question on focusing


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Feb 19, 2006
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NUS
#1
Just bought a lens from a fellow CSer, happy with its sharpness, but got one problem:

the actual focusing point is always behind where my camera is aimed at, in simple words - misfocus.

I want to know whether it's my camera's problem or the lens'. Initially I thought it's the lens' fault, because I have never had this misfocusing problem before with other lenses.

However, after some study, I begin to doubt theoratically the lens itself is able to "misfocus" at all! I mean, the lens is always doing what the body tells it to do, the lens is always focusing and giving the body the perfect point, just that the body gives wrong info on where to focus accurately. Am I right?

Really hope someone here share their explanations. Thanks in advance!
 

kietgnoel

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Dec 24, 2004
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Planet Earth
#2
What you're experincing is called "backfocus". It's usually due to the lens but can also be due to the camera body. Do you have this problem with other lenses? If not, then it's probably due to the lens. Bring it down to Sigma (I assume the lens you bought is a Sigma? If it's a Canon, bring it to Canon, etc.) to adjust it.
 

Feb 19, 2006
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#3
it's a canon lens, and how much will they charge for it?
 

kietgnoel

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Dec 24, 2004
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#4
I have no idea how much it costs. Perhaps you can call Canon. They may be able to tell you the rough cost.
 

OlyFlyer

Senior Member
Mar 22, 2006
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#5
I am not familiar with the Canon AF but before sending in the camera and/or the lens I would do some more tests. First, set the AF to spot focus in the center only, if possible. Take a brick wall (or something similar) outdoors or a book shelf full of books indoors. Important that the whole picture area is covered. Do not select a fence since the AF may be disturbed by the background and the fence pattern. Use tripod for 100% stability. Take a few shots from different distances. If they turn out to be good then nothing is wrong with the lens or the camera. Maybe the way you use the camera or the lens is not optimal for the lens. It is important to test on stationary objects. Testing on moving target is a bit more difficult.
 

OlyFlyer

Senior Member
Mar 22, 2006
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#7
Yes, it is a well known test sheet by now, but the wall test answers the back focus problem more easily and clearly. If the camera / lens focuses on the wall than there are other problems. If not than it is time to visit service and let them sort it out.
 

OlyFlyer

Senior Member
Mar 22, 2006
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#9
called Canon, recaliberate charge is $80.
Not much but I would still test the way I described before paying $80, just to make sure it is definitely the lens.
 

Feb 19, 2006
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#10
yes, I tested against my bookshelf, on my camera body and another one from a friend, confirm it's the lens. sigh...
 

OlyFlyer

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Mar 22, 2006
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#11
yes, I tested against my bookshelf, on my camera body and another one from a friend, confirm it's the lens. sigh...
Well, look at it from the bright side. At least you don't have to part from your camera, you still have something to use while the lens is in the Canon Hospital ;) and once you get it back it will be like new. Hopefully they do a complete cleaning of the inside also.
 

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