Looks like you guys got your posts in before me
Here's my overly-long explanation:
Before the digital/computer age, all Pentax lenses had an aperture ring. Literally, the point if it is to select the desired aperture when shooting.
In the 70's, the "A" setting was introduced to Pentax lenses, thus allowing Tv and P mode on select bodies. When a lens was away from "A", the camera could still mechanically feel the setting on the aperture ring and shoot with the user-selected aperture.
The use of the aperture ring dramatically changed when the *ist and *ist D were introduced in 2003. Both of these cameras had a "crippled" mount, meaning that they
cannot mechanically feel the setting on the aperture ring if it is away from "A." Up until this point, all Pentax lenses had aperture rings (K, M, M*, A, A*, F, F*, FA, FA* series). In 2003, however, the FAJ series was introduced. This series of lenses did not feature the aperture ring, thus being easier to use on the *ist and *ist D. The lack of the aperture ring, however, prevented the use of these lenses on older manual bodies.
The DA series also lacks the aperture ring, but this is justified because these lenses can't be used on the film cameras anyway.
The aperture value on current Pentax DSLRs can only be set electronically through the camera because of their "crippled" lens mounts. The crippled mount also makes manual (K, M) lenses harder to use because they need to be manually stopped down first.
To answer your second question- it is much easier to set the aperture via the aperture ring than electronically through the camera (on non-crippled film bodies). Most of those that are used to older film bodies never messed with the wheel on the back of the camera (which was actually not always available) when selecting an aperture value, but it's become necessary on the Pentax DSLRs.
Adam
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