Autofocus problem on an SLR

I've noticed recently that my Canon Rebel XT's (with 18/55 kit lens) autofocus is having problems. Just zooms in, then all the way out, without ever focusing. As far as I've known, I haven't really "done" anything to the camera. First my laptop, now this, sigh. I purchased the camera from BH photovideo in late August of last year. Was wondering if anyone has had similar problems with their Canon, and 2., if the warranty is still good. Thanks in advance.
 
If you look inside the mirror box you should see the AF sensor. Make sure there isn't any large dust specs or whatever else on it.

Aaron
 
Try cleaning the electrical contacts on the lens and body with a q-tip and rubbing alcohol. The swap shouldn't be sloshing wet, just damp.

Cycle the AF/MF lens switch a few times.

Does a friend or nearby camera shop have another, known-good, lens to swap out?

-G
 
If you don't mind paying shipping, you should send it in to Canon. Assuming you didn't get a grey-market camera, it's still under warranty -- and Canon's repair techs are very good. :) You just mail it in (insured!!) with a copy of the receipt and the warranty card, and they FedEx it back to you a week or so later, fixed. Beats trying to do it yourself, and if on the off chance there needs to be any parts replaced, they'll do it free of charge.
 
I enjoy my digital Canon, but I've had quite a bit of trouble with the autofocus system under particular shooting situations. I have one of the original 300D cameras. The two main scenes where I have problems are (1) in low light and (2) a flat no contrast scene. Heavy rain or fog can contribute to the low contrast problem. I was not able to take stills of a large elephant trunk tornado last spring due this problem. I can duplicate the situation for no contrast by attempting to take a picture (indoors) of a flat white wall. The camera refuses to settle in on a focus point, it's been this way since it was new. I've found two partial fixes for the problems. In low contrast I just go to manual, not as easy as it sounds....best to practice this before facing up with a big stove pipe. If it's late evening look for a distant light on the horizon as a focus point. I sometimes tape the lens when I find infinity as the setting varies vs the infinity mark. Make sure and remember to remove the tape if you switch back!! Also, in low light I can get by with running up the ASA from 100 to 200, 400 and so on plus slowing the aperature speed. This will work for a while until the light gets too low, then I'm forced to go manual again. Now, here's a trick I use....at least it works on my camera. In low light when the autofocus won't work switch to manual and watch the led's. Usually I can get one to blink (most often the bottom center) as it crossed the proper focus point, even through it won't go there automatically. That's where I set (tape) the lens. I've successfully shot cloud features such as wall clouds after sunset (near darkness) using this method.

Originally I had two zoom lenses, one was rather fast F2.8, but it too had issues in low light. Buying the inexpensive F1.8 50mm normal helped both late evening low light shooting and focusing. If you don't have one of these it's well worth the $80 .....and it's sharper than the kit lens 18-55mm zoom!

I had thoughts of liquidating my extensive 35mm Canon cameras and lenses, but now I've gone back to carrying one or two of them loaded and ready to go for the bad lighting and focus situations.....which now I'm able to spot in advance. Buying a higher end model won't always help either, I have friends with better Canon digital cameras and they still have these issues. I'm hoping in later models these problems get worked out, but with the "pixel race" in progress....they may continue putting out beta-models as fast as they can get them to market. I will wait until next spring to upgrade to one of the newer 10-12 megapix models in hopes of getting one with less bugs.
 
Yeah, I'll echo Gene on the auto-focus hunting. Stormscapes present a problem with gradual changes in contrast, as clouds usually do. Anymore, I just say to hell with it and shoot manual focus. However, my current 18-55mm lens has no focus markers, as it's a POS lens for manual focus shooting. I never focus on the sky itself, as it will always hunt. If using auto-focus, I almost always focus on distance ground elements near the horizon line then hold that focus until I depress the shutter fully to capture the image. My big 80-400mm lens has an even worse auto-focus hunting problem, but this lens doesn't even use Nikon's AF-S system, so it's pretty slooow AND it tends to hunt. Dick, this is really nothing new, unless it's hunting on high contrast elements other than sky, mist, fog, a white wall, or other smooth elements.
 
Mike,

It used to focus fine. What has happened, is that it will focus on something, then completely unfocus. This repeats over and over. And it usually only does it in RAW. JPEG seems fine. I've contacted Canon, and they've tried every troubleshooting problem it could have been, and still nothing. In fact, the guy had never heard of this problem. lol. Nevertheless, when chase season is completely over with, I am sending it in. Warranty is still good, but they say it will be over a month before I get it back. No loaners, or anything. :eek:
 
Mike,

It used to focus fine. What has happened, is that it will focus on something, then completely unfocus. This repeats over and over. And it usually only does it in RAW. JPEG seems fine. I've contacted Canon, and they've tried every troubleshooting problem it could have been, and still nothing. In fact, the guy had never heard of this problem. lol. Nevertheless, when chase season is completely over with, I am sending it in. Warranty is still good, but they say it will be over a month before I get it back. No loaners, or anything. :eek:

Mine will occasionally do what you're discussing here, usually with a slow lens, but yours is much worse and does need to be sent in. Perhaps there is no fix and you'll get a new camera. My daughter has been through two digital auto-focus cameras....$300 throw away cameras they are. Yet I have 25 year old Canon film cameras that are still working fine. The main problem with them is the foam padding to insolate against light and pad the shutter rots out over the years, still they work great for a long time. I've read that 50 percent of the pro's still use film.

Considering the products I've seen from all the main digital producers it seems unreasonable for me to buy the "high end" cameras until they make a robust product that can endure outdoor environments such as we have.....high moisture, wind driven rain and dust, plus bumps and jolts of a Delorme "grey road."

I'll mention this one more time.....consider the Canon normal 50mm 1.8 lens, most of you will see a number of your focus and low light problems go away.....and the cost.....chump change.
 
If you've elminated all the possible issues (bad or incompatable lens, incorrect camera setting, etc) I'd send it in. Good call talking to canon also.
With all the electronics these days, it's not worth "playing" with it and voiding the warranty.
I discovered my wide angle polarizing filter "buggers" my auto-focus also.
Good luck...
Laura
 
Mine will occasionally do what you're discussing here, usually with a slow lens, but yours is much worse and does need to be sent in. Perhaps there is no fix and you'll get a new camera. My daughter has been through two digital auto-focus cameras....$300 throw away cameras they are. Yet I have 25 year old Canon film cameras that are still working fine. The main problem with them is the foam padding to insolate against light and pad the shutter rots out over the years, still they work great for a long time. I've read that 50 percent of the pro's still use film.

Considering the products I've seen from all the main digital producers it seems unreasonable for me to buy the "high end" cameras until they make a robust product that can endure outdoor environments such as we have.....high moisture, wind driven rain and dust, plus bumps and jolts of a Delorme "grey road."

I'll mention this one more time.....consider the Canon normal 50mm 1.8 lens, most of you will see a number of your focus and low light problems go away.....and the cost.....chump change.

Thanks Gene. Yeah I just bought the 50 mm lens about a month ago, and am really impressed. Same problem with JPEG vs RAW though. I thought it might have been the lens, but it is the body. For fast moving storms, autofocus is my friend, as there really is no time for manual. This problem really came out of nowhere on a chase I went on back on July 13th.
 
infinity?

hey you are kinda touching on the issue so ill ask it here instead of new thread.. i have the canon digi rebel xt with just the lens that came with it as well. is there anyway to have it automatically focus on infinity? if not is there some lens out there with the manual focus (or auto) that has the infinity marker on the lens itself so you just line it up to infinity? right now at dark i am having to zoom in on lights, moon, stars, etc to try and get the focus right... which is annoying and tedious and hard to get 100% correctly in focus.
 
hey you are kinda touching on the issue so ill ask it here instead of new thread.. i have the canon digi rebel xt with just the lens that came with it as well. is there anyway to have it automatically focus on infinity? if not is there some lens out there with the manual focus (or auto) that has the infinity marker on the lens itself so you just line it up to infinity? right now at dark i am having to zoom in on lights, moon, stars, etc to try and get the focus right... which is annoying and tedious and hard to get 100% correctly in focus.

Brian, many of the Canon lenses do not provide the option of manual focus with a classic focus ring. On the 18-55mm you can grab the front ring and get a manual focus, but it's stiff and clumsy at best. The 1.8 normal is worse with the lens hood attached. When I was referencing manual focus I meant with a lens that offered that option. For instance, my main "tornado lens" is not a Canon lens. It's a Tamron SP AF Aspherical model: XR Di LD 28-75 mm F2.8. Although it's autofocus it has both manual zoom and focus rings with the depth of field markings. If you're going to achieve an accurate infinity focus in manual mode you will do better with one of these lenses from Canon, or an aftermarket company. With my Canon zooms I tripoded for accurate infinity, then marked the lens focus ring to that spot. I believe the "trick" I suggested with the manual focus and blinking LED will work with all Canon lenses, given proper lighting to find a object. I don't recall trying it in total darkness.

The Canon autofocus lenses will provide accurate infinity focus when they are performing correctly. That said, I've read from some users that the autofocus at infinity is not always prefectly on target; hopefully this is not a common problem. I believe astro-photographers are most likely to deal with this situation. Generally in good daylight the depth of field is wide enough to make up for these small errors though.

Gene
 
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