Does your K10D's auto-focusing ever do this?

1. In Av mode or similar, select a medium to small aperture on the camera, say F8 or F11.

2. In AF-S and SEL or single-point modes, point at a static subject and half-press the shutter button or press AF to make the lens focus.

3. Let go and press again, just to make sure it's finished focusing and doesn't need a further attempt.

4. Without moving away from the exact same part of the subject, change to the largest aperture and half-press the shutter button or AF again.

Does your camera ever slightly alter the focus or re-focus at this point (you would hear it happening) when it would not have done had the aperture selection not changed? If so, with what lenses?

Thanks for any replies. I will explain more about what I'm experiencing and wondering about this evening or later this week.

I would be particularly interested if you could test with lenses as fast as F2.8, and please repeat with different subjects.

Cheers!

I don't know as far as what percentage of the time, but it happens enough that when I know I have a static subject, I will AF, then switch to MF so that it won't refocus every time I take another shot.

I have not played with seeing how aperture affects that. I will see if I can get some time to experiment and let ya know.

UPDATE:

With help from another forum, I now suspect that this is to do with compensation for spherical abberation. Slightly different focusing can normally be required for large apertures than smaller ones. Aparently, lenses are programmed with data that the camera uses for compensation.

This seems to be done about correctly for most of my lenses, but I'm not sure for my newly aquired Tamron 17-50mm F2.8. I'm getting front-focusing quite often at large apertures.

Otherwise I'm happy with the lens. Sharpness and contrast is excellent when it is focused correctly, right from F2.8. Having a Katzeye screen is really helping.

And of course I cannot get it to re-focus, now that I try to make it happen.

Of course, no matter your selected aperture, the lens should always be wide open until you actually take the photo. So focus should not differ with your AV mode set at f/2.8 or f/11, etc... (unless you're moving an aperture ring on an older lens).

I have also heard that for most good zoom lenses that focusing should not change when zooming throughout the range. e.g. focus at 16mm, zoom the lens to 50mm and the subject should still be in focus. That used to not be the case, but at the moment focus is spot on throughout the zoom range without focus adjustment.

Could back/forward focus adjustment affect those?
Unfortunately, Ive had the internals of my camera replaced since I actually paid attention to the behavior. Plus I recently played with the focus adjustment hack. So who knows what I did.

Perhaps the behavior is a flaw that has been fixed on my camera in one way or another. : : :

What I have learned today is that focus should, and sometimes does, differ to compensate for spherical abberation - which varies with aperture. Since the lens isn't actually stopped down until the shutter is fired, the camera has to calculate what focus will be required in advance.

I'm glad that this is a deliberate feature rather than a bug. Not so glad it doesn't always work properly.

That is very interesting....can you provide a source as I would like to read up on that.

I don't know more than you can find by Googling for "spherical abberation", or reading recent threads on the dpreview forum, including http://forums.dpreview.com/forums/readflat.asp?forum=1036&thread=28076108

Sorry to plug another forum, but some good technical info does get posted there (in amongst the rubbish).

I suppose they know more that I, but logically it don't make a lot of sense to me.

I would assume that in addition to aperture, the subject distance would need to be known to correctly adjust for aberrations. There would be a few points on a graph that it would "know" about. Anything in between would be essentially guesswork (complex math, but still guesswork).

As I understand things, the sensor judges contrast sharpness for focusing.
If it were to throw that out the window and make pre-calculated adjustments,
I'm just not convinced that would give the best results.
It sounds like you'd end up taking what was otherwise critically sharp at one point, and adjusting for the rest by backing off to give an average across a larger area. The result would be a overall sharpness but with loss of some sharpness at the focus point.

I suppose thats why some don't like the idea. Not sure I do either, but I guess I would need to see before and after shots to see if it adds a benefit and to what degree it might detract from otherwise "perfect" focus.

Still, quite interesting.