Archived posting to the Leica Users Group, 2000/10/02
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]I find this debate somewhat entertaining. Forgetting for a moment that we had this discussion regarding another vehicle with the same EBL (actually, marginally higher), and forgetting that even Erwin's computations (before he added the 0.58 column) showed that just about anything can focus a 50mm lens, I come to the question Is EBL even relevant, or is it a construct? This has troubled me for a while. 1. Start from the principle that if you had 100% magnification, would you rather have a 41mm BL or a 69.25mm baseline? Pretty easy - you'd go for the 69.25 as more precise. With 0.72x finders, there's not much debate that an M6 is more accurate than a Bessa R. Move to this thought - a Leica III has the same EBL as an 0.85 M6TTL (around 58mm, if I recall). Which one has a higher precision rangefinder system? The III takes a low-BL rangefinder assembly and cranks up the magnification. The M6 0.85 takes a long-BL assembly and shrinks it. The III had a high-precision mechanism for its day, but to put it bluntly, you're magnifying what is a less precise system. I would submit that if your viewing optics were perfect, you're actually better off shrinking a long baseline (which has more precision) than enlarging a short one. So EBL appears be a misleading concept when the RF mag is over 100%, since the same EBL number could be applied to two wildly different systems. This is not even taking into account the fact that the separated VF/RF lacks the vernier focusing of the split-image. 2. Change to this comparison. 58 vs. 72 vs. 85 vs. 91 finders. They are all identical in all respects related to triangulation. I think we can all accept the trigonometry. By necessity, all raw systems must have the same resolution (if seen by the theoretically-perfect viewer). Does anyone have the hard data on is what effect magnification has on the human eye's ability to focus? Where does this come from? Is there a 1:1 relationship between size and ability to discriminate points? Or is there some minimum threshold of visibility after which all point-distance perceptions are close to equal? Do we get the same result with complex subjects that "look" to be "in focus" (that is, scores of simultaneous and instinctive point-matchings) as with simple point-matching? It would seem in this neverending debate that the utility of the EBL function is entirely dependent on the individual and would be hard to reduce to a formula that could magically determine RF accuracy. Do the functions Erwin uses reflect an average human resolving ability? This is a big assumption in all of the EBL machinations, and it is one he could explain a little more. I would assume that there is some biometric average. But there are troubling cases: for a user who has astigmatism, is accuracy better through glasses (which only correct astigmatism in broad strokes) or by the rigid contact-lens wearer (whose astigmatism is forcibly, perfectly, and completely corrected). So in sum, it would look like EBL is a questionable concept for magnifications over 100%, and that use of EBL for magnifications lower than 100% is really just a shorthand for accuracy in the hands of an average user, accuracy which may or may not be reflective of any particular individual.