Shel, I'll note that focusing with SLR's and RF's is quite different. RF's lose focusing accuracy with longer/faster lenses (Where this happens depends on the effective RF baseline length) while SLR's lose focusing accuracy with slow/wide lenses. Experience with focusing a Leica M lens does not cross over to focusing a SLR lens.
-Adam Shel Belinkoff wrote: > 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
