From: Clay Harmon (wcharmon@wt.net)
Date: 08/23/02-05:38:14 AM Z
John:
Uh, I can actually speak from experience here, instead of theorizing: I own
a 14x17 camera with a p-95 plexi ground 'glass'. I was convinced by the
maker that it was a lighter, safer and more robust alternative. I believe
all that to be true. What is not discussed is that as a focusing screen, it
bites.
The plastic has such a fine etched surface that it creates the mother of all
hot spots. Image composition is excruciatingly difficult, as it is
impossible to see the whole image from a single point of view. With my
glasses on the end of my nose, I can look at any given 3" circle on the
focusing screen with my nose pressed against it. I have come to the
conclusion that the only solution is to either purchase an enormous fresnel
to slap on the thing, or change it out for a traditional ground glass.
Another subsidiary defect is that it will flex under the pressure of a loupe
- which could potentially cause focusing errors. This is avoidable, but a
defect nonetheless in my mind.
Of course, you can't put your knee through it, no matter how much you'd like
to do so after wrestling with the damn thing.
Clay
on 8/23/02 3:12 AM, John Cremati at johnjohnc@core.com wrote:
>>
>>> N1861@aol.com <N1861@aol.com> wrote:
>>>
>>> Has anyone made a ground glass from acrylic or polycarbonate sheet,
>>> as a
>>> lighter weight alternative to glass?
>>>
> John Cremati wrote:
>
> My concern would be the warpage of the polycarbonate sheet on a larger view
> camera as well as cleaning.....I have etched decorative designs into
> Plexiglas on many occasions as commercial artwork.. I have noticed several
> things about it.. I would think the huge advantage is that is very light
> weight , resists breakage, and my guess would be that once etched it
> should produce a great image as the etching seems to be extremely fine
> grained ...
> 1) After removing the protective cover wrapping the plastic seems to have
> a static electrical charge that attracts dust. This is very obvious after
> sandblasting, during which electrical fields may even become visible on the
> Plexiglas.. I usually have to flood wash the plastic when thru etching and
> this seems to neutralize the static electricity...
> 2) the plastic will warp or expand dramatically with temperature
> change..It may be wise to mount in a floating frame...
> 3) Plexiglas may become yellow with age.. a better grade of polycarbonate
> may be in order and should be researched..
> 4) The Plastic scratches very easily ...Special cleaners are used to
> minimize this as well a keep the plastic from yellowing.. These cleaners may
> get into the pores of the etched surface and may cause streaking, staining
> or a blotting appearance...This happens so much that on my decorative work
> I always use a separate plastic cover plate over the etched surface to
> protect it sandwiching the engraving......
> 5)During the sandblast process the plastic requires several passes as it
> does not sandblast easily .. This may be the reason for such a fine grained
> appearance when finished..The plastic probably resists the sand blast action
> and only allows for minimal penetration into the plastic surface.......
> Happy Trails,
> John Cremati
>
>
>
>>
>> No, but I don't see any reason why not. Most interchangeable focusing
> screens
>> for 35mm & MF SLRs are made of some sort of plastic.
>>
>> Wayne Harridge
>
>>
>
>
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