Front Focusing: Tolerances Canon EF 24 - 105 Lens

[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!

[David]

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

[Artie]

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.

[David]

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

[Artie]

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.

[rc53]

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