Originally posted by Philoslothical You shouldn't be using m42 lenses in Av mode. Period. They will expose much more accurately in M mode, with the green button to take a reading.
Av is designed to meter wide open, and calculate the exposure for the aperture you set in camera based on that. Since you can't set the aperture in camera, it will only work wide open, and even then it won't be as accurate because the camera hasn't a clue what the actual aperture size is.
Surely the big advantage of M42 lenses, when compared to K-mount manual lenses, is that the aperture can be stopped down and they can be used in Av mode when not wide open? This use is limited, because beyond f/4 it becomes difficult to focus as you're not focusing wide open therefore light can be very limited. I have a Katzeye screen installed and beyond f/4 the prisms start to darken. But if you want to take a 50mm f/1.4 Tak and use it in Av mode at f/2.8, that's very feasible, not so with a K or M fast 50. However, at least on a K200D, that requires paint removal or film to short the contacts, otherwise the full +2EV comp will be required, and you lose positive EV comp when shooting because it can't go any higher.
To be honest, the easiest and best solution would be for Pentax to allow an EV comp of -4 to +4, and the user would be able to do as he pleases according to his knowledge of the lenses he's using.