Originally posted by stevebrot This is in the camera user manuals, though I confirmed with lenses from my own collection. Why the AF point is not selectable with non-AF lenses puzzles me, though it may have something to do with needing to know focal length for off-axis points.
Steve
I don't think this is the main reason. Because the way the camera knows when a point is in focus has nothing to do with the lens specs, it's only incoming image processing (software algorithms can easily tell if a small image crop is in focus or not). That's why some subjects are harder to AF than others, a polished glass or a mirror for example if they are clean. Also some lenses in certain conditions like the kit 18-55 lens getting too soft/blurry at 18mm are hard to AF with. That's also why it is harder to AF in low-light, as there is natural noise, less gradients, less raw image informations (less photons^^). I guess it is more an intentionnal design from Pentax engineers, selecting a specific point in MF is almost useless as you just need to aim the point you want to focus and then move back to where you want your shot centered, this is so fast to do, and that's how I used to do with old SLR MF cameras which had only the center circle on focusing screen. But if they wanted to add this functionnality, or for example add multi-focus points (having 2, 3, 4 points in focus at the same time), they could do it, this is just embedded software (they call it firmware) to upgrade.
But perhaps you're also right in the way that some special informations (but I'm not sure it needs to just be focal length) given by modern lenses may help the camera software to be more accurate when telling a point is or not focused. How exactly, I don't know, but maybe some AF lenses that are not designed for DSLR could also produce inaccurate AF, I have this issue with my Sigma AF-K zoom which seems to be a SLR AF lens, even if this lens allow me to select a specific AF point in MF mode. (I think it is inaccurate because the focal length information given is for use with FF films, so center AF point could still be accurate while other points could be miscalculated, that's why I use it only in MF or in center AF and why I'll probably sell it^^)
Anyway, if I understood what you want to do (validate an off-centered MF focused point),
for this purpose I set AF point on Auto and this does the job perfectly even if you have to often release shutter button to re-calculate while MF. I've not tested that yet, but maybe setting AF in auto and AF.C (instead of AF.S) could help you get exactly what you want with continuous focus points scanning. I'll try that.
My bad, tested agan with a MF prime, only center point can be used, just mixed with how I used my AF lens in MF mode set to Auto... Tried all options (AF.C, change AE-AF correlations), seems you just can't do it because they decided you would not.
But tests I've just done with my 24mm MF prime shows that this focus validation method in MF with MF lenses is totally inacurate (with my K200), I can get red center spot even when center point is out of focus, so I don't rely on it and will maybe just uncheck this option to focus only with my eyes... It seems that accurate AF validation or lens control can only happen with modern lenses designed for DSLR, and even then, using entry lenses in worst conditions will lead to missed focus.