[Ajs]

I have just bought a eos 450d with a 75-300mm zoom lens, i am having problems with auto focus at full zoom at times it just will not function right and just goes in and out, if i zoom out a bit it seems to be ok, am i doing it wrong camera was on full auto can somebody help. Alan

[JMitchell]

It is probably when in particularly dark conditions, or when you were using an autofocus point that wasn't a cross type, or both.
The thing you are describing the focus doing is called 'hunting', and I imagine, unless there is a fault with the lens (didn't happen to drop it did you? ), then it is probably what I mentioned above. It sounds complicated but it isn't really. If you want / need more info then just ask and I, or another member will provide

[Dabhand16]

You need to check in good lighting conditions. As Jack said, poor lighting or low contrast can prevent a lens from getting a focus lock. This will be exaggerated at longer focal lengths.

[Ajs]

Thanks for reply, its when birding and i suppose its a case of not much to focus on is it.

[Dabhand16]

Originally Posted by Ajs

Thanks for reply, its when birding and i suppose its a case of not much to focus on is it.

I guess those conditions would be challenging, but hopefully, one of our members who does a lot of this sort of photography will be able to contribute.

You might be better manually focussing on the spot you are shooting at in advance, then provided the birds are within the depth of field area they will be in focus.

[GarynLea]

Hi Ajs, what setting do you have the metering set at? If you're trying to focus on a small bird amongst a mass of foliage, then it may help to use spot metering. This will allow the lens to focus on a single point instead of many.

HTH Gary

[AQ]

Originally Posted by Ajs

Thanks for reply, its when birding and i suppose its a case of not much to focus on is it.

I would suggest that you set the camera up to use only the centre focus point and then make sure that this is accurately on the bird when you take the picture. Even this is not foolproof if there are branches in the way but it does work most of the time.