I have used the 135 lens with extension tubes. There was a manual avaialbe for the extension tubes available at the Pentax website for download. If you have the metering prism and try to meter a shot with the extension tubes attached to the lesns without setting the lens to stopped down mode, the shot will be underexposed. If you set the lens to stopped down mode and look into a metered prism, you can see the exposure change as you change the apperture setting of the lens. It won't change if you have the shutter speed set way too high or low, so you may have to experiment with the shutter speed dial to see any change. Set the lens to the apperture you want to use for the exposure to get the depth of field you want and then set the shutter speed to the exposure indicated in the metered prism. The metered prism I have is very accurate, but if you are using slide film you may need to bracket a little each way. Good luck,
Doug Webb