Autofocus prob on K20D

Am I doing something stupid?

Was out playing with K20D and my 90mm Tamron macro.
I was taking some long shots but when half pressing the shutter the central focus spot, which was on a fence post, made a "de-de-det" noise, jumped in and out of focus and refused to let me take a picture, the post was out of focus and the focus icon was flashing in the viewfinder.
I managed to get it to focus by depressing the shutter a few times.

I am at work so I haven't even looked at the manual yet.

Is it op error?

Thanks in advance

ali

Did you have the focus switch set to Continuous (AF-C)?? That would make it hunt around a bit if you were moving the camera slightly.

Also, you may have had it set to Selective focus on the back dial, which probably meant you weren't focussing on what you thought you were?

Get reading that manual Sir!

I do not think this is anything to worry about. It happens on my k10D as well.

I think the auto focus works by a little analysis that tries to peak the high frequency components of the image at focus point. This relies on there being some detail at the focus point. The subject matter, lighting, minimum aperture and softness of the lens can all influence this.

I find that if this happens I move the focus point to some obvious edge detail close to or at the same distance as the original point. Obtain the focus then recompose the shot at the original point.

I hope this makes sense,

Glyn

Nothing to worry about !......my K20 does the same,very difficult for the auto focus to pick up a point on a flat nondescript surface, my D's was just the same. As Glyn says look for an edge, or a surface with a change of colour and it will be fine!................this is with it set at centre focussing of course!........good luck Ken

The focussing mechanism relies on the contrast of the focus area. If you have trouble focussing on a horizontal edge, find a vertical one and recompose after focussing. If that fails, MF works every time.

Thanks for replies folks.

It was prob just cos of it being a fence post, seemed ok at other things.
Glad your all here, I was worried.

Op error it is then, lol

Tooks, yes it was on single focus using the centre focusing 'spot'.
I'm bad but not that bad!

Cheers

ali

Sorry! Thought it would be worth mentioning!!

I do daft things like that all the time, but then maybe the lesson for me is not to assume everybody else does!!

Posting this link just in case you haven't tried these tips

http://forums.dpreview.com/forums/read.asp?forum=1036&message=29213895

Nae bother Tooks,

I do daft things as well, think we all do, so it's always worth mentioning.

Thanks for the link Tobio.

Cheers

ali

I suppose its also worth pointing out that longer lenses will generally have a greater minimum focusing distance, if you were up close you may well have been just on the borderline.