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PostPosted: Mon Oct 06, 2008 10:05 am 
Hi,
I've got a new EOS 350D with a Canon 70-300 USM and in the past weeks I played with it taking pics of sailing competitions. The first ones were really great but at a certain point many photos were not in focus. I just don't understand why, shutter speed was fast (fixed on 1/2000), the only thing I noticed was that un-focused pics had an aperture of 5.0 or whider because ISO settings where on 200: may this be the point or may it be an AF problem?
Thank You,
Giovanni T.


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PostPosted: Mon Oct 06, 2008 1:08 pm 
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Joined: Sat Dec 22, 2007 10:43 am
Posts: 512
Location: Copenhagen, Denmark
Well the focus is somewhere, but it sounds like its in the wrong place. Several things to do imo. First of all play around with focus types. AI Servo (i think its called on the 350D) is most likely the best option, since that continues to focus on moving targets. Second you might want to try to manually select a focus point, rather than letting the the camera pick one. Im not sure, however, that this has any effect with AI Servo.

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Jake

Canon EOS 400D + EF-S 17-85mm f/4-5.6 IS + EF-S 55-250mm f/4-5.6 IS + EF 50mm f/1.8
My Photos - My webpage (danish)


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PostPosted: Tue Oct 07, 2008 7:02 am 
I think you're right! Looking closely to the pictures I can see the very narrow on-focus line and it's not always in the right place!
Thank You,
GT


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PostPosted: Tue Oct 07, 2008 11:59 am 
The fact that you have established that the focus problem occurs with photos taken with a wide aperture suggests it might be a DOF (depth of field) issue. With wider apertures your DOF becomes more shallow so accurate focus becomes more critical. And if your subjects span a certain distance you won't be able to get everything in focus because the DOF won't cover that whole area. For that you need a wider DOF and for that you need a smaller aperture. You described the focus area as very narrow and that would fit with what I described. Check the other photos taken at smaller apertures with correct focus and compare how wide the focus area is in those.

Ben
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When in doubt..... Press the shutter.


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PostPosted: Thu Oct 09, 2008 8:42 pm 
Thank you for the reply! I think I solved my "problem", I understood that F5.0 already gives quite a shallow DOF, infact the moving boats managed to come out of the focused zone! Talking with a pro I met at the sailing club I learned that if I want to keep shutter speed fixed at high speed but don't want to have a small F aperture I can still set an higher ISO! I didn't do it before because I was scared image quality would have gone down too much, but that was an issue with old film SLRs!
Cheers,
GT


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PostPosted: Fri Oct 10, 2008 8:28 am 
You're welcome.

With a DSLR you should be fairly safe at ISOs of 800 and even 1600. It will depend on how much cropping you will do in PP or whether you will be viewing these photos in full or zoomed in on your monitor. If necessary you can apply noise reduction in PP too.

It all depends on the way your photos will be used. That determines how much noise they can or cannot have.

Ben
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When in doubt..... Press the shutter.


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