Front Focusing: Tolerances Canon EF 24 - 105 Lens

I have just purchased an EF 24 - 105 lens that autofocuses 3.5 inches in front of a target at a distance of 8 feet. At a distance of 3 feet, focus appears to be within 1 inch. Should I return this lens that appears to have excellent sharpness at infinity (from 35mm all the way to 105mm) or is the autofocus inaccuracy within typical tolerances? With manual focus, sharp images can still be obtained. Another fixed focal length prime lens I own (EF 28/f1.8) appears to autofocus with no obvious problems.

As a new digital enthusiast, I am seeking wisdom of seasoned photographers. Thank you!

Hi Artie

Some more information might be useful. What Canon camera are you using; what are the aperture and shutter speed settings (and is there variation in focus distance with variation in these settings); and how did you measure the true focus distance?

Cheers

David

Hi David-

I am using a Canon 450D/RebelXSI. I first noticed this issue when focusing on geese in a grassy field at a distance of around 40 feet (f7.1, 1/250 sec, 105mm, iso 400). I found that the sharpest part of the photo was the grass a few feet in front of the geese. I consistently lock focus to the center focus point.

As a follow up test, I placed three small flat-faced alarm clocks in a staggered array on a table such that the middle clock was exactly 8 feet (measured with a steel tape) from the camera on a tripod, the front clock was placed 3.5 inches closer, and the third clock was placed 3.5 inches further away than the middle one. All three clocks were in the field of view. I then autofocused on the center clock using f4.5, 1/30 sec, 105mm, iso 400. I found upon reviewing photographs that the front clock was in focus and the middle clock was not. When I focus on the most distant clock, I find that the middle clock face is in focus. When I manually focus on the middle clock then I can get a photograph with the middle clock in focus.

The above test appears to display the same front focusing problem I had initially noticed with the geese. I have not noticed similar issues after extensive use of my 28mm/1.8 prime lens with the same camera.

Hi Artie - Thanks for the extra information. That's very useful. Two preliminary points: first, I don't have the type of lens that you have, and, second, further comments from more technically able people than me might be more helpful. However, having considered the matter as best I can I do think there may be a problem with the lens. The results of the tests that you have run should not be occuring. Your camera and its firmware seem fine because your fixed focal length lens gives appropriate shots. Your staggered clock array gives focus discrepancies far outside what might be expected with autofocus, given your use of a single spot focus point and flat faced clocks. The manual focus result implies that the optics of your lens are not at fault. Thus, there may be something awry with the focus drive mechanism in the lens.

However, one thing you may wish to check, if you have not already done so, is the Canon web site to see if this issue has been raised there, and to check if the latest firmware revision has anything to offer.

In addition, perhaps you should wait a few days to see if anyone else here picks this thread up.

Best Wishes

David

Thanks David, your thoughts are very much appreciated and I will contact the Canon website as you suggest and report any interesting feedback. It is great that you jumped in on this issue, and that this forum exists to facilitate such helpful communication.

Issues with front/back auto focussing seem quite common. I understand that auto focus with zoom lenses is difficult because of the variations of degree of zoom, and the focussed distance. Nevertheless, such a large discrepancy is tiresome; Canon should be able to calibrate the camera and lens better - on some cameras you can do this yourself, but not, I think, on a 450.

Bertie

I had a similar problem with my 6d with the 24-105mm lens with grass in front of the subject being more in focus which was glaringly obviously when I tested a half mile away shot with my new 70-300 usm is lens which was much sharper so I tried small writing at 2 metres away and found at zoom +10 micro adjustment was sharper and +6 at wide angle retested long distance shots with both lenses and at centre they were very similar with the 24-105mm having better edge focus unfortunetly not all cameras have micro adjustment unless you manually alter it.

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Originally Posted by pauluk

I had a similar problem with my 6d with the 24-105mm lens with grass in front of the subject being more in focus which was glaringly obviously when I tested a half mile away shot with my new 70-300 usm is lens which was much sharper so I tried small writing at 2 metres away and found at zoom +10 micro adjustment was sharper and +6 at wide angle retested long distance shots with both lenses and at centre they were very similar with the 24-105mm having better edge focus unfortunetly not all cameras have micro adjustment unless you manually alter it.

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dont you have a shift key/

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Originally Posted by Artie

I have just purchased an EF 24 - 105 lens that autofocuses 3.5 inches in front of a target at a distance of 8 feet. At a distance of 3 feet, focus appears to be within 1 inch. Should I return this lens that appears to have excellent sharpness at infinity (from 35mm all the way to 105mm) or is the autofocus inaccuracy within typical tolerances? With manual focus, sharp images can still be obtained. Another fixed focal length prime lens I own (EF 28/f1.8) appears to autofocus with no obvious problems.

As a new digital enthusiast, I am seeking wisdom of seasoned photographers. Thank you!

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Front/back focus calibration with the 24-105 f4L IS USM should not be notiecable "out of the box" and what little variation there is can usually be corrected using the fine focus adjustment in the cameras that the lens is designed to be used with, which are the professional or enthusiast models. As the 450D/Rebel XSi does not have this facility, and as it is possible that the auto focus calibration of this camera may vary by such an amount from sample to sample, you may have the bad luck to have a combination of camera body and lens that are both out of tolerance in the same "direction". I would return the lens to the supplier and ask to try another example, if it is still under warranty. The alternative would be to return it to a Canon service centre with evidence of the problem, and ask for it to be calibrated.

Just a thought, but auto focus will always try to focus on the closest sharp edged object. Even with just centre focus point there can be false auto focus problems in the sort of cases you mention; which is caused by the camera locking on to the 'wrong object'.

So it doesn't necessarily mean a lens fault.

Have you tried one of those focus charts where you shoot at 45 degrees to a scale. You can even make something from a ruler as a temporary test.

These tests are somewhat unreliable at best; but if you get a consistent error like this it would indicate a more serious lens problem.

There is a possibility that you need to update the firmware in you camera. More lenses is a common part of Canon firmware updates. Looking on Canon Europe there aren't any but the US or other Canon sites may have them. Otherwise an authorised Canon Service Centre could do it for you and will probably know if there are updates.

May as well mention sillies. The camera has an AF confirmation light. I initially had one or two problems on an original rebel due to not taking note of that. 300D in the UK.

The manual mentions several things that may prevent it from working - not keen on the fix though. Something suitable at the same distance.

They also suggest focusing manually. That could be done on the 3 clocks for instance and situations where objects are relatively near and DOF is fairly shallow. It would be interesting to see how well the manual focus assist works when focusing manually. The indicators should light up. Manual focusing can be improved by deliberately going from short to long and setting the middle. An interesting fact - old advice was always to focus into the subject not on it. When manually focusing there is a tendency to stop when the object appeared to be in focus. What that means is that the object is at the "front" of the focus field as seen by the eye so there is a real need to focus into subjects.

As to AF itself other than it's designed to be fast the sensors do it not the lens. The speed is a problem. They may make assumptions about how fast the parts move and the rates focus points shift at.. This might explain the content of some firmware updates. Maybe the glass in lenses has implications. Anyway my 300D has no problems with full frame lenses but the ones I use were in Canon's range when I bought the camera.

If it is a used lens I would also wonder about wear but if the focus or what ever hasn't gummed up or become rather slack it would be hard to tell without trying a known good lens.

John
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Faulty lens. Send it in.