Archived posting to the Leica Users Group, 2003/07/31
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Just got home to find that this thread is still going strong. Not for the first time, I find myself reading replies to messages that I never received in the first place. Anyway, Eric I'm not sure if you saw my earlier posts, but the main point is this. A rangefinder, such as the one in an M, has a fixed base and magnification, which sets practical limits to its ability to measure distance. This *DOES NOT* have anything to do with the lens mounted on the camera. Let us say you focus on an object at a distance p away. The rangefinder, for any number of reasons, may actually be at p +/- D, where D is a small deviation from the true distance. I think we can agree that if D is not small the RFDR is useless. D is a function of the design, construction and tolerances of the RFDR assembly and lens coupling; it may also be a (slowly varying) function of P. If the deviation D can be covered by the DOF the image appears sharp. Note that as you increase focal length and aperture, DOF reduces while D remains the same for it is a parameter of the RFDR only. At some stage the DOF will not be adequate to cover deviations from the true object distance and the images will appear to be unsharp or focussed on the wrong spot. Note that the accuracy of the RFDR is not increasing with wide angle lenses, rather the longer focal length/wider aperture lenses require a lower D than designed and built into the RFDR. In SLR's, OTOH, you are using the lens to focus through and as focal length increases, not only does the baselength increase, but the magnification also increases making the effective base even greater. The crossover occurs at around 90 mm with Leicas, but at a longer focal length with Contaxes. Hope this helps. - ----- Original Message ----- From: "Eric Welch" <eric@jphotog.com> To: <leica-users@mejac.palo-alto.ca.us> Sent: Thursday, July 31, 2003 21:43 Subject: Re: [Leica] arggghhh... rant at Keeble & Shucat... > No, that's not true. The longer the focal length the less accurate a > rangefinder is. Because the base of the rangefinder (based on the > distance between the two windows that make up its base) determines the > accuracy of that rangefinder. It becomes smaller in proportion to the > focal length as the latter increases. Thus less accurate in absolute > terms. Why do you think Leica sells bodies with different viewfinder > magnifications? > > Why is an M6 more accurate than a CL? Because of the rangefinder base! > It's wider on an M body. Why is it harder to focus accurately with a 75 > Summilux wide open than a 50 Summilux? It's in part because the depth > of field is shallower. But it's also harder to place that plane of > perfect focus where you want. Because the rangefinder is less accurate > with that lens because the "base" is narrower for the 75 than it is for > the 50 in proportion to the focal length. > > This is basic physics. But you are right that the difference in > relation to SLRs changes with focal lengths changes as focal length > changes - more or less. An SLR becomes less accurate with wider lenses > - for the same reasons. The "base" of the focusing of an SLR lens is > the width of the front element in proportion to its focal length. > > Someone ask Erwin Puts to rule on this disagreement! > > On Thursday, July 31, 2003, at 07:00 AM, Clive Moss wrote: > > > I think he means "more accurate > > than SLRs" -- which is true. If he means "more accurate in the sense > > of less > > deviation from the exact distance" - clearly untrue - the rangefinder > > has no > > way of changing its measured distance based on the focal length of the > > lens. > > Eric Welch > Carlsbad, CA > http://www.jphotog.com > > Always drink upstream from the herd. - Will Rogers > > -- > To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html > > - -- To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html