Originally posted by PaloAltoMark The lens has two aperture rings at the front of the lens. One seems to limit the aperture that can be chosen to a range. The other actually sets the aperture.
Can someone explain why this was done and how best to use it?
To elaborate on jatrax' comment, your lens has what is called a "pre-set" aperture mechanism*. One ring is used to set the desired aperture and the other ring is used as the actuator to do the actual opening/closing. There are many variations on the theme and such lenses were not uncommon for SLRs through the 1950s up and even into the early 1970s. Even today, some specialty lenses (e.g. tilt/shift) use this sort of arrangement where a coupled stop-down would be difficult to implement.
I currently own one pre-set lens, a Russian LZOS MC Jupiter-9 85/2 in M42 mount. Strangely enough, it was made less than ten years ago.
Here is a decent how-to page that include a pre-set Takumar as an example:
Preset Aperture Lenses – How They Work And Why You Need At Least One – 35hunter
Steve
* A good article for this site might be one about the four different types of aperture actuation and control mechanisms, manual, pre-set actuation, semi-auto actuation, auto-actuation, and auto-actuation with auto-control.