Why is the viewfinder @ -0.5 diopters?

[Shootist]

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Again,

 

 

Thank you Lars.

[lars_bergquist]

In fact, the Leica IV prototype did not only have a combined rangefinder. It had all speeds on a single, non-rotating dial on top of the camera, M style, 18 years before the M3. Another boon to users which was cast into the wastebasket, where it had to wait for two decades. (Well, at least one, as the prototype work on the M3 seems to have started quite soon after the end of the war.)

 

And if my eyes are not too old and rheumy, that dial was slotted for a coupled 'outboard' meter, like the Leicameters for the M3 and later cameras! If that dial is original, it is amazing—and it highlights the conservatism of the design department. –A rotating dial made such a coupling impossible of course.

 

The same old man

[jaapv]

Again, the rangefinder adjustment had nothing to do with the user's eyesight problems, if any. If this had been the case, adjustment would have been needed for both the rangefinder and for the separate finder eyepiece. After all, the same deficient eye was used to look through both.

 

And such diopter adjustment WAS indeed offered, as of 1933—by way of paired, identical dioptric correction lenses for the two eyepices, sold as a pair (code ORTUX)! Which neatly disposes of the superstition about the 'rangefinder dioptric correction', copied from book to book and from edition to edition by thoughtless writers (who obviously never considered actually using the cameras they wrote about).

 

The truth is the following. The Leica II, of 1932, had a unit magnification rangefinder. So the effective rangefinder base was the distance between the centers of the two rangefinder windows, no more, no less. This was later deemed insufficient for the 13.5cm lenses however. So with the advent of the III, and later the IIIa, the magnification was increased to 1.5x, thus increasing the effective base by the same factor.—All well and good, but now the rangefinder was a little telescope, which had to be focused for near and far subjects. PLEASE OBSERVE that the adjustment lever around the rangefinder eyepiece (around the rewinding knob from the IIIb) was marked, not with dioptries this or that, but with an infinity mark! So, if you were e.g. nearsighted, you would have screwed negative corrrection lenses into both eyepieces of your shiny new IIIa, AND then focused your rangefinder separately.

 

Now proper engineering procedure would have been to redesign the rangefinder with a longer physical base. Barnack did not do that. He used instead a patchwork approach which in fact created a new problem with the solution of the first one. The prototype called the 'Leica IV' would have solved these problems with a radically new design approach, a combined rangefinder in the spirit of the Contax II (and the later M).

 

But Barnack contracted pneumonia and died in 1936, and the IV was dropped. With the IIIc in 1940, manufacture was radically simplified; the camera was in fact totally redesigned. But Barnack's death had thrown Leitz into a coma. While manufacture was much improved, the ergonomics of the camera was left as it was, warts, quirks and aggravation and all, by a company that did not dare consider a departure from what was now a sanctified design. It took a world war to get Leitz out of the rut. And then of course the result was the M3. But—even after that, the old formula was trotted out again with the IIIg, separate rangefinder, separate dials for slow speeds and fast (rotating, needless to say!) with a minimum of concessions to user friendliness. Leitz did really never believe in new things, even though Ludwig Leitz was deeply involved in the design of the M3. The old guard was against it.

 

So this is what the lever is for.

 

The old man from the Age of the IIIf

 

If anyone is interested, there is a product shot of the Leica IV in Johnathan Eastland's Leica M Compendium. It clearly points towards the M3 in design. I wonder, Lars, did it already incorporate a banjonet lens mount?

[bill]

Thank you Lars,

 

There is something quite delightful about these little nuggets of Leica lore, particularly when they correct a misconception.

 

It's odd, but my first Barnack was a IIIc, with separate view- and rangefinder windows, slow speed shutter dial, etc. Later I acquired an older IID with no slow speeds and windows further apart. I actually found it easier and more pleasant to use.

 

I think as the LTMs developed, they got more complex and moved away from the original concept. The M3 took Leica back to that simplicity with a clean slate. I hope the M9 designers have the courage to do something similar.

 

Regards,

 

Bill

[Martin Schmidt]

With NORMAL YOUNGER eyes the -.5 diopter has no effect, your normal younger eyes can adjust to it without any problems. When "WE" get older our older eye can not make the slight adjustments as easy or at all.

 

Why does Leica set the viewfinder at a -.5 diopter. I have no idea but it has been like that for decades. It must have something to do with how the viewfinder/rangefinder works.

If you had 20:20 vision and your eyes could still focus on close up objects, didn't need reading glasses, you wouldn't need any add-on diopter.

 

I'll bet that without your glass, which are for distance (Right), you could see the eye chart OK with or without the camera in-front of your eye.

 

You are absolute right. Some is missing. The human eye can only generate plus diopters itself, not minus. Therefore (especially young people) the eye can compensate the minus 0.5 of the viewfinder. If the viewfinder was plus 0.something the human eye could not compensate the diopters.

Even "older" eyes have a depth of field of 1 dioptre. So, theoretically it should be possible to have a sharp view through a minus 0.5 viewfinder.

Regarding the tolerance it is better to produce viewfinders between 0 and minus 0.5, than having just a bit of plus diopter, which no one could compensate.

Another reason: Most people are slightly myop (that means they need minus-lens-correction). Sometimes the Doctor will not be able to measure this (letters are in 4 meter distance, not in infinity). So it is just for statistic reason to produce minus dioptre.