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Auto Focus Problems

jonathanbp 9 99 Thailand
12 Feb 2011 12:35AM

I use a Canon D50 with a Sigma 18-200 lens. Both are 2 years old, have taken 7,500 images to date with the equipement.

I'm wondering if anyone has had a problem with degredation of the autofocus? When I first bought the camera my images were pin sharp, now, not so. Does autofocus degrade after a while? Is the the lens or the camera? Are digital cameras supposed to be taken in for a 'service' once every 5,000 pics or so? I'm furstrated...sometimes it works, other times, no so.

Any tips appreciated.

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NexusImages 11 1.8k 4 United Kingdom
12 Feb 2011 1:04AM
Try a sensor clean? and a contacts clean? Smile
timbo 15 596 United Kingdom
12 Feb 2011 10:40AM
And possibly a service.
I have found with Sigma lenses that after a while they do appear soft and in some cases badly soft. Sent to sigma who serviced them but never really happy afterwards so tend not to use them now.
filmforever 10 748
16 Feb 2011 9:59AM
I'm sure there must be many reasons why autofocus systems don't "autofocus" on a consistant basis.....poor light, lack of contrast on the subject etc.
It's ironical that the circumstances where you need autofocus the most...i.e. fast moving subjects....are the very times it's likely to let you down.

One thing I have noticed, the autofocus on my old film Nikon F90x's is far quicker and more reliable than on any of the digital cameras I've owned.
Is there something in the digital process that makes autofocussing more difficult then with film??

Maybe someone with technical knowledge can answer this one?
discreetphoton 13 3.5k 20 United Kingdom
16 Feb 2011 10:13AM
partly it will be down to the difference in depth of field with full-frame, which allows for better edge contrast in your F90.
User_Removed 8 1.4k England
16 Feb 2011 10:17AM

Quote:digital process that makes autofocussing more difficult then with film??

Depends on the implimentation.

Most cases its the same.

But if the film camera had an analog autofocus, e.g. sonar or another implimentation (phase detection) then it would be much more responcive than a digital contrast detection algo, which obviously has to be processed - and is not normally priority.
LenShepherd 10 3.6k United Kingdom
17 Feb 2011 9:36AM

Quote: When I first bought the camera my images were pin sharp,

First clean the lens and body contacts - then do an AF test of a good AF target, ideally using flash as the main light source, and camera on a firm tripod. Shooting this way can you get very sharp images?
If not, providing your subject is one the AF can accurately detect, your camera or lens needs a repair.
If your camera AF is good in a static test maybe you now shoot different subjects beyond the AF capabilities of the camera and lens.
Turning to film bodies v digital - there is no AF speed difference with a good contrast target between my F5 and my D3s using single are AF
The D3 can detect a much lower contrast target than the F5, and in 51 point AF the D3 slows up because it is processing more information than in single area AF.

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