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Trouble focusing
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Hi – i have a Canon 5D MKIII and I am currently using a 70-200mm F2.8. I am having a problem at 70mm getting a focus on anything in the distance, for eg today I was trying to photograph a lighthouse in the distance but at 70mm the lens (auto focus) couldn’t get locked on at all, as I started to zoom in gradually I continued to try to get a focus point but it wasn’t until i was at 135mm before the lens was able to focus on the lighthouse. Even using manual focus and zooming in on live view at 70mm i still couldn’t get anywhere near an acceptable sharpness at all. At 70mm i was able to focus perfectly on subjects that were approx 20-30M in front of me but any further and I lost it. Even the field in front of me the lens got to 100mm zoom before it was able to lock onto a focus at the end of the field (but at 100mm it still couldn’t see the lighthouse until i got to 135mm). Anyone have any idea what’s happening? The lens is 2 yrs old and has been treated with kid gloves, no bangs or anything. I have tried the lens with the distance slider at 1.2M-Infinity and also 2.5-Infinity but it’s no different. Maybe this is an issue for Canon?
Thanks a million Pete I will try that – although I’ll prob send the camera off to a professional outfit for cleaning as I don’t think I’d have the skill to do that myself. Will let you know, much appreciated.
Actually Pete no that’s not the problem, I just tried the lens on another Canon body and it’s doing the same thing. Cleaned the contacts on the lens but no joy – was worth a try!
Auto focus normally needs a good percentage of contrast in the subject in order to “lock onto” it. If the lighthouse in the distance and in hazy conditions, it would be difficult for auto focus to lock on, especially if the subject was small in the frame. The same could occur if there was shimmering from heat rising in front of a distant subject. Check which auto focus points (in your camera) are active. Normally, the center point is more sensitive than the others, but if you were using “all” the points (at the time), the camera won’t know which point that you want to use & will jump from one to another. Try setting only the central point & see if that works. If it does lock on, recompose the scene with the shutter button half pressed & then shoot. Closed down apertures can also affect how auto focus works if there is not enough light reaching the active point.
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