[pure-silver] Re: Neither My Hassy Nor View Camera Have This Problem
- From: Helge Nareid <hn.groups@xxxxxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Fri, 29 Oct 2010 22:31:21 +0100
On 29/10/2010 21:31, Richard Knoppow wrote:
>
> ----- Original Message ----- From: "Helge Nareid"
> <hn.groups@xxxxxxxxxxxx>
> Hi Helge, its been a long time, I'm glad you are still following
> this list.
Hi Richard. Yes, I'm indeed following this list, mostly in lurking mode,
I'm afraid. I don't do any silver-based photography anymore, I'm afraid
- partially due to a lack of access to a darkroom and partially because
I don't have the time to devote to it.
I also don't work with optics any more on a daily basis, so I'm getting
a bit rusty in that field as well.
> I am not sure what was used in the motion picture cameras. I think
> pellicle mirror finders were installed by an after-market company.
> Mirror finders became popular around the 1960s when zoom lenses began
> to be widely used.
Pellicle mirrors make a lot of sense when you have to work with existing
lenses - as I indicated in my previous post, using prism beamsplitters
would require a complete redesign of the entire optical system.
> There were pellicle beam splitters used in some of the one shot
> color cameras made from about the late 1930's. I think these used
> gelatin similar to the material used for filters. I don't know how the
> silvering was done. Vacuum deposition of either silver or aluminum
> would require heat and I am not sure if the gelatin would survive.
> Silvering can be done chemically so that was probably the method.
Interesting. Having had reason to look into the optical properties of
gelatin, I found that it has a surprisingly (to me at least) high index
of refraction, typically around 1.56. That means that any deviation from
flatness will have detrimental effects when placed in the optical path,
and also that any significant thickness can cause problems. Gelatin also
doesn't have much in the way of tensile strength, so keeping it flat can
be another significant problem. Also, when used for filters, it is
normally protected by laquer on both sides in order to protect it from
moisture - all adding up to the thickness of the assembly.
The normal substrate material for modern pellicle mirrors is mylar in
single-digit micrometer thicknesses (as somebody else has already
posted). It has high tensile strength, so a very thin substrate can be
held flat by tension. In those thicknesses, the less desirable optical
qualities, such as birefringence, don't matter all that much. However, I
would still worry about polarisation effects - without looking it up,
the Brewster angle is normally around 55 degrees for most optical
materials, which is bit too close to 45 degrees for my peace of mind.
Which is one reason for my preference for placing the sensor in the
reflected path rather than the transmitted.
> Beam splitter prisms would simply be impractical for these cameras,
> however, they were used in the color separation Technicolor camera.
I know that I have seen diagrams of cine camera lens systems using
prism beamsplitters, but as I recall they were 8mm systems. Since the
glass weight of a cube beamsplitter goes as the cube of the size, I am
not surprised that they aren't common in the professional cine formats.
> One of the Technicolor cameramen I talked to long ago told me that the
> company was extremely secretive about the splitters (and other things)
> so that they were removed from the camera when it was out of service
> and kept by the cameraman. He told me he slept with it under his bed!
They would be very complex optical systems, so I'm not entirely
surprised. I have a vague recollection of having seen a diagram of a
technicolor beamsplitter system, and it was not a trivial piece of
optical engineering.
> The main advantage of the pellicle is that the finder image does
> not flicker and that its easier to set up the camera for certain
> special effects.
To be pedantic, a prism beamsplitter system would have the same
advantage - but as already mentioned it has severe size and weight
penalties, and the optics will have to be specifically designed to
incorporate the extra glass thickness.
- Helge Nareid
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