John, There was no "original contention." What started all this was my comment that I found the ST 105/2.8 easy to focus when stopped down to f8.0 when used with the istDS. It was just a simple comment reporting my personal experience with a specific lens on a particular camera, and my pleasure in finding how nice and easy it was to use an old screw mount on the istDS. No contention that this experience is/was transferable to other people, cameras, lenses, situations. Later I tried the 35mm/3.5, and found it about as easy to focus. Just another comment reporting my experience, with that lens, on the same camera. I guess my ignorance of the laws of physics and the science of focusing must have skewed my experience. I just didn't know that longer lenses are easier to focus. Had I known that, perhaps my experience would have been colored by that knowledge, and I'd have found the 105mm easier to focus than the 35mm ;-))
Neither Bill nor I contended that our experience would be true for other lenses, other situations, other cameras, nor were we trying to refute the laws of physics. However, JCO, and now Mr Papenfuss, Ph.D., PPSEL-IA, claim that longer lenses are always easier to focus, although Mr Papenfuss at least has the good sense to add a rather long list of qualifiers to his argument. Never did I think that this thread would get so contentious, and that it would run for so long. And now Mr. Papenfuss, Ph.D., PPSEL-IA has joined the fray and gotten things going again. All the scientific evidence and book learnin' in the world cannot change the experience Bill and I had, regardless of what the laws of physics says. Of course, Mr. Papenfuss was careful to qualify the heck out of his comment, which is as it should be, because there are numerous variables in the real world, which, unfortunately for some people, is where we have to live, work (and photograph) these days. Now, were we to try the test in a lab, eliminate all variables, use scientific measuring tools, remove the human element, it may be that the 105mm lens could be proven to focus easier or more accurately than a 35mm lens. It's also interesting to note that some fast, long lenses are notoriously difficult to focus well or quickly, and that wider lenses are easier to focus. A case can be made using the Leica M75/1.4 or the Leica M 90/2.8. For many people both lenses require a lot of practice to focus accurately, and focusing a 28mm lens is much easier for many, if not most, Leica M users. So, where does that fall into this "discussion." Oh, wait, no one said anything about rangefinder lenses ... they are the exception to the laws of physics and scientific testing and discourse. Shel (a man of no letters) > [Original Message] > From: John Francis > > Everything else being equal (aperture, contrast, resolution, > > helical gear cut, etc), a longer focal length (e.g. 105mm) will have a > > higher "focusing sensitivity" than a wide angle (e.g. 35mm). That's just > > plain physics. > > But that still doesn't necessarily make them easier to focus, which > I believe was the original contention. In fact in at least one way > it makes them harder to focus - it's too easy to overshoot, or to > focus on the wrong place (especially if you are trying to pre-focus > in anticipation of a moving object coming into your composition). -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
