Re: Leica-Users List Digest V1 #34

[Gary J Toop <gtoop@uoguelph.ca>]

On Fri, 19 Apr 1996, Eric Welch wrote:

SNIP

> 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>
> 
	I am willing to stand corrected on my terminology.  There was a 
very interesting discussion of this topic on "rec.photo.darkroom" a year 
and a half ago or so and I think that the term "diffraction" was used, 
but I could be mistaken.  Since I haven't been able to dig up my copies 
of the original discussion, I cannot say for sure that "diffraction" was 
indeed the term that was used and, of course, those using it could have 
been mistaken.  I do stand behind the explanation of the mechanics of the 
effect, though.  My apologies if the terminology confused anyone.

Gary Toop