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Old Dec 18, 2005, 2:32 PM   #1
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Hi everyone,

I had 2 questions.

1. When I see MANY pictures online (from DSLR with similar lenses) or any images from P&S cameras, it seems as though majority of the image is in focus whereas whenever I take pictures I have the center focused but then even someone's ears could be out of focus (in the bokeh?) though generally the face area is ok and then it's just the body. I was wondeirng how people manage to have such a huge focus area? Also wanted to add that I don't notice much difference stopping down? (increasing f/#) either.. also increasing the f/# of course makes it harder to take handheld pics indoors.

I know I'm using a large aperture which causes small DOF but a person's body is pretty much on 1 plane. I also use centre point AF all the time because.. that's just what people suggested and it makes sense to me. Is it possible the centre point AF is causing the top/bottom and left/right of the person/group of people to be out of focus?

2. What is the focusing mode that refocuses on a moving subject so you can, for example, take a picture of a moving car and have the car still but the background blurred? (I don't have my manual with me)

Thanks a lot for the responses!

Doug
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Old Dec 18, 2005, 3:16 PM   #2
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The Depth Of Field (DOF) must increase when you increase the f-stop. You might not be noticing, but it does.

There are 4 contributing things to the size of the DOF:
The larger the f-stop (smaller aperture) the larger the DOF.
The shorter the focal length the larger the DOF.
The further away the subject the larger the DOF.
The larger the sensor the larger the DOF.

I would suggest reading:
http://www.dpreview.com/learn/?/Glos...f_Field_01.htm

It has a depth of field calculator on it, and I'd suggest calculating how big the DOF is for the situation you're trying to photograph. You'll be surprised how small the DOF can be.

A person's body is very definitely not in 1 plane. It is very easy to have a DOF that is smaller than the length of someone's nose. And if you focus on face, you just might be focusing on their nose. It is a very common problem to have only part of a face in focus. Stop down (increase the f-stop) and you'll get more DOF. F8 or more is not uncommon.

The reason to use the center AF point on most Canon cameras is because it is more accurate with better lenses. And it is a cross-hatch sensor, meaning it detects contrast going horizontal or vertical, and all Canon camera AF systems work off contrast detection.

Could you describe the settings/situation you're shooting? Focal length, f-stop, subject distance, camera?

2. AI-Servo always tries to track what the subject is doing. AI-Focus switches between One Shot and AI-Servo when it thinks the subject is either stationary or moving. I don't recommend that setting. BTW, you can download the manual for every Canon camera off the web. Check out http://www.canoneos.com. If you can post to Steves's you can view the manual in your web browser.

Eric
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Old Dec 18, 2005, 3:35 PM   #3
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Thanks a lot for the response! the things you've said have already explained a lot of my problems!

I'll definitely take a look at the artcile you posted shortly!

Thanks again!

(and I can't believe I forgot to check for an online manual! argh!)
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Old Dec 19, 2005, 8:44 AM   #4
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Glad to help.

Guess what? The Canon flash manuals are *not* on their web site, but the camera ones are! So their web site isn't exactly complete in that area. I just happen to know because I've downloaded it before.

Eric
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