How to determine the diopter rating of a eyepiece attachment?

[Steven Lee]

I have several Leica viewfinder correction attachments like this. They're expensive and I don't use them. However, I have no idea what their rating is. Most likely somewhere between -0.5 and -1.5 but I can't be sure. They are unmarked. Is there a way to determine their rating? Thanks.

 

[Donald Qualls]

Simple method: use the detached diopter to focus an image of the sun, and measure the focal length. Diopter strength is 1/focal length (in meters). So if you measure, say, 250 mm focal length, that's +4 diopter.

For negative lenses, combine the unknown with a known value positive -- measure and calculate as above, then subtract the known positive strength. If you used the +4 as above, but got a combined focal length now of 1 meter, +1 diopter - 4 diopter gives a -3 diopter for the unknown.

 

[Steven Lee]

@Donald Qualls Thank you!

 

[Donald Qualls]

I learned diopter arithmetic when doing macro closeups with a Kodak Pony 135, ca. 1972. It's useful to know.

 

[miha]

Simple method: use the detached diopter to focus an image of the sun, and measure the focal length. Diopter strength is 1/focal length (in meters). So if you measure, say, 250 mm focal length, that's +4 diopter.

For negative lenses, combine the unknown with a known value positive -- measure and calculate as above, then subtract the known positive strength. If you used the +4 as above, but got a combined focal length now of 1 meter, +1 diopter - 4 diopter gives a -3 diopter for the unknown.

This is by far the best knowledge-sharing I've seen on Photrio in ages. Thank you, Donald!

 

[Donald Qualls]

Thanks!

 

[MattKing]

One caution if you intend to sell the item.
In many cases, the diopter lenses are marked with, and sold as, the diopter that results when you combine the lens with the optics in the camera.
So if the camera optics present with +0.5 diopter, an accessory diopter lens of 1.0 diopter strength won't be sold and marketed as a 1.0 diopter lens, it will be sold and marketed as the 1.5 diopter (0.5 + 1.0) accessory.

 

[xkaes]

While Donald is "right on" as to how to do it, with a viewfinder accessory lens it's pretty difficult for a one-man procedure.

It's hard enough to hold a close-up lens and measure the focal distance, but a viewfinder attachment is much smaller and the focal length is going to be long. It's easier if you have a setup with bright light coming in through a window into a dark room -- where you can place the accessory (fixed in place some how) close to the window. Then place a sheet of white board/paper at the expected distance -- around 3 feet -- and move it forward and back until the image is sharp. Then hold it in place and measure it exactly.

Maybe this is easier with THREE people!!!!

 

[Steven Lee]

One caution if you intend to sell the item.
In many cases, the diopter lenses are marked with, and sold as, the diopter that results when you combine the lens with the optics in the camera.
So if the camera optics present with +0.5 diopter, an accessory diopter lens of 1.0 diopter strength won't be sold and marketed as a 1.0 diopter lens, it will be sold and marketed as the 1.5 diopter (0.5 + 1.0) accessory.

Yes, that is the point. I would assume that a buyer knows this, especially because this information is always listed when they're sold, maybe it's a part of Leica sales collateral. Their stock finders are -0.5.

 

[reddesert]

In my experience, which does not include Leicas, eyepiece diopters are not marked with the diopter value in isolation that Donald describes. They are marked with the diopter value that results when combined with the optics of the viewfinder.

What Donald describes is the correct way to measure the diopter value of for example a closeup lens or a pair of reading glasses.

A typical viewfinder in isolation produces an image at an apparent distance of about 1 meter. You can add a lens to this to adjust for vision, and the camera makers will typically label the lens as the correct output diopter (so if it produces an apparent image at a distance of 0.5 meters, good for a person who wears +2 glasses, they'll label it +2), but the actual focal length of the additional lens is different from 0.5m.

I measured a few eyepiece lenses for Bronicas and they had focal lengths in the 8-12 cm range, which would make them around +10 diopters, but of course they weren't really for +10 vision, they were labeled in the -1 to +2 range. So I think this is harder than it appears. You may need a diopter with known labeling and focal length for comparison.

Edit to add: This may be somewhat different for RFs and SLRs, since you look through an RF while you look at an SLR screen. But they both include optics to project an image at some finite distance, so I think it's still not guaranteed that the "+1 diopter" measures 1000mm focal length.

 

[MattKing]

Yes, that is the point. I would assume that a buyer knows this,

My experience in sales tells me not to assume buyers know anything in particular. After all, they may have learned everything they know from the internet!
I'd provide both the actual power of the lens, and the resulting diopter if used on a Leica.

 

[Steven Lee]

@MattKing good point!

 

[cowanw]

I expect you might find a friendly optician and have him try various lenses in the big glasses they use, (better one, better two) looking not at the wall chart but through the viewfinder at the wall chart. Add up the various diopters including your own and you should have the system diopter. You would need to know the internal cameras diopter, likely the one metre above and you can figure the eyepiece.

 

[Mr Bill]

They are unmarked. Is there a way to determine their rating?

Steven, do you have the Leica that they will fit?

 

[Steven Lee]

@Mr Bill Yes, they should fit all post-Barnack film Ms, including the M7 and M6 that I have. But I have switched to contact lenses instead of diopter-corrected attachments.

 

[skahde]

That optician should have the same device my optician has: Insert any lens, read value from display, done.

 

[Mr Bill]

@Mr Bill Yes, they should fit all post-Barnack film Ms, including the M7 and M6 that I have. But I have switched to contact lenses instead of diopter-corrected attachments.

Hi, I can't say for sure how they rate the diopter attachments, nor have I ever tried digging deeply into the subject. But here's what I'd probably try... assuming that you have an SLR camera available:
- put the slr lens right up to the Leica eyepiece and focus the SLR on the "image" the same way as you would with your eye. (You might try focusing on the bright frame image instead.) Once the SLR is thusly focused do not disturb its focus setting.

- Next, find out out, in the real world, what distance the SLR is focused at (move the SLR to a distance where it is focused on some object, then measure distance from front of lens to that object). This distance is the same as that of the "virtual image" as seen by your eye when looking into the Leica viewfinder.

- If you repeat this process with each of your eyepiece diopters you can measure how the "virtual image" distance is affected by each of your attachments.

As I said, I don't know exactly how they relate this to the eyepiece attachment but this virtual-image distance is actually the important thing going on. And it does NOT necessarily correlate to the actual focal length of the eyepiece lens attachment. (This virtual-image distance will change if the eyepiece can be moved a little closer or farther.)

FWIW I've used a similar method looking at SLR camera "virtual distances" just for fun; in this case the focus screen itself (or markings on it) can be used as the focus point. I'm presuming that the rangefinder viewfinder will work the same way but I've never actually tried it. (Ps up till then I sorta presumed that the SLR cameras DID use a nominal 1 meter distance to the virtual image; various literature suggests this. But after measuring it on a few cameras... well, I don't presume this anymore.)

The significant discrepancies going on in this thread are because some people are using the term "diopter" as a way to describe the actual focal length of a lens. (This allows closeup lens calculations, etc.) But there is apparently another common usage which is only related to the apparent distance the human eye sees through an eyepiece. Again, I only have a slight familiarity with this usage.

 

[Steven Lee]

@Mr Bill and @Donald Qualls and others: thank you! I figured this out, they are both -1.5. I managed to find the invoices for them, so the need for optical experiments (unfortunately) disappeared.