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I have a Canon 1000d, and the standard lens EF-S 18-55mm. Recently I was out taking photos and I've noticed that if I zoom in fully, the lens can't auto focus. I can still take shots using manual focus at full zoom though.

It will just try and find focus constantly buzzing back and forth for as long as I hold the shutter. I was in a bird hide and was holding the shutter and moving the camera around and it was doing it constantly until I pointed it at something nearby.

Does anyone have any idea on why this might be happening? I'm really worried that I've damaged the lens in some way. Or does my kit just need cleaning?

Any advice would be great. My camera lives in a camera bag when not in use, so I don't think it could be dust. I also have a clear filter to protect the shooting end of the lens.

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2 Answers 2

up vote 5 down vote accepted

If you are unable to focus on any subject at any distance when fully zoomed, then it could certainly be a hardware issue.

It is more likely, however, that the autofocus is simply not able to find a clearly defined subject in the area where you are shooting. This can happen for a variety of reasons, so I'll just list a few common ones:

  • too close to subject - lenses have a minimum focus distance, which should be listed on the lens
  • low contrast - the AF system needs to have a high enough contrast subject that it can tell where the edges of objects are
  • low light - this causes low contrast
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It could be that, here is an image I took, picasaweb.google.com/neon1024/TestwoodLakes#5575831054259189618 - do you think the contrast could be off? That's post Lightroom though! –  DavidYell Feb 21 '11 at 13:39
    
I could certainly see that being the issue. The edge of the lake or or the white tree above the bridge have good contrast, but most of the wooded are does not. This scene might be a good time to focus manually. –  chills42 Feb 21 '11 at 14:23
    
Also, at the settings used in your example (f/10 and 5mm lens) the hyperfocal distance is 52 ft. That said, it looks like the edge of the water is probably not to far off from that, so you could use that as your focus point and everything should be in decent focus. –  chills42 Feb 21 '11 at 14:28

The 18-55 is at f/5.6 at the long end. Not coincidentally, f/5.6 is the smallest aperture that Canon's AF system (except the top of the line models which keep going until f/8) will actually work at - officially, it simply quits trying if the lens is f/6.3 or darker. Some superzoom f/3.5-f/6.3 lenses cheat, and lie, and say that they are f/5.6 to keep the AF trying. In short, the AF does not have an easy time of it at f/5.6, and the 1000d is a low-end camera with a not particularly brilliant AF system.

A lens with a larger aperture, like a 17-50 f/2.8 style zoom, would give more reliable autofocus. A camera body with a more fancy autofocus system would have an easier time of it with the 18-55. As it is, you must simply deal with it by either supplying more light in the form of an external flash which will project a grid-pattern onto the subject to give the AF something to bite on (this only works at close range, obviously), or do your best to point the autofocus at something that is distinctive and has good contrast against the background.

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Or use manual focus :) –  fmark May 17 '11 at 16:26
    
Not much fun on an 18-55... designed for manual focus it is most certainly not! –  Staale S May 17 '11 at 21:49

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