Yet another M diopter question

[adan]

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The prescription for my glasses (shooting eye - right) has been changed. It is great for general distance viewing, but is blurry when I look through my Leica M eyepiece.

My old Rx is just right for the M viewfinder (focusing patch is clear and sharp).

Old prescription was for sphere -1.75. (no other corrections)

New prescription is for sphere -2.75. (no other corrections or changes)

Assumption - since the difference between the old and new prescriptions is an added -1.0 diopter exactly, a +1.0 Leica screw-in diopter would be the correction I need for M viewing with the new glasses.

Good assumption, or too simplistic?

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[erl]

Your argument makes good sense IMO, but the acid test is, front into a Leica Shop and test drive the variations available. This will give a fool proof decision. Of course, the shops don't always have a set of diopters available.

[jdlaing]

Try going to a shop like Walmart or Walgreens and hold an inexpensive pair of reading glasses in front of of the viewfinder and see if it clears up. They should have a +1.0

[adan]

5 hours ago, erl said:

the acid test is, front into a Leica Shop and test drive the variations available

Ahh, you mean like - hop a jet roundtrip from Denver to the East or West Coast, where there are actual Leica stores?

I've yet to see any store that actually stocks a full range of Leica diopters (at $215 each - $AU330) for demos any more, anywhere within 1000m/1600km of Denver. 😜

[jgeenen]

If I am not totally mistaken, you need a diopter that is close to your prescription, in other words a -2 or a -2.5 diopter.

negative values indicate shortsightedness. With your old prescription you should have been able to see sharp on closer distances - may be up to TV distances. For anything else (eg. driving), glasses should have been a must have. Now, with a -2.75 prescription, blurriness should start already a few feet away. the Leica viewfinder creates an image that looks like 2m away. 
 

jdlaing‘s idea is not bad. Take your Leica to your optician and ask him/her to place one of their testing glasses close to the eyepiece and view through that combo. The value that works best should be the one to order from a Leica store. As written, my guess is -2 to -2.5.

 

[Jeff S]

On 8/25/2025 at 12:26 PM, jgeenen said:

Take your Leica to your optician and ask him/her to place one of their testing glasses close to the eyepiece and view through that combo. 

 

Exactly what I’ve done, and often recommend here.  Optician was happy to offer use of trial diopters. Math is one thing; actual experience is better.  Order the Leica one that suits.

I keep my glasses on to correct for distance and astigmatism (prescription sunglasses for light sensitivity), but aging eyes require the addition of a +.5 diopter to optimize focusing. Math wouldn’t tell me that.