Re: [Leica] M6 correction lenses

[Patrick Markham <markham30@home.com>]

on 8/14/01 2:26 AM, Frank Filippone at red735i@earthlink.net wrote:

> OK..... Here goes my ignorance of eyeball optics......
> 
> My normal right eye long distance prescription is 1 diopters.  Is that MINUS
> or PLUS ?
> Do you add or subtract -0.5 to that number to get the correct lens for the
> camera to come out right?
> 
> Do we have an optician and or opthamologists better yet, 2 opticians etc. in
> the house that can double check the results of this statement to add or
> subtract ?
> 

<Snip>

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.

Herb Kanner is on the right track with his response. 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 is Herb Kanner right that we should 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.

I hope this information will help.

Patrick Markham