>on kmp.bdimitrov.de I saw the specs of the 200 f2.5 lens. It says that >the filter ring is 77mm, but this means that the lens cannot be f2.5, >as that would mean a minimum 80mm filter ring! Can anyone explain? BTW. >Isn't it save to have some margin between the filter ring and what is >calculated by dividing the focal length and the aperture. I'm thinking >about robustness against vignetting, especially when using a filter.
>But I agree that this inner, effective diameter can't be bigger than the >outer one. Such I guess that Pentax cheated - the number is incorrect. The 200/2.5 is, of course, ~supposed~ to be a little faster than the 200/2.8. They both use 77mm filters, but the actual diameter of the 200/2.5's front element is definitely a little wider than that of the 200/2.8's front element. (The circular "frame" around the outer edge of the 2.8's front element is definitely more restrictive than is the thinner "frame" on the 2.5.) However, it does seem that the math cannot support the 200/2.5 really being an f/2.5 lens, although it may indeed be slightly faster (f/2.7, say - <g>). As for "cheating" by Pentax, well, it would not be the first time that tech specs have been dreamed up in a marketing department, would it? (<g>) After all, when the lens was first introduced, the Pentax Lenses and Accessories booklets of the time stated: "In testimony of its role as a leader in the field of optics, and ever mindful of the needs of the professional photographer, Asahi Optical has introduced the first 200 lens with an f/2.5 maximum aperture. This ultra high-speed telephoto lens is well suited for available light photography, such as indoor and nighttime sporting events. Even when used wide-open, its 6-element, 6-group optical design ensures high contrast and resolution, as well as attractive out-of-focus highlights." So, with a marketing "blurb" like that, how could Pentax possibly let a little math stand in the way of "its role as a leader in the field of optics", right? <g> [As an aside, the same kind of "fuzzy math" must have been at work in the labeling of the Takumar Bayonet 135/"2.5", with its meager 52mm filter thread (unlike the K 135/2.5, which has a 58mm thread).] The A* 200/2.8 and the K 200/2.5 share the same optical design, along with the K 135/2.5, and no other Pentax lens has the same construction as these three lenses. (Judging by the qualities of these three lenses, I'd have to say that it was a very successful design.) As for comparing the two lenses optically, I think that the K 200/2.5 is virtually as sharp as the A* 200/2.8, though it is a little longer, a little heavier, and perhaps a little more rugged. The 2.8 does have an LD element, I guess, but I don't think this appears to be overly significant at 200mm (as it starts to be by 300mm or so, in my experience. There might be just a very slight edge for the 2.8 in color details at high magnifications, which may be due to the LD glass. The 200/2.5 seems to focus a little more stiffly than does the 200/2.8, but this is simply because the 200/2.8 focuses "softer" than most of the other manual focus Pentax lenses. (This focusing difference is analogous to the difference between the K 200/4 and the A 200/4.) I was still in possession of an A* 200/2.8 when I obtained my mint 200/2.5. I liked the 200/2.5 so much (call me old-fashioned - <g>) that I ended up selling the 200/2.8, and I've not ever been displeased with the change ever since. Fred - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
