Hi folks, I played around today with various lenses and thought I'd report back the results.
F50/1.4, FA24/2, 77/1.8, Sigma 28-200 3.8-5.6 (AF), Tokina AT-X 80-200 2.8 (MF) As expected all lenses gave correct exposure. With the Sigma lens the focal length was recorded but not the lens name. The Tokina was described as "A lens" but no focal length or aperture value was given. SMCT 85/1.8 and 300/4 using M42->K converter These gave correct exposure in Av mode if use in stopped down (manual) mode. The f-stop is not displayed but the speed is, and as you stop down further the speed decreases. It is all perfectly workable. SMCT 6x7 135/4 (with #3 tube) and SMCP 67 165/2.8 using 6x7->M42 and M42->K converters. Once again, with the lenses set to manual stop down metering in Av mode worked just fine. M35/2 In Av mode it won't stop the lens down, so regardless of what you set the aperture ring to it is always wide open. This means that the metering is correct and the picture is correctly exposed. In Manual mode it will stop the lens down so you need to get the correct exposure. There is no need to swap between modes. Hit the "green button" (that is the official pentax name) and you will get the correct exposure for wide open, then adjust your shutter speed accordingly. Note: If you take an A/F/FA lens off the A setting on the aperture ring it will behave as if it is an M lens. You won't see the aperture value in the viewfinder and you can't set it from the body either. Basically the M42 and 67 lenses are just as usable as with any other K mount body, so no loss there. The K and M series lenses are usable, but it is a bit more kludgey than I would like. With rapidly changing lighting it would be a pain, otherwise I can live with it. Paul Ewins Melbourne, Australia