Re: TECH-3D digest 219

[T3D <vidi@xxxxxxxxxxxxx>]

  I sent this to Photo 3-D by mistake yesterday, it was meant 
for this forum.  I thought I'd bring it back here anyway in 
case there were those who don't keep up with all the miscellanea 
of P3-D.  

  I got a reply from John B. refering me to the LensCombo.xls 
Spreadsheet I had downloaded from the FTP site some time ago,
thanks John.  I had looked at that pretty late in the evening
and couldn't see where the parameters given would concern me.
I must have been half asleep already though because I just 
reviewed it now after John's reply any I now see what's really 
going on there.  I figured I could get the 32.55 combination 
focal length by adding the 500mm lens, but what I didn't notice 
from the LensCombo.xls work was the effect on S'.  But now it 
seems (unless it's just too late again) that there is no 
combination of lenses that will allow me to do what I want.
If I can't change S' which is built into the camera, then it
seems there is no value of F1 or d (with f2=34mm) that will keep 
S' at 34mm!  Am I missing something?  Intuitively I know I can
add a lens to a camera and change my distance to the object and
come up with a focused image at some greater Mag.  Why can't I 
see this here?  Hopefully someone will enlighten me when I read 
your reply tomorrow and I will feel better about myself, but 
for now.... it's not working!

    (here is what I posted to P3-d)

   I am contemplating making a modification to a Nishika 3-D 
Camera, which was given to me, to use as a stereomacro camera. 
I would like anyone interested here to verify some things 
I've measured and calculated.

   Given (from Camera Spec. Sheet): Focal Length = 34mm
Measured f No.: f/15 and f/22 (2.26mm and 1.57mm Dia. apertures)
Measured Lens Spacing: 18.5mm Center-to-Center

   Given the lens to film distance (s) is probably 34mm, I need 
to determine the focal length of a supplimentary lens to add in
front of each camera lens to make an object at 2.5 feet (762mm)
focus again at this "s" of 34mm.  So, if S=762mm and s=34mm, we get
a required focal length of 32.55mm.  To change the 34mm lens to 
an effective focal length of 32.55, I figure I need a 538mm lens
(500mm closest choice) about 10mm away from the 34mm lens and 
this will give me the the s of 34mm that I can't change and allows
me to focus on an object which is about 2.5 feet away.

   Now, given this, I figure I can use three different pairs of 
the Nishika lenses to capture fairly accurate macro images.  
If I use lenses 1 and 3, I get a lens spacing of 37mm which I 
figure is just right for an object at 2.5 feet with infinity 
in the background.  I can use lenses 1 and 2 for objcts as 
close as 18 inches (if the depth of field of the lens at 
f/22 will let me), and use lenses 1 and 4 for an object as 
far as 4 feet away.

   Does this all seem right to you?  Can anyone tell me if I'll
ever see the focus I want at the 18" and 48" distances?  And the 
final question, how do you figure these will look in the half 
frame format of the Nishika?  I'm used to viewing full frame 
from my slidebar 35mm camera(s).  Am I going to be dissapointed
once I go through all this trouble and view the images for the 
first time and say "oh...., well that didn't work".  I've taken
quite a few macro images with my Micro-Nikkor and they really 
look great.  But I think part of the effect is the impact of the 
full frame filling my field of view.  Any thoughts.

Thanks,

John Vala



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