AF hunting problems

For a while now I've wondered if others have more trouble locking on focus with the Canon 500 f4 and 1.4 x TC....mine is coupled with a Canon 7D. Without the TC it still hunts sometimes, but most lenses will to a certain degree, more so in low light naturally enough, but with the TC attached it can often be frustrating as was the case today.

Scenario today .......I was trying to lock focus onto a bright yellow bird { Golden Whistler } with the 1.4 x TC on the 500 F4 from a distance of approx 20 feet, the focus distance limit switch was set accurately and no oof foliage or the like between camera and subject. I missed many opportunities to get a shot of the subject today even when the bird was perched openly in good light, it should of been an easy shot to get. Taking the TC off resulted in locking on much better, but wondering if this should be the case or not ?

Phil,
How many AF points do you have enabled? If one, do you have AF expansion on? I have seen the behavior you describe with AF expansion on when there are other subjects nearby that a similarly lit (ot brighter), like leaves, or branches. I have observed this with both 7D and 1DIV with 500 mm with and without TCs. My solution is to use one AF point with no expansion.

Roger

Thanks Roger, and yes I already use a single focus point with expansion off, so not sure that's the problem.

Having had the problem myself Phil in all three 1D series cameras I've owned (even had Canon look into the problem with no positive outcome) i'd put it down to the camera.

As frustrating as it is I've come to except thee cameras have a mind of their own at the worst possible times.

Richard

There is usually a distracting element causing hunting in cases like those Phil discussed. The actual AF zone is quite a bit larger than the rectangle one sees in the viewfinder and has fuzzy boundaries. Thus, any similar or slightly brighter object next to your intended target can cause the AF to lock onto the unintended object. Another cause of hunting is something in front of your subject, like a twig or blade of grass. Even though the object may be very out of focus, the AF system sometimes tries to lock onto it.

Roger

In situations like this, I sometimes find it helpful to set the "Lens drive when AF impossible" custom function (C.Fn III-4 on the 7D) to 1 (Focus search off). You may need to manually focus to somewhere in the ballpark to get started but then you may have better luck not losing the AF lock on the subject or at least getting it back much more quickly if and when you do lose it. I'm assuming here that the distance to your subject is not varying dramatically. Of course, you run the risk of missing a shot if a different subject suddenly appears at a much greater distance that the one you are currently working. You also need to make sure to set it back to 0 when you are done with this situation or you will be wondering why your lens will no longer focus when you point it at a distant subject. It's not a perfect solution and I use it as a last resort but it can be a big help in certain situations.

Thanks folks, I'll tinker with it and try a few things

The AF should lock instantly in the condition you describe above unless we are missing something about the scene or your camera/lens is defective.

Can you post an example where you had hunting problem?