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Old Mar 19, 2005, 7:55 PM   #1
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Cameras like the Canon Ixus 500 (S500) and the Sony Cybershots (and I'm sure many other) allow "prefocus."

If you "prefocus" the camera on one subject (by aligning it in the centre) and then move the camera (whilst still holding the shutter button down) so that the prefocused subject is no longer in the centre of the image, does the original subject remain in focus?

Thanks :-D
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Old Mar 19, 2005, 8:10 PM   #2
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Yes, the focus will remain the same, but exposure may not be appropriate for the new scene composition. Many cameras have exposure lock buttons to help with that, or you can try checking exposure on several parts of the scene and setting it manually (if the camera allows).
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Old Mar 19, 2005, 8:22 PM   #3
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What is this feature actually good for then? Is it just to reduce lag time?
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Old Mar 19, 2005, 8:49 PM   #4
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If the scene is evenly lighted, and/or if you want your exposure to be keyed on the focus point (say, a person standing on the edge of Grand Canyon), it makes sense. You can prefocus on the person, then move the camera slightly to put the person at the side of the frame, letting in more of the background scene.
On the other hand, if you prefocus on a dark main subject, but then move the camera so that strong highlights come into the frame, you'll probably end up blocking out the highlight areas. That just comes with the territory.
No one said it would be easy It requires practice and some experience to figure out how you want the exposure to appear. But the prefocus function is definitely beneficial in many cases--as you suggested, if nothing else, it takes less time then for the shutter to operate when things are moving quickly.

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Old Mar 19, 2005, 8:57 PM   #5
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Ah, I see. Thanks a lot for the explanation. :-D
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Old Mar 19, 2005, 9:33 PM   #6
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Many digital cameras have a continuous focus mode. It burns a little more battery but makes the pre-focus a really great tool. My Oly C50 doesn't have continuous focus and I have more blurred or poorly framed pictures of rugrats than I would like to admit with it. My other cameras have continuous focus and it is a night and day difference. I can pre-focus for the fast shutter but it will stay in focus if the subject moves.

It is never tested or even mentioned by the testers. I suspect they take their static test shots for reviews and then grab their DSLRs for their personal pictures. From comments I've read there is a difference in continuous focus capabilities between cameras. No way to know from the reviews though.

A quick perusal of the S500 review didn't turn up a continuous focus mode, so you are probably in the same boat I'm in with my C50. I try to pre-focus to get the fast shutter release but have to release it and pre-focus again if the target moves. I really miss having it when I'm using that camera in dynamic situations.


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Old Mar 20, 2005, 5:19 PM   #7
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I haven't got my new camera yet but I'm looking at a few cameras including the Sony P200. The Sony does seem to have this "continuous focus mode" however Sony call it "Monitor AF mode."

I'll try and look to buy a camera with it because it does sound useful. :-D
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Old Mar 20, 2005, 5:33 PM   #8
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Recon wrote:
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What is this feature actually good for then? Is it just to reduce lag time?
Yah...I really do believe that it is good for reducing the camera's thumb twiddling time. I agree with that.

Also, I think that there may be times when you have to pay more attention to the pre-focus, such as when you're doing very close-up shots in macro mode. If we use those low aperture 'values', and we prefocus on something really close to the camera, then moving the camera by rotation, or just unwarily shifting the camera back or forth (hand held condition) could get us out of focus again. Just depends on the situation I think.
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