively (e.g to f16 or f22) then you will probably see a slight overall reduction in sharpness due to diffraction, but at f5.6 and f8 this will not be a noticable effect.

There should be no focus shift on a lens like the 70-200/4. It's something you might see on a really fast (f1.2) prime, but not on a relatively slow lens (and f4 is slow in this context).

A noticable decrease in sharpness when stopping down from f4 to f5.6 and f8 is not something that can be explained in a well designed lens like the 70-200/4 and there should be no focus shift with aperture either.

If you are sure that your observations are correct, I'd say the lens is probably defective in some way and I'd return it for exchange or repair.

[scotch196]

Thanks Bob, appreciate your advice and confirmation that this should not be occurring on a lens like the 70-200 f/4.

I'm pretty sure from my tests that the lens is faulty and have sent it to a Canon repair center under the warranty. Was fortunate in being able to speak to someone there to explain the problem as I see it, so hopefully they'll be able to sort it out, otherwise the supplier will need to replace it.