Originally posted by reeftool The camera can and does display f/3.2. It doesn't offer it as a maximum aperture in the mid 20's mm range as some of the published reviews claim. DP's review states that f/3.2 is the maximum DISPLAYED aperture at 24mm on the Canon version they did their tests on. Our Pentax bodies only display f/3.2 as a stop between f/2.8 and f/3.5 at 17mm - 20 mm (appox. as there is no marking on the lens focal length scale between 17 and 24). At 24mm, mine displays f/3.5. I'm not going to loose sleep over this. I looked through Sigma's advertised tech specs and it only mentions the minimum aperture of f/22 and no mention whatsoever of the exact largest aperture at any particular length other than the assumed f/2.8 at the shortest and f/4 at the longest.
With the F 50mm 1.7, I can see the following: 1.7, 1.8, 2.0, 2.2, 2.5, 2.8, 3.2, 3.5, 4.0, etc.
Obviously, this is not a camera limitation but rather a lens one. It would seem strange that a lens released some 20 years before my camera was even an R&D project would be better at relaying f-stop information to the camera than one released last year, even if the lens is a third-party product. (They surely tested the lens on a K-5 before releasing!) I'd find it hard to believe that Sigma would be so lazy not to report fractions of f-stops that the lens is actually doing, although that could be true.
Also, I don't think it's necessarily true that f stop on variable aperture zooms necessarily increases linearly. It really depends on how the lens is designed. A constant aperture lens has magnification elements that move as you zoom the lens, and proper placement ensures the correct effective f-stop is attained. Certainly, a simple lens with only a single magnifying element would necessarily have linear and continuous f-stop increments, as the magnifying element would be moving from point A at the wide end to point B at the short end (closer to the lens), inherently reducing the magnification factor. But if the lens contains a few of these elements that move over shorter distances could achieve piecewise-constant f stop. I'm not an expert on lens design, but I'd like to think that sheer laziness of improper reporting of f-stop is not the explanation for what we are seeing here.