The way A lenses work is quite low-tech, but consistently gets the job done as you've already observed. The contacts on the lens mount only tell the camera what the maximum aperture is, and if the "A" setting is selected. Also, the aperture stop-down lever controls the aperture diaphragm in a standardized manner, such that the same amount of travel will yield the same number of stops no matter what lens is mounted. Thus, whenever you dial in an aperture, the camera can calculate exactly how far it needs to move the lever in order to get the desired aperture setting.
The "A" setting allows the stop-down lever to move from the minimum aperture to the maximum aperture, much like the F22 setting. This lets the camera select any possible aperture setting when it goes to take the shot.
The aperture mechanism is only operated when the shutter is open. At all other times when the lens is mounted, the camera forces the aperture control level all the way up to the wide-open position.
BTW, modern lenses employ the same principle
That's why a DA lens can theoretically be used on an old A-era body such as the Super Program.
So what's the difference between DSLRs and film SLRs, you might ask? Well, you may have noticed that on the back of your lens, there's a flange that moves whenever you turn the aperture ring. Unlike DSLRs, film SLRs also had some hardware that could sense the position of this flange, thus being able to tell what f stop was selected on the aperture ring. That's why on film cameras, you could use the aperture ring instead of / in addition to in-camera menus to select the aperture setting. This isn't possible on digital, which means that if the aperture ring isn't set to "A", you need to stop-down meter. This is only an inconvenience when using M or K lenses, which is why so many people wish Pentax would "de-cripple" the K-mount used on DSLRs.
Adam
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