If the camera thinks it's focused at distance x, but actually it's focused at distance x + y, where y is some small additional distance, you'll get out of focus shots unexpectedly.

In my test shot above, the camera was pointing at where it says "this text should be perfectly in focus", and the AF was giving me the green focus confirmation hexagon.

Clearly, it's actually focused several mm further back than that, at around the 6mm mark behind where it thinks it's focused. Hence the term 'back-focus'.

In the custom menu, there's an adjustment which can be saved for several lenses separately. I've never understood why the amount of back (or front) focus would be different for different lenses, but there it is.

There's really no more to it than that, in practical terms.

Nothing to do with depth of field - it's a question of where the actual focus point is, relative to where the camera thinks it is.

Actually, I lie - it is relevant to DOF, but only in the sense that if you stop the lens down, more of the distance range appears sharp, so any back or front focusing is masked.

You're very welcome, hope you enjoy it. Thanks in turn for your prompt payment.

F5.8. It doesn't get any wider open at the long end.

Edit: PS, I forgot about the troubleshooter forum, sorry, I probably should have posted this there.

Before I start fiddling about, please could anyone suggest a rough amount of adjustment to correct this much back focus:

[IMG]http://i162.photobucket.com/albums/t244/chris5gd/Problems/IMGP0889.jpg[/IMG]

It's the 55-300 on the K-5, 300mm at a range of about 5 feet.

Thanks in advance!

I was certainly not intending to be aggressive - if that's how I came across then I apologise. I tried to explain how it had appeared to me in plain terms, nothing more.

I've noticed it do that, but usually with moving objects that I'm not tracking well enough to keep the centre AF spot on the target. Then it hunts horribly, and yes, I feel the pain too when it hits the stop.

I switch to auto-AF point for things like that, which gives it a bigger target to aim at, as it were. Rarely had the problem after discovering that.

TBH, I think it's the quick shift that makes all the difference.

To me, it seems a little quicker on the K-5 than the K10D, but not massively. Once you get it close, the AF seems to be able to do the last bit very quickly.

Whereas if you force it to spin the focus wheel from a long way from target, it takes a while, simply because it has so far to go.

Well it clearly is in this case - it's a fantastic image, and I wouldn't dream of taking anything away from it.

Well fair enough. I don't know anything about any Flickr groups, I was just calling it like I saw it. I take exception to being accused of aggression (not by you, I hasten to add!!) - people get too angry at the drop of a hat these days.

I'm not clear how you think I'm being uncivil.

It struck me that the stated purpose of Dr Orloff's post was a little disingenuous. I may be wrong, or I may not. I expressed an opinion, based on the impression I gained, to the effect that blatant showcasing on a C&C thread, giving nothing back, is a little 'off'.

That's all. I'm not sure how that's uncivil. I'm just getting the impression that I'm a bit of a soft target here, since I've had the temerity to stick my head above the parapet.

Yes, I think this happened with the Sigma P-TTL flashes from a couple of years back.

It's probably why Metz have made their high-end units firmware-upgradable.