Same problem I have with the Ixus125 HS. I wnat to fix exposure time at 1/1000 s but don't seem to see where we can fix aperture.... It would ideally be f2.7 with ISO varying to compensate different illumination...
Anyone have an answer?
Most of the less expensive Canon P&S cameras - including all the IXUS models - do not have a variable aperture that the user can control. The f-stop reported is only a function of the zoom lens position. You can't change it as it's physically fixed - there is no mechanism in the camera to vary the iris size.
As a small consolation, all the camera with fixed aperture have an Neutral Density filter that can be inserted, giving about the equivalent of 3 f-stop decrease. That's what was causing the occasional dark pictures noted in the original post in this thread.
Here's a script that will let you play with various priority modes ( Tv, Av, M, AutoISO ) if you want to fool around a bit :
CHDKplusI recall from your previous post that you fly UAV's. Perhaps this will help :
KAP UAV Exposure Control Script as it manages exposure for precisely your situation.
