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AI focus AF

Ines Lojna Funtak , Apr 23, 2007; 12:45 a.m.

Hello. I was happy for getting my Canon 20mm 2,8 but very dissapointed because my picture looks unsharp. Finally, I saw one "little" mistake I made, just yesterday evening. My AF mode was on AI not on one shot. Could that be the reason for not getting sharp images? What is the minimum exposure time taking pictures handheld with my 20mm lens? Today I intendend to go to Canon service to resolve my "not good" lense. Thanks for the answer. Ines

Responses

Brian Coy , Apr 23, 2007; 09:42 a.m.

Try shooting images at a fast speed on single shot autofocus. If they don't look good then you might have a problem with the lens, if they look good then the problem is technique.

Mark U , Apr 23, 2007; 03:09 p.m.

It's unlikely that AI focus would cause loss of sharpness. With static subjects, it should focus just the same as One Shot provided you don't recompose after focussing. You don't say what camera you are using, or what film or ISO. Some information about the shutter speeds and apertures you were using might also help diagnose your problems. It's possible that the lens you have is defective, and if it is new rather than second hand, Canon should be able to fix it for free under warranty.

Nadine Ohara - SF Bay Area/CA , Apr 23, 2007; 03:53 p.m.

On the other hand, AI Focus and even AI, can shift the focus (to usually the background) unintentionally, and cause one to think one's images are out of focus. AI Focus is especially bad, because if you focus/recompose, and then move the camera slightly, or something in the background is moving, the focus will shift to the moving object or the background because the camera "thinks" your subject is moving. However, you would need to verify that by checking your images. If part of the image is in focus (even if not the intended part), then autofocus mode can be the cause. If the whole image is out of focus, it is possibly a handholding problem or it could be the lens. If you are using a crop camera, the shutter speed guideline is 1/30th. For a full frame camera, 1/20th.

Steve Dunn , Apr 24, 2007; 03:17 p.m.

On the other hand, AI Focus and even AI, can shift the focus (to usually the background) unintentionally, and cause one to think one's images are out of focus.

Good point. If your unnamed camera has such an option, turn on the viewfinder display of which focus point(s) it's using; that will tell you where it thinks the subject is before you take the picture. This is only an approximation, as the AF sensors are often substantially larger than illustrated in the viewfinder (and can therefore focus on things which are not within the boxes in the viewfinder), but it will at least confirm that the camera is focusing on something in the same part of the viewfinder you want it to.

If this is digital, some of the software that came with the camera should be able to show you which focus point was used. Out of the software that came with my 20D, for instance, EOS Viewer Utility has an option to superimpose the AF boxes over the image and mark the active one(s) in red. This can be useful if you're examining an image, are unhappy with sharpness, and want to check which focus point(s) is/were used.

If you suspect that the problems could be due to handholding at too slow a shutter speed, test it. Put the camera on a tripod. Use flash as the main light source (flash bursts are typically in the neighbourhood of 1/1000s), or use a remote release the self-timer (so that you don't shake the camera at the moment of exposure by having your hand pressing on one of its buttons). If this yields sharp images, the lens and camera are working properly, and the problem is your handholding technique and/or choice of shutter speeds for handholding. If taking your hands out of the equation still yields unsharp pictures, then there is something wrong with the equipment; it may have AF problems or the lens may be out of alignment, for instance.

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