Thanks, Mark! You took the words right out of my mouth! We've noticed the same
thing -- we can polish off the degraded area of the prism, but there's always a
line between the silvered and exposed glass. Only a new (or undamaged) prism
looks
new.
Mark Dapoz wrote:
> On Thu, 19 Jul 2001, O.C wrote:
>
> > The problem described that occurs with the foam and prism is that the foam
> > rots and becomes bound to the prism surface. When stuck on the glass it
> > alters the glass to air interface that is required for the total internal
> > reflection to occur. What you see instead is the foam through the glass.
> > Thats why cleaning the prism of the gunk and restoring the original glass
> > air interface will restore things.
>
> In theory what you describe is true, but I've found in practice that's not
> the case. I have a prism which is about 80 0e-silvered. I can get it
> to work reasonably well if the prism surface is perfectly clean. Unfortunatly
> problems arrise at the sharp edges of the prism (at the top and around the
> eyepiece). At these points you can clearly see a faint line in the
> viewfinder.
> No amount of cleaning will make these lines disappear. I suspect the silver
> backing which is applied to the prism somehow "fixes" this problem. A prism
> with no silver backing is still useable, it's just not as good as one with a
> silver backing.
> -mark
>
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