Aperture control with old K mount lenses

[MattGunn]

I apologise i this has been covered before but I couldn't find anything obvious by searching old listings.
I have been steadily accumulating old lenses (both M42 and K mount and a few others for conversion). With any of these that has a lever to select between manual and automatic aperture it is easy to set this to manual and then I can stop down that lens manually and take photos in Aperture priority without serious problems (but some inconvenience).
However some of these lenses do not have the facility to set the aperture to maual and so the camera opens the aperture fully by default (as it does with KA and newer lenses) and then allows it to close down to take a picture. In this case the metering has been done with the aperture open and when it closes the images are under exposed (Damm the crippled K mount). The only way around this that I have found is to open the back of the lens an disconnect the aperture lever. Now the aperture is always at the setting selected on the aperture ring irrespective of what the camera tries to do. Allthough I have no porblens with doing this (as I can alway put them back if I need to) I am wondering if I have missed a trick. Is there a better way to resolve this problem?
Surely, as the mechanism which operates the aperture in the camera is not mechanically liked to the shutter in the new DSLR's, it would be possible for the manufacturer to program the camera to allow the aperture to close a fraction of a second before the shutter opens. This would allow the camera to meter with the lens stopped down and thus give correct metering in spite of the crippled mount! Anyone know anything about reprogramming the cameras...

[MrApollinax]

When you take your pictures do you press the green button/AE-L button prior to shooting? With manual lenses (SMC (K), M) you need to press that button for the camera to stop-down and meter correctly. Aperture control with A lenses or newer is supported by Pentax DSLRs. Here is a link I use for reference a lot for shooting manual lenses.

K100D (Super) Pictorial guide to using manual lenses [imgs] [Page 1]: Pentax SLR Talk Forum: Digital Photography Review

[MattGunn]

This is exactly what I was after, I HAD missed a trick.
As they can program the camera to do pretty much anything it does seem strange that they have not put in an option to do the stopped down metering automatically in aperture priotity mode. Thanks for the tip, I can put all my lenses back together now...

[MrApollinax]

No problem. You need to keep in mind that in Av mode all of your SMC (K) and M lenses will always meter and shoot wide open regardless of the Aperture setting on the ring. You will need to work in M(anual) mode for the stop down metering to work.