I am about to get my eyesight tested for intermediate range vision, in the 3-6 feet range, as I notice I can see things less sharply than I used to.

I might as well do the tests at the same distance as the virtual image in the M6 viewfinder is at, since rangefinder focussing is the most critical task that I put my eyes to at this distance.

Questions:

1. Does anyone know, definitively, the distance the viewfinder virtual image is at? The eye doctor needs to know before he tests my eyes.

2. And would I get the same power diopter as my prescription, for the eyepiece, or add/subtract 1.0 for the power of the viewfinder system itself?

-- Mani Sitaraman (bindumani@pacific.net.sg), July 28, 2001

Answers

It's downhill from here on.

-- Bill Mitchell (bmitch@home.com), July 28, 2001.

Mani: When you speak of a viewfinder power of 1.0, I wonder if you mean one diopter, since a power of 1 would be no power at all. I would have guessed that the power of the finder would be equal to its magnification: .91, .85, .72, or .58. I don't expect there's any diopter power to the finder. If there were, folks with normal vision couldn't see clearly through it. I think. Unless someone knows otherwise, I think you should have your eyes corrected for normal distance vision. To see what a closer correction would do, try looking through the finder with your bifocals.

There's a real similar thread going now, BTW, with a title "Diopter Compatibility."

-- Bob Fleischman (RFXMAIL@prodigy.net), July 29, 2001.

In other words, I think the diopter lens you install on the camera should be the same as your distance prescription. I think this question came up once before, though, about whether the finder itself has some power that needs to be taken into account. So maybe there is something to it. I don't know why that would be the case, though. Maybe someone will have the definitive word on this.

-- Bob Fleischman (RFXMAIL@prodigy.net), July 29, 2001.

I belive I canīt answer non of your questions here, what I recomend about your eyes is to give them some rest, eat well, specialy fresh vegetables, and iīm sure they are going to get better.

I belive prescription lenses makes our eyes dependable.

-- r watson (al1231234@hotmail.com), July 29, 2001.

I am 95% certain that Nikon manual cameras (F, F2, F3, FM2, etc), Leica M's, and Hexar RF viewfinders have values of -1 diopter. For example if you need a +1 diopter correction for these cameras and want purchase the makers correction lenses, you order a "0" diopter viewfinder correction eyepiece. If on the other hand, you want to get your option to cut you a diopter correction lens to add to the viewfinder assembly then you ask your optician for +1 diopter lens.

-- sait (akkirman@clear.net.nz), July 29, 2001.

sorry for the spelling and other mistakes in my previous posting, it should read: "I am 95% certain that Nikon manual cameras (F, F2, F3, FM2, etc), Leica M's, and Hexar RF viewfinders have values of -1 diopter. For example if you need a +1 diopter correction for these cameras and want to purchase the makers correction lenses, you order a "0" diopter viewfinder correction eyepiece. If on the other hand, you want to get your optician to cut you a diopter correction lens to add to the viewfinder assembly then you ask your optician for +1 diopter lens. "

Best method is to take the camera with you to the optician and find what sort of diopter adjustment makes you happy. If you want the camera manufaturer's viewfinder correction than you have to take the viewfinder's original diopter measurement into account as I have explained above. On another note, If your perscription glasses are "progressive" that is no correction on the top and getting more correction towards the bottom of the glass frame then it is easy to use the camera + new diopter combination without having to take your glasses off. However , if you getting standard glasses then you might have to take them off to look through your camera + new diopter combo. I hope this all makes sense.

-- sait (akkirman@clear.net.nz), July 29, 2001.

Sait and Mani,

There are a couple of things I didn't quite understand about your posts.

First of all, I don't understand talking about a zero diopter "correction" lens. For one thing, zero diopters represents no lens power at all. The diopter number of a lens is the reciprocal of its focal length in meters. So a +1 diopter has a one meter focal length; a +2 has a FL of 0.5M (because 1 divided by 2 equals one- half. A positive diopter lens has a convex surface and so is thicker in the center than the edge. A negative diopter lens is concave and thus thicker at the edge. Now, imagine that we have a lens made of some flexible material that we can morph to any surface shape. Let's start with a +2 diopter and morph it to say, -2. Notice (in your mind's eye) that as it passes through the transition point from positive to negative, the shape has to pass through flatness on its way from convex to concave. At the instant of flatness, the power is zero diopters. Thus zero means no power at all. It has to be a flat piece of glass. Adding plain glass to your finder won't change anything. Thus, adding zero diopters to your finder or to your eyeglass prescription changes nothing.

My next point is that If Nikon (to take the example given) really does give its finders a -1 diopter power, there must be a reason for it. Since negative lenses make things look smaller, I suppose this might be done to reduce the image size in the finder until you can see it all. Maybe that's how Leica makes the finders for the various (de)magnifications like 0.58, 0.72, etc. Of course, the diopter power would then need to be different for each finder, not -1 across the board. Now let's consider this. When we look through the finder with a personal corrective lens prescribed for our eyes (only), what we want is to see the finder image just the same as another person with perfect vision would see it. So we only want to correct our vision, not change the finder.

Now, if your personal prescription is for, say, -3.25 diopters, and you add a +1 "correction" then what you have really done is to change your own presciption lens from -3.25 to only -2.25. Is this a correction, or an error? Once we put on our glasses or contacts, we see with normal vision. If we add the correction lens to the camera instead, then we see into the camera with normal vision. You wouldn't put a +1 lens on your camera if you had normal vision. The whole business would not even occur to us. For this reason I don't understand adding a +1 diopter once you have added the exact prescribed lens that gives you normal vision.

I'm not saying I'm right. I'm just saying I don't get it. Maybe we could hear from any opticians who read this.

-- Bob Fleischman (RFXMAIL@prodigy.net), July 29, 2001.

WOW! Guys, it's not that complicated. First things first: the "virtual" distance in the M finder is approximately 2m (6ft 8in). Second, the eyepiece of the M6 is a -0.5 which is what reputedly Leica's research told them the "average" user needs to clearly see at that distance. The eyepiece correction lenses screw into the standard eyepiece, they do not replace it. True, the correction lens' magnification adds to the built-in -0.5 but contrary to what has been written in many books, the correction lenses are marked so as to indicate the *final, additive* correction. Thus a -3 correction lens gives a -3 total correction, *not* -3.5 (-3 + -0.5). If your eye doctor says you need a -2 or a +1 or whatever to focus clearly at 2m, then that's the correction lens you want to buy. They're available only in 0.5 diopter increments, so ask your eye doctor in which direction to round-off if you're in-between.

-- Jay (infinitydt@aol.com), July 29, 2001.

For Bob Fleischman: You are correct in your assessment re: adding eyepiece correction lenses if you also wear glasses. Normally it's an either-or situation. But there is one instance where a correction eyepiece can help even when wearing glasses. For a nearsighted person (a negative diopter Rx) who gets to the age where bifocals become necessary to see at close distances as well, a correction eyepiece along with glasses is a big help. Someone with a -4 distance Rx might need a bifocal of +2 (final Rx= -2) to read at 12- 18 inches, but neither that nor the -4 in the upper part of the glasses permits critical sight at the 2m virtual finder distance. So by placing a lesser + correction (perhaps a +0.5 or +1) in the eyepiece and sighting through the distance (upper) part of the glasses, the correct Rx for the 2m distance is achieved. Luckily, the close vision recedes gradually from middle-age, so it takes a while before the distance Rx alone won't work at 2m, plus the rangefinder requires much less sharp vision than a reflex ground glass. All my SLR's have adjustable diopters in the finder; so far the M seems to be fine without any correction although I've been wearing bifocals for a couple years now.

-- Jay (infinitydt@aol.com), July 29, 2001.

OK, so now does anyone know of a strategy for astigmatism, as opposed to near and far-sightedness? My eyes are average for distance, but need a fair amount of correction for astigmatism. I got the idea of hanging a little pendulum-leveled appropriately cylindrical lens to rotate on my eyepiece :-) but before I do that, is there a more common solution, besides wearing specs, or do I have a great idea here that I could sell? :-)

-- Michael Darnton (mdarnton@hotmail.com), July 29, 2001.

I also have astigmatism. There are two solutions. Leica used to make a mount into which you could install a custom-Rx lens made up by your optician. You might search for one of those...and if you can find one, then search for an optician who is willing to make your Rx up in that tiny little size. The second option which is slightly less precise but much more practical, is to ask your eye doctor for what is referred to as the "spherical equivalent" for your Rx, to the nearest half-diopter, and then use that correction lens in the eyepiece...if it is available. In my case the spherical equivalent is one diopter more negative, which is beyond the correction Leica provides. But maybe you'll be luckier.

-- Jay (infinitydt@aol.com), July 29, 2001.

My apologies, in my earlier posting I stated that, Nikon and Leica viewfinders were -1 diopter. That should have read -0.5. It is interesting that I need +1 diopter adjustment for Nikon & Hexar RF but can use Leica comfortably without any corrections.

-- sait (akkirman@clear.net.nz), July 29, 2001.

Something else for eyeglass wearers to consider is the fact that eyepoint can vary with diopter. Nikon says that at the default setting of -1 diopter, the eyepoint of their adjustable N80 finder is 17.5mm. At 0 diopter, however, the eyepoint is 18.5mm, making it quite a bit easier for a glasses wearer to see the corners of the frame at the cost of some minification. Anybody happen to know the eyepoints of the M finders?.....

-- david kelly (dmkedit@aol.com), July 29, 2001.

Thanks, Jay, you have cleared the whole thing up for me. I started to realize I was missing something after reading Bud's posts on the "Diopter Compatibility" thread. Somehow I didn't realize that the main issue was correcting for presbyopia (the inability us older people have to focus close). I should have known it from Mani's question, in the thread title, about the right distance to have one's eyes tested.

Also, I was assuming that the finder image was at infinity. Now that I realize it's around 2 meters, it all makes sense. You would add a diopter correction to achieve focus at that distance, that's all. If using the camera with glasses or contacts, you could just add the 0.5 diopter to the camera. If fitting a prescription lens to the camera, you (the optician, really) would add the half diopter to your normal prescription.

No wonder I can't see to focus anymore, as well as I used to.

By George, I think I've got it!

Thanks again,

-- Bob Fleischman (RFXMAIL@prodigy.net), July 29, 2001.

Sait, Bob, Jay,

Thanks for all the light you have shed on the matter.

2 meters, it is.

I won't be getting the custom lens ground for the astigmatism I have, because I do rotate the camera between vertical and horizontal quite a bit-and glasses are not the solution I am looking for with a .85M6 :-)

Bill, LOL :-D

-- Mani Sitaraman (bindumani@pacific.net.sg), July 30, 2001.