M9 Viewfinder Effective Viewing Distance

[pnoble]

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What is the effective viewing distance to the frame lines and focus area, with the -0.5 diopter eye piece factored in?

 

My glasses, middle range, are optimized for around 0.5m viewing distance. What if any diopter adjustment should I add? If I want to use without glasses how would I calculate the appropriate diopter adjustment?

 

Optimal visual acuity for manual focusing seems like a good idea!

 

I know this subject was dealt with in the past, but I couldn't find the thread by search, and I think there was anyhow a divergence of opinion at the time.

 

Considering the likely average age of Leica owners I'm surprised Leica doesn't publish an FAQ or white paper on the subject. Of course adding a variable diopter adjustment like every other camera manufacturer would be an even better idea.

 

I'd just get the MS-MAG 1.5x variable adjustment eyepiece and be done with, but between the total inability to then see 28mm frame lines, and the fact that they are not allowed to sell it in the USA for rangefinder use due to Leica patents (!), I thought I'd better go the Leica diopter route if needed.

[Bo_Lorentzen]

A single diopter which works well for you is always the cleanest way to do this.

 

Personally, I have used glasses since I was a teen, and simply leave them on my face, sandwiches the glasses between the camera and my face.. eventually they suffer, but by then its probably time for a new set anyway.

 

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[Guest EarlBurrellPhoto]

The Leica viewfinder is not like an SLR where there's a ground glass "set" at a "virtual distance". The Leica viewfinder is rather like a telescope. Thus whatever eyeglass prescription you wear is added to the -0.5 and the result is the diopter lens you need. That is, if you wear single-vision glasses and do not have astigmatism. Astigmatism can be dealt with by choosing a diopter according to your "spherical equivalent" (ask an optometrist or optician what that would be for you). Those of you who are at the age where you need reading glasses or bifocals (probably a large majority) will find that one diopter will not correct the viewfinder for subjects at infinity and at the nearest focus unless your prescription for reading is very mild.

 

Side note: the viewfinder LEDs et al are "set" at a fixed "virtual distance".

 

(Courtesy of my opthalmologist, who is an amateur photographer and Leica user).

[jaapv]

The virtual image of the framelines is set to a distance of 2 meters. The easiest way of finding out what works for you is to go to your optician and hold his try-out lenses between your eye and the viewfinder until you find the strength that works best. In a pinch you can use throw-away reading glasses at the petrol station for this

[pnoble]

Thanks, Jaap!

 

I assume the rangefinder element is at the same virtual distance, yes?

 

And if I'm using glasses corrected for long distance, with the effective distance of the frame lines and rangefinder element at 2m, and the image itself at actual distance (?) this should work? What effect does the -O.5 diopter eyepiece have on otherwise corrected long distance?

[jaapv]

There is discussion about that. I think it is at the same distance as the framelines, Lars argues convincingly that it is at the same distance as your viewfinder image. Use the try-out method. For me, with reading glasses at + 2.25, the best diopter is + 1.0.

[lars_bergquist]

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Earl is right. The strength of the eyepiece (minus .5 diopters) means that the frame lines and the LEDs (which are on the same moving mask) are seen at a virtual distance of 1m x .5 = 2m.

 

The actual rangefinder limitation aperture in the mask -- the edges of the rangefinder patch -- is also a hole in the same mask. This means that the edges too are at 2m virtual. But the image in the rangefinder is not a separate image on a matte screen inset in the hole; that is SLR thinking. You see *through* the hole, as when you see your garden through a window.

 

What you actually see through that window is two superimposed images. One is the very same viewfinder image that you see through the large finder window. This is an image seen through an inverted Galilean telescope. "Inverted" means that the thing does not enlarge, but 'minify', in this case .86x. When you look through a telescope, all objects are not at the same optical distance, because you see real objects at different real distances, not a matte screen at a fixed distance. (This BTW is why we normally must focus telescopes. That is not necessary here because of the minification.)

 

The other, moving, superposed image comes through a Keplerian or 'astronomical' telescope; a prism turns the image right side up in this case. This rangefinder telescope too has a minification of .86x (natch) and the same reasoning applies to that. Think binoculars, not matte SLR screens.

 

Here's an aside. The Leica II of 1932 had a rangefinder with unit magnification. So the natural accommodation (that's the technical term) of the shooter's eye took care of focusing, or his specs did. With the Leica III of 1933 the magnification was upped to 1.5x, and now there was a focusing lever around the RF eyepiece. Without it, you would get an unsharp RF image at close distances.

 

As for me, I am badly presbyopic. I would need more plus diopters in a correction lens than Leica will supply. But I use progressive specs and I can use the normal eyepiece with perfect clarity of the finder image. BUT -- when I place the spec lens so that the scenery (AND the rangefinder images) is sharp, then the framelines are not perfectly sharp. They are plenty sharp enough, however. Remember again, this is natural -- you are seeing different objects at different distances.

 

The old man from the Age of Bifocals

[pluton]

While the previous discussions are helpful, I thought I'd point out that the scene image in the RF is an aerial image, and it does have, effectively if not theoretically, an 'apparent distance'. I know this because my vision, with glasses, is corrected for infinity, yet to see the scene image in a Leica finder, with glasses, I need a slight + diopter(+.5 or maybe +1); this indicates plainly that the practical apparent distance of the aerial image is closer than infinity, probably in the neighborhood of 2 to 4 meters effective. Final analysis: As has been stated, try different diopters yourself to decide which is best.

[lars_bergquist]

Pluton -- of course the rangefinder image (and the combined image in the RF patch) has an apparent or virtual distance! The point is however that this distance is not determined once and for all by the position of a matte screen, as in a SLR, but is dependent on the actual distance of the object or objects viewed. And as they do vary, out there, the apparent distance too varies. That is, if the object is close, then the apparent distance is closer than when the object is far away. This is actually the reason why we mostly cannot see both the scenery and the framelines equally sharply -- only when the viewed object is at a distance that would produce a 2m virtual or apparent distance. Am I clear?

 

Second, I find that when stating the magnification of the M9 finder, I inadvertently swapped the figures, writing '.86x' instead of the correct value '.68x'. Me, an old proofreader! But we were always told that no-one can adequately proofread a text he has written himself, because he will 'know' what he had written ...

 

The old man from the Age of the Red Pencil