Need guidance on diopters

[Rick]

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This question comes up often and seems to always spark discussion. Jaapv's answer always seems to be the best one. You need to go to your optometrist and have them help you decide which power is correct for you and it needs to involve using trial lenses in front of the eyepiece of the camera to determine the best lens power and you need to look at a distant object.

 

The factors are too multi-factorial to come up with one correct suggestion. Some of these factors include; your age, your accommodative reserve and amplitude, your glasses correction, the amount of astigmatism in your correction, which viewfinder magnification your M has and other factors.

 

The concept of the frame lines and/or the focus patch being set at 2m is stated by Leica somewhere, but I'm not sure that this is related to the fact that the finder has a "power" of -0.50. I understand that the focal length of 0.5 is 2m, but I don't know how this would place the virtual distance of the frame lines at 2m. Maybe the -0.50 is in front of the frame lines in the optical path and has nothing to do with the straight through image at all. I don't think anybody really knows where in the optical path that effect happens (i.e. after the two-way mirror or in front of it or somewhere in the center image path) and, if it has anything to do with the -0.50.

 

Another thing, I believe that if Leica increases the accommodative demand by -0.50 in the whole finder image, they are doing so to compensate for instrument myopia that is induced when our visual system looks into an instrument and believes that the objects it sees inside are closer than they really are and thus tries to focus. We all do this. This extra -0.50 might be used to compensate for this phenomena and it would help to make the image in the finder appear sharper. As we get older it might or might not effect your own personal experience depending on which way you lean, near or far sighted. So, you really need to use trial lens at your optometrist to know for certain.

 

Also, the image size is reduced (by .68) in the M9 and that affects accommodative demand on your visual system. It reduces your need to focus at near. So, if you get it right for distance the near is less critical. Again, you need to trial lens this.

 

Hope this helps.

Edited February 27, 2011 by RickLeica

[lars_bergquist]

Well. The –0.5 dioptries strength of the M eyepiece means that the frame lines and the edge of the rangefinder patch are at a virtual distance of 2m. This happens to be the same as the virtual disance to he finder screen of my old Olympus OM cameras.

 

With the OM, this is the end of the story – the finder image is a flat surface at one distinct distance. Not so with a rangefinder camera. For while the finder info (= the mask with the line cutouts and the LEDs) are at virtual 2m, this is not necessarily the case with the finder image of the subject, out there. There is a discrepancy here, because you see the subject directly, not as an optical image projected on a matte screen. The optics becomes very complicated, but remember that what you have to see with absolute clarity is NOT he frame lines or the patch edges, but the image(s) within the patch. Your focusing depends on how well you make these two images coincide. So if you test, this is what to test for.

 

I have no reserve of accommodation left (I am a distinctly un-accommodating old badger) and I am also badly oversighted. My progressive eyeglasses solve that problem nicely for me. When I test focus, I usually do it at the critical short-to-medium distances.

 

The old man from the Age of the Artillery Rangefinder

[Rick]

Lars, you are confusing me with the OM camera stuff. Can you keep it to just the M rangefinder. I'd like to understand where in the optical path of the rangefinder that the -.5 is introduced. Can you explain to me where in this diagram of the M rangefinder it is placed? Thanks...

 

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[jaapv]

An SLR screen and a RF image are essentially different. An SLR projects an image on a screen, a RF projects a virtual image. I agree with Rick, let's stick to the optical viewfinder/rangefinder, it's complicated enough.

[Rick]

Had to kind of scramble around here to follow your changes to your post Jaapv. I agree with you.

Edited February 27, 2011 by RickLeica
One must follow the changing times...

[jaapv]

See edit:p

[Rick]

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The dentist hand, quicker than the optometrist's eye tooth.

[Viv]

That's nonsense.

 

If your eyeglasses are +1 dpt then you will need a correction lens for the camera's eyepiece of +1 dpt as well. As simple as that.

 

You may be forgetting that the standard viewfinder is minus 0.5 diptres.

 

The Leica website advises the addition of 0.5 dioptres to your prescription.

Edited February 27, 2011 by Viv

[stunsworth]

Viv, as mentioned earlier in the thread, if that's really the case people with 'good' eyesight would need a +.5 dioptre correction.

[Viv]

Viv, as mentioned earlier in the thread, if that's really the case people with 'good' eyesight would need a +.5 dioptre correction.

 

Yes, you have a point.

 

I was relying on Leica's advice.

 

Speaking personally, my distance correction is + 0.5 dioptres. The standard viewfinder gives me a slightly blurred image. I use the +1.0 correction glass from Leica.

[01af]

You may be forgetting that the standard viewfinder is minus 0.5 dioptres.

No, I don't.

 

 

The Leica website advises the addition of 0.5 dioptres to your prescription.

And that's wrong.

 

You may get away with 0.5 dpt above or below your prescription value ... depending on your eye's combination of myopia, hyperopia, and/or presbyopia, and on how accurate your prescription value actually is. Prescription values tend to be meant either for far or for near distances but for using the rangefinder you need good vision at medium distance. It's not correct to blanketly add 0.5 dpt to your prescription value just because the viewfinder's eyepiece has a -0.5 dpt preset. That preset is there for a reason and must not be compensated for.

[hlockwood]

Lars, you are confusing me with the OM camera stuff. Can you keep it to just the M rangefinder. I'd like to understand where in the optical path of the rangefinder that the -.5 is introduced. Can you explain to me where in this diagram of the M rangefinder it is placed? Thanks...

 

[ATTACH]245512[/ATTACH]

 

Thanks for that diagram, RickLeica. It's very helpful in understanding the optical paths forming the real and virtual images. It is interesting to "focus" while alternatively covering the VF window and the RF window

 

Harry

[gpwhite]

:confused:Where do we differ? If they were readily available it might be practical to try them out - but almost all dealers have to order them. So the try-out lenses of the optician are your best bet.

 

Gee, Jaap, I did not mean to be difficult . I just had a hard time imagining how to hold up a test lens from the optometrist against the viewfinder opening so that I could get a really good look through the viewfinder.

 

But my main point was that I have some consternation about why it is necessary to use such different diopters (for the same scene) across the naked finder, 1.25X and 1.4X magnifiers. I think there must be some complicated optical effects that are in play here and are affected by the variable of distance from the focus patch to the point of focus. I think this is RickLeica's point from one of his posts in this thread.

 

Peter

[lars_bergquist]

Lars, you are confusing me with the OM camera stuff. Can you keep it to just the M rangefinder. I'd like to understand where in the optical path of the rangefinder that the -.5 is introduced. Can you explain to me where in this diagram of the M rangefinder it is placed? Thanks...

 

[ATTACH]245512[/ATTACH]

 

The reference to a SLR should not be confusing. The actual optical path between the eye and the screen (via the pentaprism) is so short that you cannot focus on the screen image. Hence the eyepiece, which is in fact a magnifier, a positive lens that seems to move the subject (here, the finder image) out to a larger virtual distance. With most SLR cameras, this virtual distance was 1m, with the OM cameras 2m. So all the magnification/dioptries are in the eyepiece itself.

 

With an M, it's more complicated. The total magnification/dioptries are the sum total of the optical stuff between the eye and the frame mask ("frame assembly"). The pertinent parts are the eyepiece itself, and the non-labeled lens assembly between the mask and the beamsplitter prism (with the "two way mirror"). When you add a correction lens – a more correct term than "a diopter" which is either a measuring unit or a gizmo with a hole in it, used e.g. as a riflesight – this is added to the total.

 

I hope this answers your question.

 

The squinty old man

[Sayantan Dutta]

So if one requires correction of +1.5, he should order the +2.0 or +1.5 diopter for M10 ?

[newtoleica]

2 hours ago, Sayantan Dutta said:

So if one requires correction of +1.5, he should order the +2.0 or +1.5 diopter for M10 ?

I really wouldn't order anything without trying one first.... it needs to be comfortable and accurate or using an M will be a frustrating experience. if there isn't a Leica store nearby you can trial standard correction lenses (for presbyopia) at an opticians or try the cheap ones you get in a pharmacy.

[dpitt]

4 hours ago, Sayantan Dutta said:

So if one requires correction of +1.5, he should order the +2.0 or +1.5 diopter for M10 ?

There is one simple test that always seems to work for me. Place an object with fine details at 2 m, then look at it directly (without camera) with your glasses. You need to see it 100% sharp. if not, these glasses will not work well for focusing your M. For example, my +2.0 reading glasses could not be used to see at 2m. They were intended for use at 30-60cm for reading books. These glasses were too strong for the RF. I ended up with +1.0 working best with the RF.

Even if your test at 2m works, it still is best if you can try first before you buy.

Edited December 7, 2023 by dpitt

[Pyrogallol]

This thread is 12+ years old, until revived now. I have just added a +2 dioptre lens for my M cameras and it has made a noticeable difference to focussing. Pity Leitz stopped providing a variable eyesight adjuster when they changed from screw to bayonet cameras.

I have added the same for my Nikkormat and Fuji X-Pro 1, which both use the same 19mm size lenses.

Edited December 7, 2023 by Pyrogallol

[jaapv]

I am not sure that the adjuster on rangefinder Barnacks is primarily meant for eyesight correction. I think it is meant as a distance compensation  

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Hallo,

Look here: Leica M

[bideford]

Just to qualify my experience.  In my right eye I am -2.  With a -2 diopter all is good without any compensation for any rangefinder calibration.  As it is when wearing glasses without the diopter.

Given that I am only -2 and the suggested Leica correction is 0.5 I would expect to notice any difference more than anyone with a greater eyesight correction.

Sometimes I suspect we overthink things.

James