Archived posting to the Leica Users Group, 2003/07/31
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Eric, Karen is correct and you are not. As long as the DOF is greater than the rfdr accuracy, focus should be fine As long as the depth of field (using whatever COC you select) covers the cumulative inaccuracies/tolerances of the optical rangefinder & mechanical coupling, matters are Ok. The absolute focusing ability of an M rangefinder *does not* depend on the focal length of the lens mounted on the camera. As you correctly state it only depends on the baselength. However, the ratio of focal length to rfdr base length you mention below is an erroneous metric; note that aperture is not mentioned. It works, superficially, because as focal length decreases (at a constant aperture), DOF increases making cumulative rfdr error less of a factor. The reason why 90 mm is usually taken as a crossover point between RFDR & SLR has to do with (surprise!) baselength. Calculate the effective baselength of a 90 mm Summicron R on an SL, say, assuming it is a simple lens (hint: 90/2 = 45 mm) and compare this to the effective baselength of, say, an M6 0.72 and it will become clear. Ask yourself which will have a longer baselength when focusing a (theoretical) 35mm /f0.5 lens, an SL or an M6? By the way, I cringe every time I see someone say "Basic Physics" on the web. Regards, Akhil - ----- Original Message ----- From: "Eric Welch" <eric@jphotog.com> To: <leica-users@mejac.palo-alto.ca.us> Sent: Thursday, July 31, 2003 01:36 Subject: Re: [Leica] arggghhh... rant at Keeble & Shucat... > Sorry, Karen, but you are flat out wrong. There is no such thing as > accuracy being covered by depth of field. Every lens has a focusing > plane where the optical image is at its best, and even in the "cone of > light" of the depth of field (based on the standards of the photo > industry, that tends to be any part of the cone less than 1/30 of a > milimeter) the sharpness falls off as you move away from that optimal > plane of focus. > And the very FACT that at about 75mm to 90mm SLRs becomes more accurate > in focusing than rangefinders is proof. This is a well-known fact, > espeically amongst Leica folk, since we've debated this issue for > decades (the debate being where the SRL becomes more accurate than the > rangefinder). The focusing of a rangefinder camera becomes more > accurate at shorter focal lengths. And it has NOTHING to do with depth > of field. It has everything to do with basic optics and physics and the > very principle of how rangefinders work. The wider the rangefinder > base, the more accurate that rangefinder becomes. > And the shorter the > focal length, the wider the rangefinder base of a given camera becomes > in proportion to the focal length. > > These are just examples I pulled out of thin air, but the principle > applies: > > At 90mm, say the rangefinder base is 100mm. That's nearly a 1:1 > relationship. At 50mm, the relationship becomes 2:1, base to focal > length. At 24mm that relationship becomes 4:1 base to focal length. > It's clearly vastly more accurate in focusing at 24mm than it is at > 90mm, because the relative rangefinder base to the focal length is 4 > times larger. And it's THAT base that determines focusing accuracy. > > You can argue that for most people that's not important because they > shoot the "f/8 and be there" hyperfocal method. I only use that in > riots and other fast-breaking news situations. Because that technique > might end up putting the optimal focus plane far away from where I want > it. But when you're shooting a 35mm Summilux ASPH vs ANY 35mm SLR with > a 35mm lens at 1.4, the rangefinder is going to be clearly superior to > the SLR. But at 90mm, the SLR will be more accurate than a rangefinder > camera at the same aperture. > > Basic physics. > > On Wednesday, July 30, 2003, at 06:49 AM, Karen Nakamura wrote: > > > So the focusing *accuracy* of a rangefinder is constant, it's just > > that that accuracy will be visible with certain lens combinations. > > Eric Welch > Carlsbad, CA > http://www.jphotog.com > > Good judgment comes from experience, and a lot of that comes from bad > judgment. - 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