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Old Aug 19, 2005, 8:56 AM   #1
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I have never been confident using manual focus on a camera. According to my eye-doctor I have a spherical distortion of the eye whereas the dioptre adjustments on a camera only ever compensate for simpler cylindrical eye deformations. What this means is that if I am to have any success getting sharp focus manually I need to wear either glasses or contacts. What this really means is that I only ever rely on the cameras autofocus.

But earlier on I was experimenting to see how my ability to focus a camera manually compared to that of the cameras. I started to get some pretty strange results.

My main lens on a Canon 10d camera is a Canon 28-135 IS, a lens which I have grown quite fond of. When, at 28mm, I focused manually on an object several feet away from me and then flipped over to autofocus to see how my accuracy matched up I was not surprised to see quite a difference. The camera said focus was almost 2 metres beyond where my eye was telling me focus was.
Strangeness started to creep in when I tried focussing manully at the 135mm end. This time what my eye and what my camera was telling me matched up perfectly. I wasn't too surprised yet, a larger more detailed image should be easier for me to judge? What I saw that was very strange was that the autofocus distance at 135mm matched what I had manually chosen at 28mm. In other words the autofocus was differing by 2 metres between 28mm and 135mm.

Now i've tried out all sorts of different things. I've tried the same (admittedly half-assed) tests using different lens, both manually and using autofocus, at wide angle and zoomed in, and the distances I am getting match with the reading at 135mm pretty consistently. The Canon 28-135 at 28mm is the only odd one out where it says focus is actually behind where everything else says focus is.

Now I know some of the first things people are going to suggest is to check the dioptre adjustment is appropriate; but I also went through the same test changing the lens onto a less advanced Canon 300 film camera (which doesn't have any dioptre adjustment control) and the results were the same.
Also another thing I should mention is that I never moved my position, always focussing on the same point and from the same place and using the same (central) focussing spot in the camera.

Now I need to take photos to compare sharpness but can anyone suggest why there would be a difference in the autofocus between the 28 and 135mm focal lengths?

I just want to say i'm not in a panic about the lens back-focussing (if that is whats happening) just curious about it is all. Any suggestions?
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Old Aug 19, 2005, 10:27 AM   #2
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I just made a proper test. Set the camera up on a tripod and measured the distance from the tripod to the subject with a tape measure. I used two different lens, a Tamron 17-35 set at 28mm and the Canon 28-135 set at 28mm. Aperture was fixed at F4.0 with mirror-lock-up and cable release used. I took two shots with each lens, one manual and one Autofocused and noted the approximate distance the lens gave each time.

Yes, I have way too much time on my hands!

So what I expected to see from the distance readings, given what I mentioned earlier, would be that the two shots from the Tamron (manual and Auto) and the manual shot from the Canon would match the tape measurement closely. I expected the Autofocus shot from the Canon to be well off. This is exactly the result I got, the Canon autofocus shot measured the distance some 2.5 metres behind where everything else said it should be.

What I expected to see when I transferred the photos to the computer would be 3 pretty sharp images and one that was focusing somewhere well behind the subject.

What I didn't expect would be that the Canon autofocus shot would be damn near the sharpest of the lot! My own two manually focused shots weren't bad but were still noticeably less sharp then the two which were focused automatically.

This confuses me!

All I can think of to explain it is that the distance scale on my Canon 28-135mm is inaccurate, while the autofocus mechanism itself works perfectly.

My conclusion is that I should shut-up and trust my camera to know a sharp image better then I do!
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