Originally posted by Adam If the fl changes while focusing it might need to do that, but is it that camera or the lens?
You mean the focal length? It's the DA 50 1.8, other lenses seem to work just fine. It's an abrupt action, you can hear and feel it. The lens stops down slightly (or opens up). Basically it varies between 2 different apertures. At 1.8 it stays fully open, but from 2.0 to 22 you can observe the effect. We've tested 2 K-5, a K-3, a K-50, 2 DA 50 1.8, DA 18-55 AL, Tokina 80-200, and IIRC the 31mm Ltd and DA 35 2.8. The only lens that had the problem was the DA 50 1.8, both had it. Also, only the K-5 and K-3 (all of them) showed the effect... but only with the 50 1.8. The K-50 as I described earlier has that weird thing where it always goes back to a default aperture when not recording a video... when you start shooting it will go to the selected aperture and start recording. No change of aperture was noticed at the "default" aperture.
I'm curious if this has been noticed before, if it is a bug (maybe Pentax can fix it? Though it is not really a big issue... you may loose a bit of light depending on where you have focused to before starting to record, but it shouldn't be a big issue). Maybe other lens/camera combinations have the same issue?
Reading your comment again... do you mean that since when you focus the focal length does change slightly, it will adjust for it? Sounds possible, but why only on the DA 50? Other primes should have the same issue? And does it really make sense to adjust for the tiny difference in focal length when you may change the FL again once you've started recording (and the camera wisely doesn't touch the aperture at that point)?