Originally posted by kevbirder I'm not sure what difference the type of lens makes?
The deal with the lens is that with "modern" lenses (ones with exposure automation A, F, FA, FA-J, D-FA, and DA, with or without "*"), the system can automatically figure out the optimum aperture to use, and can also read the current aperture if youv'e set it yourself and figure out how much to scale back the flash if you've got a large aperture selected. So you don't need to experiment with stopping down the lens yourself and either guessing or using the guide number formula - that's all automated. With m42 and other older lenses, the flash *always* fires full power, and the camera has no way of setting an aperture for you, so you've got to figure it out yourself.