AFAIK all Pentax DSLR's before the K-3 cannot move the aperture blades while the shutter is open - a limitation imposed by the mechanical design.
So it cannot adjust the aperture diaphragm to maintain a constant F number. I think it should have been possible though for the actual aperture to be properly indicated as the lens is zoomed in and out.
I've tried the same with my K-30 and it seems to behave exactly as you described in the opening post as far as not moving the aperture blades is concerned. But it does change the F number shown on the LCD accordingly with the change in zoom to indicate the real aperture of the lens. So it seems to me just a minor software error in the K-5ii which they could easily fix if someone notified them about it.
The K-3 behaves correctly, maintaining constant real aperture as the lens is zoomed by changing the diaphragm opening in sync with the zoom change. The exception to that is of course when the aperture is at its end stop and cannot open/close further, in which case the displayed F number then changes accordingly.
The one thing which is not 'perfect' is that the blades open/close in steps rather than smoothly. I think the steps are about 1/3 of a stop but not sure.
Originally posted by fgaudet Might be a stupid question but how can you see if the aperture blades are moving or not?
Easy answer - just look through the lens, possibly shining a light into it - it is very easy to see the aperture blades and whether or not they move.
On the K-3 you can even hear them move.