Auto/Manual switch on Takumar?

Nick, it's nice to hear you're using this fine lens! I have this and a few more Takumar lenses and love them too! At any rate, the Auto/Manual switch cannot be used with the adapter or type camera you're using. This was originally there to control if the lens would be in stop-down metering mode (manual) or not (automatic.) As you know, the aperture on a modern SLR typically remains wide open until the moment of exposure, when the aperture is closed down to the aperture you selected in order to make the correct exposure. The lens stays wide open during viewing to make focusing easier and so the screen is not darkening and lightening up all the time. On the original Pentax cameras these lenses were made for, you could use the lens automatically as described or you could set it for manual and this is how it works on your Canon no matter what. As you adjust aperture, you see the change in the viewfinder. This is useful for DoF preview. So your process should be: 1--set the lens to 1.4 2--focus 3--set the lens to the desired aperture 4--snap the photo This process is called Stop-Down Metering because you have to stop down to meter. Notice that when you remove the lens, without that pin depressed, and set it to A and change the aperture, the diaphragm inside the lens does not move. But when you set it to M, the diaphragm moves accordingly. When you depress the pin, the diaphragm moves regardless of it the lens is in A or M. This should help you understand what is going on. If you want to see what this lens is like when used in automatic, go to KEH.com and buy yourself a Pentax Spotmatic F 35mm SLR! Enjoy!