[MF3D.FORUM:1139] Re: Intro, Deviation and Viewers for a 43mm taking lens

["Michael K. Davis" <zilch0@xxxxxxxxxxxx>]

George,

Great explanation!

At 12:52 AM 8/23/00 -0400, you wrote:

[snip]

>>   Magnification = (10 * x^-1) + (250mm / viewer FL)  
>>
>>again, where x is the apparent distance to the image.
>
>>OK, I have jumped from the frying pan where I didn't know how to calcuate
>>Magnification to the frying pan where I don't know how to measure the
>>"apparent distance to the image."   Joy!
>
>That depends on how you focus the viewer.  Try increasing the
>distance between the lenses and the slide as far as you can
>go but still see things clearly.  Now you have focused the
>viewer at infinity.  Now, reverse this process.  Reduce
>the distance between the lenses and the slide.  You will
>start accommodating your eyes in order to see a clear image.
>A LARGER CLEAR IMAGE.
>
>When you cannot see a clear image any more then you have
>focused the viewer at your closest accommodation distance.
>This distance depends on your age.  If you are as old as I
>think you are :) then it should be no shorter than 10 inches.
>So you've got your +1 factor.  But my 8 year old son can 
>focus his eyes at the tip of his nose.  For him this factor
>is even larger!  BOY, DOES HE SEE A LARGE IMAGE THROUGH
>THE STEREO VIEWER!!!!
>
>Hope this helps your calculations.
>
>George

That makes SO MUCH SENSE!  I can't believe I missed that.  I just measured
my closest focus (printed text in average indoor lighting) - a whopping
13-inches.  
Yes, I'm 43 and failing!  

So, when I focus the viewer as close as I can I'll be at an apparent
distance of 13 inches.  So simple!  THANKS!

Using the formula that I use:

Magnification = (10 * x^-1) + (250mm / viewer FL)  

I would get a magnification of 3.97 with 78mm lenses or 6.32 with 45mm
lenses.  Hooray!  That will cascade into some other math I'm doing!

Hmmmm... maybe I should use the 10-inch figure to accomodate people with
better eye-sight than I have.  OK, no wonder everyone is using Mag = 1+250/FL.

Thanks again!

Mike