[dragra]

sometimes the camera is just searching for focus.. even it is a bright day.. it is not a photo is out of focus after taking.

guess i need to read up a bit how the autofocus of the DSLR works so i have some clues.

RTFM please

[RedMist]

Your brains have to be online with the camera AF-mode, I mean you ought to "tell" the camera what you're after, what's your point of interest. If you point it to the plain sky it might be tough, except there is some cloud or BIF which it can have a lock on.

I think most people realise you need some contrast to focus on but I've just been using my K30 in London today - started with 18-135 lens, no problems for 3-4 hours, switched to my 40mm Ltd that has been giving me problems and at first it was OK for photos of my kids at close range but I pointed it at some statues 70-100ft away and immediately wouldn't lock focus, pointed it at some buildings and again it wouldn't lock focus, plenty of contrast nothing like a solid white or blue sky

For me I only get the problem with 40mm Ltd using Phase Detection when trying to focus > 50ft. Switching to release priority won't help much if it's not in focus.

To clarify, live view Contrast Detection does work OK but it's quite slow with the 40mm Ltd so would prefer to use the ViewFinder.

Maybe my 40mm has a problem with infinity focusing? If that was true then Contrast Detection would have a problem also surely -I need to do a proper test sequence at various distances

[RedMist]

RTFM please

Not sure the manual gives the exact details of how Phase Detection works which is what I think he was referring to, it's a bit too much physics for most people

[SteveB]

Not sure the manual gives the exact details of how Phase Detection works which is what I think he was referring to, it's a bit too much physics for most people

Phase detection works a bit like our 2 eyes do. Look past a subject and you see 2 subjects, align the 2 and you're also in focus. Phase detection can be caught out by repeating patterns - I remember going boss-eyed looking at some terrible flowered wallpaper in my bedroom when I was a kid - the pattern repeated......

[RedMist]

After some testing today it looks like my 40mm definitely has an infinity focus problem, anything beyond 30-40ft and it refuses to focus, at first I thought CDAF was OK but although the focus seems to lock it's not in focus and manual focus doesn't help either.

Up to 30-40ft focus is fine, no BF or FF which is the range where I normally use it anyway for low light indoors or close range portraits so I hadn't really experienced the problem much previously (only had the lens 5 months, 2nd hand).

This is maybe a different problem from what the OP was having problems with.

After some searching this looks like something that can occur with some pentax primes, for example another thread here describes the same problem with the 21mm Ltd

Looks like I'll have to get the small screwdrivers out or pay to have it fixed