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M eyepiece correction lenses
- how to estimate?

Wondering which diopter lens to use to correct the M eyepiece? In 2001, optometrist Patrick Markham noted the following:

I'd like to do my best to set the record straight on this subject as I am a serious Leica user (both M and R) and an optician with a college degree in optics and 21 years of professional experience.
 
[…] Corrective lens corrections for the M6 should be based primarily on the spherical portion of one's spectacle prescription. If your prescription is say a -1.00 sphere (i.e. a simple spherical prescription with no astigmatism) the correct lens insert for your M camera would be a -1.00 D. Easy enough. If your prescription involves a small amount of astigmatism (usually -0.50 D or less as the second number in your prescription) which means your prescription looks something like -1.00 -0.50 X 90, then you would ignore the astigmatism and your insert would still be a -1.00 D.
 
Prescriptions with higher amounts of astigmatism (say -0.75 to -1.25 D) would need to take into account half of the astigmatism in the corrective lens insert - which is known as the prescription's "spherical equivalent". This is done by doctors as follows. Take the sphere portion of the prescription (the first number) and add half of the value of the cylinder or astigmatism portion (the second number) to arrive at the spherical equivalent. For example: -1.00 -1.00 X 90 would yield a spherical equivalent lens insert of a -1.50 D. Any questions about your particular prescription and the best viewpiece lens insert can be e-mailed to me at <markham30@ home.com>.
 
Why should we ignore the inherent negative lens value that's in the M viewfinder? Simple. The M viewfinder system was given a negative lens value in order to accommodate Leitz wide angle lenses because minus lenses make images seem smaller. However, it's important to stress that the optical masters at Leitz have perfected the M6 viewfinder system so that the image appears as a 20/20 image for one's eyesight who needs a minus 0.50 D lens in order to see a perfect 20/20. For example, I need a -0.50 D lens in order to see 20/20. With a -0.50 lens I see fantastic in the distance - but the image size is now smaller for me.
 
Because of this Leitz designed different viewfinder optics systems in order to accommodate it's different lenses. M3's have a less minus viewfinder system with a large image size (0.85 the size of a plano lens) but their viewfinders can only accommodate 50mm, 90mm and 135mm lenses. Hence the birth of the M2 with its .72 image size (i.e. more minus viewfinder system than the M3) to accommodate wider angle lenses like the 35mm.
 
One last point here - if you wear glasses make sure you are wearing the very latest spectacle lens designs in order to take the best pictures you can. Why would one have a top of the line Leica camera and be shooting images through inferior eyeglass optics? New aspheric lens designs with high quality Zeiss or Crizal anti-reflective lens coatings will make a HUGE difference in the way you see - larger visual fields with much sharper images, well defined colors with a drastic reduction in glare, etc. I only mention this because the optical industry has done such a poor job of informing consumers that these breakthroughs are finally here for our use.

Wondering how to correct for astigmatism? Patrick Garner noted the following on the greenspun.com Leica forum in March 2002

[…] There no diopters to correct astigmatism from Leica. My optometrist says that making diopters to correct this common condition would be difficult because the glass itself would have to be uneven & set (or aligned) precisely inside the finder. The technical difficulties may be what deters Leica from trying-- BTW, Hasselblad is identical, so this is not just a Leica problem…

John Collier added the following details:

Leica never supplied eyepiece correction lens for astigmatism because there would be too many different combinations. Leica did, and still does, supply a mount into which you can have your optomtrist put in a custom lens to suit you. The lens element can be rotated when you switch from horizontal to vertical framing. Some people find having to rotate the eyepiece irritating and do not bother with astigmatism correction.

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