AF Inconsistency - Advice Needed

I have a 17-55 f/2.8 IS lens that I used to be very happy with.  Some time ago I found myself using it less and less because too many shots were coming in out of focus.

A while back I upgraded from a 40D to a 7D so it occurred to me that the lens might have been better matched to the 40D.  I read about AF microadjustment techniques here and tried it out on my 7D to see if I could fix it.

With my camera on a tripod, this chart taped to a box and a measuring tape running past the box I took some test shots.  I set the lens to 55mm, AF, IS, the camera to Av f/2.8 ISO 100, placed the camera about 1.4m from the chart, enabled mirror lock-up and used a remote shutter cable, checking the images I was capturing at 100% on my laptop.

Before I even got to making AF adjustments I noticed major inconsistencies in the focus distance.  The depth of field was roughly 5cm +/- 1cm on the measuring tape, but consecutive test shots would be 4 or 5 cm apart in focus position.

I thought that the IS might be interfering so turned it off, then the first couple of shots seemed consistent, but then I got a pair that were 6cm apart.

At that point I gave up on the idea of microadjustment because something much more troublesome was going on.  I tried switching to my 85 f/1.8 2.1m away from the chart to check my methods and it turned in shots that consistently varied by less than 1cm.

Do I have a faulty 17-55 lens on my hands or have I missed something obvious?

Thanks in advance for any advice,

Sam

Hmm..

The best tool to use to find a problem is substitution, which you did. Here's what you did and what you found:

1. Only a single lens is affected
2. The affected lens fails across two bodies
3. Other lenses work OK on both bodies (I assume)

Considering the above, my conclusion is that your lens has a problem. It does happen. I assume you had the camera in One Shot, and that the test target was well lit?

Rich

http://www.iceninephotography.com

Gravi wrote:

Hi,

Hopefully I can help you with an easy solution: it might be that you have some dust in front of the AF sensor on your camera body.

Put the camera in sensor cleaning mode and use a blower to clean the AF sensors in the bottom of the mirror chamber. Then test again.

If you are lucky it was dust and AF will be consistent again. Now let's hope it is consistently spot on as well

It appears that the OP is having a problem with just this lens. This sort of rules out the AF sensor. You may be right, though. It's worth blowing out the mirror box, just to be sure.

Rich

http://www.iceninephotography.com

salamandersam wrote:

I have a 17-55 f/2.8 IS lens that I used to be very happy with.  Some time ago I found myself using it less and less because too many shots were coming in out of focus.

This lens should be a very consistent AF performer. The test chart you are using is not ideal from an auto-focus standpoint. Try testing on a image that is more similar to real life subject matter (such as a piece of fabric or even a cardboard box that has been printed on). And make sure the test is carried out under bright lighting conditions. If you are using the center AF point, it can become confused under low to moderate lighting conditions when using a lens of f/2.8 or wider. I try to use one of the AF points surrounding the center point under less than ideal conditions.

When you put it on a tripod and used liveview, did you remember to block the eyepiece?  That can and will cause inconsistent results.

A really good way to check a lens for consistent focusing is to use FoCal Pro.  It sets the lens to infinity or to minimum distance before every shot (you set the options)

You can easily see if the lens is focusing consistently, or if user error might play a part.  There is little doubt that a damaged lens can focus inconsistently.

BTW, they all vary occasionally, but should be reasonable.