How to avoid back-focus problem when focus-recompose?

Hi guys

I just realized when you are using a telephoto lens to shoot a portrait, 'AF first and then recompose' brings a back-focus problem. Could you explain how I can lessen the 'out of focus' effect? MF?- -!

Thank you.
Peng

MF is the answer I'm afraid, either that or put the feature in question (eyes or whatever) directly on one of the AF points.

With DA lenses the answer is to AF and then tweak the focus manually using the Quick Shift feature. No need to change to MF to do this. The exception within the range will be the 18-250mm and the DA-L lenses.

If your AF is accurate, you shouldn't have a back focus problem when you focus and recompose. Your AF might need to be adjusted. If your camera pre-dates the K20D, or any model that doesn't have a focus adjustment, you should send it with the lenses to Pentax to have it re-calibrated.

On the other hand, if your AF is up to spec, maybe your technic is not.

You don't have to send it back to Pentax. You can enter debug mode and calibrate the focus yourself

http://www.pentax-hack.info/documents/debug.html

with the modset file on your SD Card root switch the camera on with the card door open to enter the debug menu. Once the debug mode is enabled he AF calibration appears as an extra parameter in the regular set up menu.

The AF calibration you set will remain in force even when you leave debug mode. For pre-K20D cameras it applies to ALL lenses. I leave my K10D permantenly in debug mode so I can alter AF cal everytime I switch to certain lenses.

Hope that helps

That's not true i'm afraid, unless the DOF of your lens isn't flat... telephoto fisheye maybe?

Here is an image of the problem to make it clearer.



If you've an AF with multiple focus points you can also of course use the correct focus point or can't you do that?

Does it help to use the menu option to disable focussing on half-press of shutter release and use the AF button to focus on the subject? You can then re-compose without the camera trying to focus again.

Just a thought

Myles

The camera won't refocus if you keep the half pressure on the shutter release.

99% of the time, the error is going to be so small, it won't be noticeeable.

That depends on so many things i don't want to say a percentage.
I do agree with you that the error is small most of the time but if you've only millimetres of playing room that small error will be noticeable.
Just try to recompose with macro photography or portrait with a very fast lens that gives a very small dof.

Or indeed, with extreme wide angle lenses.

Sorry to resurrect an old thread, but I'd like some more info on this please.

My K10 which ( admittedly) has been dropped, has an occassional focusing problem...or does it?

I have to use lenses with an A ring, but I can live with that. The main lens I use is an SMC Pentax - F 28-80mm. I find that many times the AF can't find focus with the lens all the way out at 28mm. I have to zoom i slightly, re-focus, then zoom out and set my aperture to give a slightly longer DoF to cover the fact that the focus point has now moved.
Is this a known problem with this lens, or is my camera at fault.

It's not a major issue, but can be a little frustrating at times. If it is as simple as calibrating the AF, then I'm willing to give it a go.