Canon 17-40L back focusses at 17mm?

i just noticed this but my 17-40L back focusses when zoomed out to 17mm. focus gradually gets better as you zoom in. its perfect from approx. 25mm-40mm.

if i zoom to 17mm, acquire focus and take the shot, its consistenly back focussed compared to when i zoom to 40mm, acquire focus, then zoom back out to 17mm and take the shot.

any ideas? or is this by design?

Not all zoom are parafocal. Meaning that refocus might be requirred when changing mm.

so youre saying this is by design?

ok i just googled what parafocal meant and no, this is not my problem. if i acquire focus at 17mm, its back focussed. if i acquire at 40mm, the focus is perfect. why is that? or is this by design?

thanks for the quick reply btw.

I see what you mean, and I actually intended to make a thread addressing this. It's a habit I've developed whenever I use a zoom. I always zoom in all the way, acquire focus on the subject, and then zoom out as necessary to compose. I never compose before focusing just because it seems like there's more room for error for the autofocus at the shorter ends of zooms.

Now I don't know if your 17-40L is back focusing at the short end or not, but it seems like focusing at the shorter end of zooms isn't as accurate. Then again, I suppose that just makes sense.

thanks chris. yeah its weird. ive tested this phenomena on my 30D and 5D and both exhibit the same behavior with the 17-40. im wondering if this is by design.

never noticed this issue on my other zooms.

roeddel wrote in post #4879709
thanks chris. yeah its weird. ive tested this phenomena on my 30D and 5D and both exhibit the same behavior with the 17-40. im wondering if this is by design.

never noticed this issue on my other zooms.

I don't have this issue with my 17-40, neither on the 350D or 400Ds I owned, and neither on my current 40D.

However, I focus differently with this lens, especially for landscapes, than I do with any longer lens.

I focus 1/3 the way into the object I want in perfect DoF. That does the trick every time. Oh, and it does focus correctly with a test chart.

I use the same technique with my EF-S 10-22.

HTH, kind regards, Wim

thanks Wim. you said that it focusses correctly on a test chart. does your 17-40 focus properly at 17mm? or do you have to acquire focus at a different focal length then shoot at 17mm? thanks for the reply!

roeddel wrote in post #4879709
thanks chris. yeah its weird. ive tested this phenomena on my 30D and 5D and both exhibit the same behavior with the 17-40. im wondering if this is by design.

never noticed this issue on my other zooms.

If you only notice it on your 17-40L, then your lens may have its own flaw, though I'd rather not jump to that conclusion so quickly. I would assume that your lens is parfocal, which would explain why you can zoom in to 40mm, focus, and not have any issues zoomed back out to 17mm. I don't think varifocal SLR lenses would function the same way.

Can you do two tests on a chart? Maybe one where you focused at 17mm and the other where you focused at 40mm and zoomed back out to 17mm.

i havent dont focus chart tests but real life tests prove that when i focus at 17mm - it backfocusses and when i focus at 40mm and zoom out to 17, focus is perfect.

i wonder if anybody else with the 17-40 have this problem.

"noobies argue about bodies, pros argue about glass, seasoned veterans let the pictures argue for themselves." Can you show us a pic of what you're getting? How close are you to the subject?

the proof is in the pudding mah friends: this was shot at about 10" from the front battery for which focus @ 17mm was acquired.

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OK I will test that with my 30D + 17-40L.

Cheers,

Sam

thanks sam!

Right, well I am not convinced your test methods are totally suitable, but I had a go anyway!

My lens seems fine. I set it up rested on the table, MLU, remote release. I dofocussed the lense manually then let it AF.

I did notice that if I moved from the 2nd battery to the 1st battery the AF did not refocus?

Here's the pictures, no sharpening other than in camera.

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