...or neglibible:

Contax and Nikon rangefinders had the same mount but a slightly different back focus. The wide-angles were interchangeable (thus the new Voigtsinas lenses also useable on both) but the longer the focal length, the more incompatible they were.

Now why does the Konica Hexar RF's initial back-focus problem affects the wide-angle Leica lenses more than longer lenses?

-- Andrew (mazurka@rocketmail.com), May 19, 2002

Answers

While wider angle lenses have greater DOF, they also require a much smaller amount of lens movement to focus. Hence, a back-focus issue can be a bigger problem on shorter lenses. But it ultimately depends on wether or not the back-focus error is within an acceptable DOF range or outside of it, and this will depend on the lens, the actual back-focus error and the individual photographer's threshold for focus-error -- or more accurately, COC ("circle of confusion") -- tollerance.

Personally, I have a very low tollerance for point-of-focus errors, so when I focus on the eyeball, and the nose looks sharper it drives me nuts. This is the type of issue I had with the Hexar and my M lenses. The eyes were still acceptably sharp by themselves, but the nose was sharper, and hence made the eyes look soft -- at least to me. In a landscape or scenic image, this would go un-noticed. But with a close subject and wide apertures, it is unacceptable -- at least for me ;-)

Cheers,

-- Jack Flesher (jbflesher@msn.com), May 19, 2002.

Jack, your problem sounds more like a cam follower arm adjustment problem than a back focus issue. I've had that problem on a couple of Leicas - I'd bet it's the single most common adjustment performed during a CLA.

-- Paul Chefurka (paul@chefurka.com), May 19, 2002.

The problem with Contax vs Nikon is not back focus--it's the number of degrees the two manufacturers selected for the mount to turn from infinity to 1 meter. One turns slightly farther than the other, and since rotation directs the RF. . . . That's why the problem expresses itself at near distances, and why a W/A lens can manange to work fairly well. Whether Nikon got it wrong intentionally, or because they were idiots is the question. Canon seemed to be able to do it all correctly, as has Voigtlander, and Konica, for whatever reason, didn't. Hence, I've chosen to reward Canon and Voigtlander with my money.

Willfully creating a problem, or idiots? Neither scenario wins me as a customer, regardless. The obvious answer is, since they didn't get it right, why buy their product, or fret over the problem? I'm doing neither. Let them die the natural death of the inept.

-- Michael Darnton (mdarnton@hotmail.com), May 19, 2002.

the difference between the nikon rf and contax is simply the pitch of the helical. this does result in a slightly longer thro for the contax. the difference was quite intentional. nikon offered "c" series versions of its lenses for contax. i believe it paid a fee for this privilege. i think the reason nikon injected the difference was to avoid a legal battle with zeiss. as you say, the difference gets swamped by DOF up to 35mm. as for the new voigt CS lenses, it may interest you to know that they are optimized for nikon (although with only 21, 25 and 35 offerings, it doesn't matter. i should also point out that voigt was as bad as the rest of the companies when it was german and making cameras. are prominent lenses interchangeable with anything other than some of the russian cameras??

-- roger michel (michel@tcn.org), May 19, 2002.

This may be a chicken or egg question totally uninteresting to Leica readers, but since my theory involves one standard rotational unit for every lens in the series, but yours involves a different incorrect helical in exactly the same manner of incorrectness for every lens in the series, I'm going with my idea.

However, I suspect that the real culprit is more connected to history, that the Nikon RF and inner bayonet mount was calibrated for the Leica-standard 51.9mm nominal 50mm lens that Nikon had long been making before they started making Contax copies, whereas the Contax mount and RF were designed for the Contax' 52.8mm (or whatever exactly it was) normal lens. That is, there's no error, and no intent to be different--merely the desire to follow the direction in which they had already been going with the lenses they already had in production.

I've read a lot of Monday morning quarterback explanations, but I'll bet that's the one that really makes sense.

-- Michael Darnton (mdarnton@hotmail.com), May 19, 2002.

By the way, the implication of that scenario is that the helical is most likely the same, but the Contax 50, being longer in reality, actually requires a bit more rotation to focus to the same close distance. I suspect all the people who've said the helical is different never tried to fit a Nikon inner bayonet on a Contax to confirm their idea. Wanna bet?

And then, as we know with Leica, Nikon, and Contax, all, other lenses manage their coupling to the RF by mimicing what the 50 does, whatever the default 50 is.

-- Michael Darnton (mdarnton@hotmail.com), May 19, 2002.

And while I'm flogging this dead horse into horseburger, a 1mm difference in nominal FL means about a 2% error, so I'd suspect that the Contax lens would require a throw about 2% longer. I believe the throw is around 280 degrees, and 2% added to that would be around 5 degrees added--and I do think I remember that the difference between the two systems is on the order of 5 degrees, right, anyone?

-- Michael Darnton (mdarnton@hotmail.com), May 19, 2002.

that mite make sense except for several facts: (1) with the built- in helical design, the essential thing with lenses of different focal lengths is to get the infinity back-focus distance rite at infinity. this would be accomplished by altering the mounting point of the helical in the camera, not the pitch or throw of the helical (however you wnat to look at it); (2) a contax 50mm works fine in a nikon rf AT INFINITY. it's only nce you start focusing closer that problems crop up. and this is not a dof issue as using the fast 50 wide open will show you; and (3) all lenses BUT the 50 have their own helical - - there is no reason that nikon (or contax) couldn't have made these lenses cross compatible. they didn't, with nikon producing a separate loine of contax cmpatible lenses.

-- roger michel (michel@tcn.org), May 19, 2002.