Diopter / Prescription question

[kzphoto]

It seems for the past few months now that I'm squinting much more when using my M camera. I think my vision is getting worse. At 29, this is a sad thought! Regardless, I have my written prescription from my optometrist but I'm unsure which diopter I may need (if I need one!)

My prescription reads as thus:
-0.25 -0.50 x94
-0.50 -0.25 X55

I'm right eye dominant.

As I understand it, the M viewfinder is configured to -0.50. If this is the case, should not my viewfinder should be configured to my eye without diopters? As it is I don't read glasses, but I'm having trouble getting critical focus on my M and I already use a 1.25X mag. Any feedback would be welcome.

(The camera was just serviced as well. Distance focusing works, I can attest to that.)

[whitecat]

I would put this question to the doc. I have done this before and he told me what I needed. And yes, the viewfinder is set to -0.50.

[john_s]

Originally Posted by kzphoto

-0.25 -0.50 x94
-0.50 -0.25 X55

Which of these lines is for the right eye?

You have a bit of astigmatism which can't be corrected for a camera in a simple way. If you're near a Leica dealer maybe you could try a -0.5 diopter. At your age I would have guessed that your eye would have enough accommodation to use the camera without a diopter.

How is focusing without the magnifier?

I assume that your x1.25 mag is not adjustable. I have a Megaperls x1.15 mag which has an infinitely adjustable diopter from something like -2 to +1.5 or maybe more. It's very suitable for my eyes (I'm 63).

When it is said that a camera's viewfinder has a built-in diopter of -0.5 or -1.0 or whatever, it doesn't mean that it's set up for a person who needs -0.5 or -1.0 to see distance: it's set up so that a person who sees perfectly at distance can see the virtual image which looks as though it's a meter or two away through the viewfinder. If you need correction to see distance, theoretically you need a diopter to focus this kind of camera.

[edit] My previous paragraph is misleading. As a later poster pointed out more clearly, the viewfinder system is set so that the user sees the focus image at an effective distance of 0.67m (Bronica), 1m (Nikon) or some other distance, maybe 1.5 or 2 metres (Leica?), depending on the camera. The following quote from a Bronica website expresses it clearly:

"Bronica finders are designed to simulate a 26.3" eye to focusing screen separation. If you normally have blurred vision at this viewing distance without glasses, you may consider replacing the standard diopter eyepiece with one more suitable."

26.3" is 0.67m.
1/0.67 =1.5 which is where the 1.5diopter comes from.

[kzphoto]

I believe the first is my right eye -- that's how the prescriptions are usually written.

As for the rest, I'll make my way to one on Saturday. Give this a try. I've been really curious to see if it makes a difference. I played with a Contax G2 and was able to dial the diopter up and down. Looks like it actually made quite a difference.

Thanks for the info everyone!

[awilder]

As an optometrist, your right eye prescription converts to a spherical equivalent power of -0.50 dipoters, thus the built-in -0.50 diopter power should work okay. I wouldn't expect the uncorrected astigmatism to be much of an issue as it's rather low and getting a special diopter for the eyepiece to correct the astigmatism might be expensive and hard to obtain but I think Leica does make one. Getting prescription spectacles would probably work best if you're having problems with the viewfinder sharpness.

[pagpow]

Originally Posted by awilder

As an optometrist, your right eye prescription converts to a spherical equivalent power of -0.50 dipoters, thus the built-in -0.50 diopter power should work okay. I wouldn't expect the uncorrected astigmatism to be much of an issue as it's rather low and getting a special diopter for the eyepiece to correct the astigmatism might be expensive and hard to obtain but I think Leica does make one.

+1 here. I'm not an optometrist but have similar problems so I've been dealing with diopters a lot, in some cases by trial and error.

1) While I have astigmatism, it is also relatively low and allows correction with simple diopters.

2) I need much more spherical correction than you do, but i am between two integers. My point: Correcting within half a power seems to work just fine.

Not much help, of course, if you continue to experience problems, but thought I'd give you a data point from experience.

[richardl]

Leica did manufacture a screw in eyepiece with astigmatiic correction (the centre of the eye piece rotated freely to position the astigmatic correction). Leica has since stopped producing this. I got my last one from DAG Camera Parts. The next step was to find a shop that would grind the lens, and insert it into the eyepiece, no easy task. Monokel Optik in Berlin did mine, send me a PM if you want further details on them. I have never heard a conclusive story on the -0.5 diopter that is camera native. Whether to factor that correction in or not. I think a call to Leica Germany would probably solve that quickly. In my case I simply had my diopter return my sight to normal, and did not care about what the camera does after that.

[payasam]

I am surprised that you should have trouble over a mere half dioptre. Before cataract surgery I managed to focus SLRs, arguably more difficult that RFDR cameras, while needing rather more correction.

[srtiwari]

To find one's vision inadequate ONLY when using a camera seems unusual, and makes one wonder if it isn't the instrument at fault. Significant refractive error would be evident even otherwise. Also, if astigmatism is in fact the problem (as in my case) soft contact lenses might offer the best solution. And it is better that your right eye is the dominant one, since that is the one that will be corrected for distance.

[Dwig]

Camera VFs, like your Leica's, are setup not for someone with perfect distance vision, but instead, for someone with perfect vision at roughly 2m (6.6ft). This is the -0.5d native VF diopter.

Your eyeglass prescription is for 20ft (~6m). Someone only 29 years old is not likely to need reading glasses. That lack of focusing range doesn't usually become significant until your mid-40s. Still, you could have some problem that requires a different prescription at 2m. My 58 year old eyes have the nasty issue of the astigmatism rotating and changing in strength as I focus closer. This make bifocals almost impossible to grind; they usually only allow for differences in the spherical value with both lenses sharing the same astigmatism correction.

Your prescription is for as much astigmatism correction (second two numbers, the strength followed by the angle) as spherical (first number). You should revist your eye doctor and ask him to refract you at 2m to see if there is any significant difference, particularly in the astigmatism correction. In the end, you may find that an eyepiece attachment that refocuses the VF to infinity (0.0d) so that the distance correction in you glasses works properly.