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I have a Canon 500D with the kit lens. Recently I have observed that its not able to focus at higher zooms but it works at min zoom. To get it to focus, I have to zoom out or recompose and it works. Why would this happen?

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A few important questions for background — when you say recently, do you mean the problem started recently and was fine before, or that you only noticed recently? And second, can you give an idea of what focal length range the problem occurs at? Finally, do the images seem sharp at high focal lengths if you focus manually? – mattdm May 19 '11 at 8:38
It started happening recently only, it used to work fine earlier. It usually happens around the range 35-55mm or somewhere around this. I never tried to focus manually when the problem happened i just tried to recompose or change the focal length and it worked. – intoTHEwild May 19 '11 at 9:00
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Have you checked the EXIF info for some of the old photos that focused ok to make sure you're dealing with similar conditions? Low light is a killer for autofocus. It might also help to describe the behavior you're seeing when autofocus fails (ie, focus hunts back and forth). – D. Lambert May 19 '11 at 12:26
yes that is what exactly happens, the lens elements move back and forth and they cant focus. – intoTHEwild May 20 '11 at 5:01
Perhaps a dumb question, but what are you taking a picture of? It's difficult to take a picture of a black cat -- I was able to use auto-focus sometimes, but had to resort to manual focus at others. As other say, the amount of light you're gathering matters too, and these factors can combine together. Separately, have you considered taking your camera & lens into a brick & mortar camera store and asking one of their people to just take a picture or three and see what they think? They'll probably be willing to do that for you. – khedron May 23 '11 at 14:03
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2 Answers

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If this is the f/3.5-5.6 lens, then as you zoom out the max aperture is reduced ( f/3.5 at 18mm, but f/5.6 at 55mm). Autofocus will struggle in lower light at the f/5.6 end.

You can try to find something high contrast to focus on, or use manual focus if you have to.

If you focus at 18mm then lock focus and zoom in, that won't necessarily still be in perfect focus at 55mm.

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i guess that makes some sense, could there be any other thing? – intoTHEwild May 23 '11 at 3:54
If it works fine in bright light but has a problem in low light, then I don't think there's anything wrong with your lens. I think the rule of thumb is that AF works well up to f/4.5 or so, beyond that it's going to struggle in low light. – MikeW May 24 '11 at 1:56
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It sounds as though one of the autofocus cables inside the lens is about to fail - this is a very common problem on the kit lens and the initial problem is failure to autofocus at certain zoom settings followed by complete failure to autofocus in one direction once the cable has totally broken. It is a ribbon cable and replacements can be found on ebay for about two dollars. However the fitting of it isn't easy. You have to take the lens apart and would need tweezers, glue (possibly) and watchmaker's screwdrivers. Alternatively camera shops repair them for around $100

This youtube video shows how to replace the cable

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And for which lens is this valid? Since the original question doesn't mention a specific lens this answer seems a bit vague. – Håkon K. Olafsen Feb 3 at 13:41
@HåkonK.Olafsen Tge question does mention the Canon 500D kit lens, which is the 18-55mm f/3.5-5.6 IS. – mattdm Feb 3 at 15:39
Some shops sell "kits" with camera and separate lens, not just the kit Canon ships, so it's not clear. – Håkon K. Olafsen Feb 3 at 20:13

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