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Manually pre-focus to the distance you'd expect then the auto focus won't have far to move/lock on. Only turn the IS off if you are using a tripod. The auto focus works by detecting a difference in contrast so if the conditions are not compatible i.e a BG the same as the subject it will 'hunt and peck'.
Make sure the auto-manual focus switch on your camera is set to auto (A) and also make sure the auto manual focus selector on the lens is in auto (A) as well. If your lens contacts are dirty then this will affect the function of the lens as well. Clean those with either a lint free type cloth and alcohol or methylated spirits.
The Auto-Focus on the lens is damaged. I have had the exact same problem with my 18-55 and I ended up buying a new one because I take shots at weddings and I can't afford to manually focus each photo. You can still use it if you switch the small knob to MF and then use the focus ring for manual focus.
there is couple of possibilities like wise when you zoom lens manualy and can you heard the hard noise then could be AF unit itself faulty. AF unit gear is gone.
could be AF shaft broken thats why not focusing
could be AF Flex broken thats why not focusing
for that please bring to canon service center
It is because the lense is only an AF lense. The d40x doesn't have an on-body auto focus motor so that needs to be on the lense. I have a d60 and have the same problems. You can use the manual focus on the AF lenses; however, to use auto focus, you need to buy an AF-S lense. It's disappointing I know, but that's the price we must pay for saving on our body lol
I have the same problem. I can make it temporarily work by manually turning the focus ring until it is out fully. Then it will auto focus and take photos until I turn the camera off or it times out. Then I have to repeat the process. It's annoying, but at least it works for me. Try it and post to let me know if it works for you
I experienced the exact same problem with my 100-300mm lens and I was never able to get it to work correctly again. In fact, attempting to use it further seemed to exacerbate the problem.
The further you zoom, the wider the aperture opens allowing more light for the camera to focus properly. If the aperture mechanism is damaged, the auto-focus process may not work as it should; which would explain why the auto-focus feature works only when at 18mm.
Regardless, I strongly recommend you discontinue use of the lens until a professional technician can examine the lens and figure out what's going on. I'm sure you can contact the manufacturer to open a support case or you can find a camera store that offers repair services.
This is because the camera will not expose a picture until the auto focus is locked. If you are shooting in dimly lit conditions, the AF might have a hard time obtaining a lock. Using the flash as auto focus assistance many time helps in this condition.
Another reason might be that you try to focus on even surfaces with low contrast. The auto focus work by sensing contrast. Even surfaces, such as a white wall or the sky, have very little contrast, therefore the auto focus will not be able to obtain a lock.
The solution here is to point the AF sensor at something with more contrast, such as an eye, a pole, or basicly anything that have edges/lines.
I hope this help you!
Did it ever work on this camera?
I have seen some Sigma lenses that are auto focus on canon film cameras but then when put on the digital they dont work right. If you have used it on this camera then make sure the lens is clean.sometimes dirt on the front wil make it try focusing on the dirt. Next try focusing in manual focus to make sure the focus is working. Then take the lens off and then put it back on and make sure it clicks into place.
Let me know if none of those things work. also make sure to try focus on something close and somthing far away.