Was RE: [Leica] M8 Impressions now Eye stuff

[drodgers at casefarms.com (David Rodgers)]

Richard,

Many thanks for that very good explanation. I know you've posted similar
before. But I appreciate the effort to post again.

DaveR

-----Original Message-----
From: Richard Clompus [mailto:rclompus@mac.com] 
Sent: Thursday, November 16, 2006 11:54 PM
To: Leica Users Group
Subject: Re: Was RE: [Leica] M8 Impressions now Eye stuff

Hoppy, let me try to shed some light on this subject.  :-)

If your dominant eye is -0.50 diopter, that means you have a mild  
amount of nearsightedness (myopia) in that eye.  This will create a  
very mild amount of distance blur in that eye.  If you're other eye  
is fine (no blur), you probably won't notice any blur when using both  
eyes together.  Your brain will automatically use the clearer of the  
two eyes so that your "perception" of the world is clear.  I've seen  
many patients over the years that had quite blurred vision in one eye  
but were not aware of it until you have them cover the clear eye and  
ask them read the eye chart.  Then they seem shocked.  Brains tend to  
adapt well to blur in one eye which helps with survival.  Also keep  
in mind that a little blur looking straight ahead has no impact on  
your peripheral vision.  Peripheral vision is always blurred and is  
essentially there to detect motion - like noticing the saber tooth  
tiger sneaking up on your left side.

Many camera systems are designed so that the user must accommodate  
(or focus) a little bit to see the distance.  Human visual systems  
seem to prefer this minimal focus requirement.  In many cases, just  
putting something close to your eye stimulates your visual system to  
focus a little.  If you are less than 40-45 years old, your eye will  
focus this small amount automatically without you being aware.  The  
camera's optical system presenting infinity at a virtual point 2  
meters in front of the camera.  You eye must focus 0.50 diopter to  
see this point in space clearly.

When you combine the mild nearsightedness (-0.50 diopter) in your  
viewing eye with the -0.50 diopter focussing requirement of the  
viewfinder, you might think you would need a -1.00 diopter correction  
lens.  Well, it's not that simple.  You would be way over-corrected  
with a -1.00 diopter eyepiece lens and although the viewfinder might  
seem clear when first viewing through it - you would not be  
comfortable.  Your eye's focussing system would be working too  
hard.    If you're over 45 years old, you would definitely be  
uncomfortable.  If the rangefinder is blurred when viewing without  
your glasses, I'd recommend you start with a -0.25 diopter (if  
available) or -0.50 diopter corrective eyepiece lens to produce clear  
vision through the camera's rangefinder.  Use the least amount of  
minus lens power to clear the image in the rangefinder.  Many  
eyepiece corrective lenses only come in 0.50 diopter increments.  You  
have probably been selecting lenses that are too strong for you.   
Better to be under corrected than over corrected.  Weaker is  
better.    Fortunately you don't have astigmatism.  That's another 3  
paragraph explanation.  :-)

I know this is a long winded answer but I hope it helps understand  
some of the aspects of vision and photography.

All the best,

Richard