On-lens Aperture Question

My apologies if this is a basic question, but it's something I've wondered for a long time....

On lensws with an aperture ring, instructions always say to set the on-lens aperature at its minimum f-stop (i.e., largest f-stop number) if you want to set the aperture with the camera. This has never made sense to me, because, at least to my logic, one would want to set the lens to its maximum f-stop (i.e., smallest f-stop number). That way, it seems logical that the camera could impose smaller f-stops as desired.

Could someone explain this to me? Thanks in advance!

Hi and welcome.

Some fully mechanical lenses are used with stop down metering, meaning that you open the lens all the way (maximum aperture, smallest number) to focus, then stop down to the correct aperture for the exposure. Some older 35mm lenses are this way, and large format view cameras are like this too.

Newer lenses have automatic apertures that stay open until you take your shot (they close down right before the shutter opens, then open back up). The light meter knows what you've set the aperture ring at (ex f8) but the lens stays open (ex f1.4) because of a mechanical interlock.

Because of that mechanical interlock, stopping the lens down all the way (minimum aperture, highest number) allows all apertures to be used. If you stopped it only down to f8, it could only use apertures from wide open to f8. You can't do this because you'll get an error message (fortunately) but if you could override that safety feature this is probably what would happen. I don't recommend doing this, but there's a spring-loaded lever you can slide on the back of the lens that opens and closes the aperture. If the lens is wide open, it won't move. Stopped all the way down, you can move it all the way from wide open back to minimum aperture (how the lens will be if it's off the camera). That's how the camera sets the aperture.