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Old 26-11-2010, 10:10 PM
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hotspur (Chris)
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Back Focus issue with70-200 2.8?

How does one go about looking at if there is aback focus issue with a lens,and then getting it looked at?

I have a 70-200 F2.8 canon,and its a very fine piece of glass,I am happy with images,but around 10% or a little less,seem to have back focus.I have noticed when using its not as good as focusing as say my 400 L or my (17-55 F 2.8 which is the best I have ever had for focusing).

Recently I took some images at my son's year ten formal,my work friend asked if I could take some of his daughter,I took some in the hall,some were quite good,I then took some of the girl outside against a tree,but all 6 or 7 images were like the three posted,focus on the tree,I aimed at her eye.

The ones inside hall,pic 4 look ok,the only major difference,I had soft box on flash out side.

Any thoughts,about how I could think about this issue,most likey operator error-99% of lens issues are 98% user error.But the 17-55 F2.8 almost out guns the 70-200,this should not be so.

Images are as is out of camera.
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  #2  
Old 26-11-2010, 10:42 PM
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dugnsuz (Doug)
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Hi Chris - Which AF mode are you using with the Camera/Lens?

I ask, as from the shots it looks like the focus is 'centered' on the foliage rather than the girl's face.

I found that going from the multiple point AF mode to the single central AF point sorted out my focus issues as the DSLR didn't pick a point outwith the centre of the image.

When I bought my 300mm f4L I couldn't get a really sharp shot until I switched AF modes - then problem solvered!

Doug

ps..that's probably too simple, but it's all I got!!

Last edited by dugnsuz; 27-11-2010 at 12:32 PM. Reason: punctuation!!!!
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Old 27-11-2010, 01:34 AM
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Octane (Humayun)
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Doug,

Simple, but, spot on.

If the lens had a backfocus issue, it would typically show up in each image.

Judging by the sharpness in the final image, I'd say you just need to switch your AF point to the central focus point (typically the strongest performing point) and use the focus and recompose technique. Be careful when using shallow depths of field as the focus and recompose technique may not work for gross movements.

H
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Old 27-11-2010, 11:57 AM
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acropolite (Phil)
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Chris even if the lens has a back focus issue, the 50D allows lens focus calibration. There are several sites with instructions on how to calibrate your lenses to the 50d body, you can calibrate individual lenses and the camera will recognise and apply the calibration when the lens is changed, do a google search.

Last edited by acropolite; 27-11-2010 at 03:21 PM.
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Old 27-11-2010, 01:38 PM
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DavidTrap (David)
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I was talking to my father in law - he had a focus issue with his Nikon D300. They could "electronically" adjust the focus - but only in the workshop.

Nonetheless, I tend to agree with earlier posters, I think your autofocus might be picking the wrong target. Have a play again with your lens and watch which focus point it is using. My camera isn't flash enough to have the face recognition stuff, but I prefer to use the central focus point and recompose.

DT
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Old 27-11-2010, 03:27 PM
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hotspur (Chris)
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re focus

Thanks for your thoughts chaps.I always select the focus point.in these 3 images,I selected the right focus point and used the camera vertically.I was using the 450d,so suspect the Humayan is right-the centre focus point is the strongest one,also being a entry level SLR it may not have very strong focus points apart from centre.

I may have selected the wrong point,as I was heading to car to put gear away.so slightly rushed.

Here is another image this time from 50d and 17-55 lens it has had some slight adjustment in P.S.

Cheers Chris
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