Re: Leica-Users List Digest V1 #34

[Eric Welch <ewelch@gp.magick.net>]

Gary J Toop wrote:
>emulsion and bounce off the back of the film, whether there is a black
>backing there or the dyes used in 35mm film.  On its way back, some of
>this light will strike and expose some of the silver grains, rendering
>the image slightly less sharp.  The light bouncing off of the backing
>will be refracted, but as it passes through the emulsion, some of it may
> be diffracted.  These effects are small and I don't think that they 

No, light bouncing off the film back is not refracting. Refraction is 
when light passes from one medium to another (from air into glass for 
example) and it changes direction (bends). 

Diffraction is when light scatters becuase it's hitting the edge of 
something. Neither of which is what you're talking about. The reason the 
image is slightly unsharp because of "halation" is that the grains of 
silver are being exposed with non-imaging light. It's creating a halo 
around the grain - thus the reason the backing on film is called 
anti-halation backing. Which is the same function of the black inside 
the paper on 120 film.  

So you have the right concept, here, but the words need some adjusting. 
<g>

-- 
Eric Welch
Grants Pass, OR