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[MF3D.FORUM:1126] Re: Sputnik ground glass


  • From: Tom Hubin <thubin@xxxxxxxxx>
  • Subject: [MF3D.FORUM:1126] Re: Sputnik ground glass
  • Date: Mon, 21 Aug 2000 11:57:22 -0400

PABigelow@xxxxxxx wrote:
> 
> Hello,
> 
> Has anyone sucessfully replaced the
> "ground glass" Sputnik viewfinder lens
> with a ground glass panel?  If so, where
> does one find suitable ground glass?
> So far, in my searching, local glass shops have:
> 1) Sandblasted glass (too coarse)
> 2) Acid etched glass (not coarse enough)
> 
> Best regards,
> 
> Paul Bigelow

Hello Paul,

It would be interesting to put numbers on the scattering of the glass.
That would make it easier for you and others to compare scattering
materials. You could use a 1mm HeNe laser beam or a pocket laser pointer
to do this.

The laser or pointer divergence would be very small. Probably a fraction
of a degree. So you could ignore that divergence. Point the laser at a
white wall, screen, paper or whatever. Then place the glass in the light
path. The laser light will be scattered over some area on the screen.
Measure that diameter and the distance from scattering surface to
screen.

Usually the diameter is measured to the 50% power point relative to the
intensity at the center. That gives you the "full viewing angle" as used
for LEDs and other light sources. You would need a light meter to
measure this (50% is 2 stops). As a rough guess, just use a ruler and
your opinion as to the size of the circle of light.

Then give me the distance from the glass to the screen and the diameter
of the circle of light on the screen and I will give you the full
scattering angle in degrees. 

Or you can take half the diameter at the screen, divide by the distance
from glass to screen, take the inverse tangent, then double that. That
will give you the full scattrering angle in degrees. 

Tom Hubin
thubin@xxxxxxxxx