20D Focusing Problem

[brian_h4]

<p>Hi. I have a 20D with the Canon EF-S 17-85 zoom. It has been awhile since I last used the camera. Batteries are charged and I go out to shoot and the lens seems to keep searching and won't lock on, especially at full zoom. At wide angle, it seems to be ok. Can you tell me what I'm doibg wrong? Thanks. </p>

[www.philwinterphotography.]

<p>Is the focus mode set to AI Servo?</p>

[nico_lilien]

<p>Perhaps a dark environment and/or a low contrast subject... On the other hand you mention decent AF at wide angle.</p>

<p>Hmmm, does anything other than the focal length vary between these shots? <br /></p>

<p> </p>

[brian_h4]

<p> Thanks for the replies.<br>

The lens is set to AF with stabilizer on. The camera dial is set on the green square. IS there another setting that I'm overlooking?<br>

No lighting is good, outdoor sunlight. </p>

[nico_lilien]

<p>It is actually not that difficult to get a good functioning camera confused. I'm not entirely sure if this applies to your 20D though:</p>

<p>When I set my 10D to 'green square' it has to auto do everything: pop's the flash and starts to select AF points that I normally don't use.</p>

<p>When I select a low contrast subject eg. an even coloured carpet or a milky glass door it starts hunting for a good AF point. This is of course not directly comparable to your situation as this was done indoor wit a 50mm lens. But it does show that using the auto mode isn't always helpful. Sometimes subjects are easier captured using the central AF point.</p>

<p>Your lens probably uses a different aperture at the 'wide end' compared to full zoom. This difference affects the light available for obtaining AF.</p>

<p>May I suggest setting the 20D to M and the center/single AF point. Set the aperture to the largest value indicated on the lens barrel.<br>

Just use any shutter speed setting that gives a decent result (we're not in a contest).</p>

<p>Now I'm curious if you still encounter a difference in AF lock between zoom and wide angle.</p>

[brian_h4]

<p>Thank you for taking the time Nico. I hope you are right (that I am simply confusing my camera). I will have to break out the manual to implement your suggestions. And will post the results. Obviously I'm not versed in the operation of my own camera and rely heavily on its auto capabilities. I would like to maintain the auto mode but change the focusing point to the center of the viewfinder. Thank you again.</p>

[nico_lilien]

<p>Your problem keeps getting my attention, apparently it puzzles me. After a good night sleep I'm not so sure that manually selecting an aperture will provide equal conditions for the AF system.<br>

Your lens will probably use the maximum aperture opening during the focus process, it will only close down to the needed or selected aperture during the actual exposure. This is why I think my suggestion to select the aperture manually won't help to determine the cause of your focus issue.</p>

<p>I think you can't select a single AF point and use the auto mode at the same time. But you can use the P (program) mode to do some tasks automatically and set some features manually.</p>

<p> </p>

[steve_dunn2]

<p>The aperture you select is completely irrelevant to autofocus. As suggested above, AF is always done with the lens at its maximum aperture; the lens only stops down for the exposure itself (or, of course, if you activate depth-of-field preview).</p>

 

<p>Switch to P (or any other mode which gives you control) and manually select the central AF point and one-shot AF mode. Try focusing on something that's well lit and has some sort of high-contrast feature for the AF system to lock onto. If your camera still won't focus reliably, then something is definitely wrong, though it may be hard to say if it's the body or the lens without having an opportunity to try a different lens on the same body or the same lens on a different body. If the camera works reliably in this situation, then start changing the above items (one at a time) until you find some sort of change that makes the problem occur again.</p>

 

<p>I suggested P mode because it is more or less green box (auto) mode plus the ability to take control over things. If you reset the camera to its factory defaults in every way and set it into P mode, it works almost identically to in green box mode - but then you have the option of overriding some of its settings, which you don't in green box. So if you want to learn a bit more about how to use the camera as more than a big, heavy point-and-shoot, but don't know where to start, P is a good place to start. It will seem familiar, yet allow you to experiment until you become more comfortable.</p>