Originally posted by Just1MoreDave Now that I read it again, it needs some qualifications. I was thinking of two points that don't apply to every lens or camera combination.
First, the aperture rings rarely click at every possible half-stop, so using just the aperture ring, you sometimes will not have a setting where you need it. I'll agree that a 20 year old A50/2 only dropped twice may not hit every third-stop between f2 and f2.8 with perfect accuracy, but the ring only clicks at each full stop. If you want f2.4, the camera body can probably do that at least as well as trying to put the ring between clicks. On many lenses, the ring clicks at an unmarked position - e. g. the M50/1.7 clicks between f1.7 and f2.8 (probably f2). An A lens doesn't have that ambiguity.
Second, on a crippled-mount body, the lenses with an A position work like they were designed to in the A position. That is, having the aperture adjusted by the lever. Once you go off that A position, the mechanism within the lens is used. It is probably more a feeling than fact that these may produce different results. I haven't tested it myself. If you're not using a crippled-mount body, it probably doesn't matter as much.
I think the statement would be harder to defend if I had not included the qualifier phrase "between lenses". That's where the first point can be important. On the lens tests I've done, I always find some lenses without a setting at certain apertures. But if I had a lot more "A"-type lenses, I might be complaining about the false impression of accuracy given by the viewfinder numbers.
Thanks for the clarification. This answer makes more sense. I hope you don't mind that I bolded the last sentence.
I was taught that process accuracy is how close the delivered value is to that which was expected. Precision is the variance around that value for multiple attempts. I would expect that aperture accuracy might vary between lenses, even for the same model based on care taken at time of manufacture. Precision is determined by a combination of design, build quality, and wear from usage.
With an "A" lens, aperture lever travel is supposed to be proportional to the aperture area. X amount of travel will result in Y percentage increase/decrease in aperture. Given that the amount of travel is never more than about 6 mm full open to full closed, it is amazing that these things work at all, never mind to 1/3 stop increments. Factor in the required manufacturing tolerances to translate that motion through a system of levers and rings with no run-time feedback mechanism...I think you can get my drift. Both accuracy and precision are difficult to attain and are the product of both body and lens build quality.
Compare that to the traditional aperture ring with fairly large movements that translate the setting through fairly simple mechanisms and mechanical detents.
Now as far as performance of an Pentax-A series lens both on and off the "A" setting...
The aperture ring on my A 50/1.7 is so clunky, I don't think I would use it on any of my pre-A bodies! Assuming it works correctly, my understanding is that the aperture lever executes full motion for lenses off the "A" setting with the aperture being determined by the mechanical "stop" for the ring position. When in the "A" position, there is no stop (i.e. minimum aperture). Same mechanism for both methods with different means to determine when the blades stop. Accuracy and precision for this case is also determined by build quality, though the task is somewhat easier and extends to the lens only.
Thankfully, both film and digital sensors have plenty of exposure latitude!
Steve