[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 19:58:06 +0100
On 29/10/2010 19:15, Richard Knoppow wrote:
>
> ----- Original Message ----- From: "Gerald Koch" <gerald.koch@xxxxxxx>
> To: <pure-silver@xxxxxxxxxxxxx>
> Sent: Friday, October 29, 2010 10:34 AM
> Subject: [pure-silver] Re: Neither My Hassy Nor View Camera Have This
> Problem
>
>
> Many years ago Canon made a camera called the Pellix which was a SLR
> with an
> immovable mirror. Since the mirror took up 30% of the light entering
> the lense
> the aperture sizes were larger to compensate. I don't know if they
> refered to
> these augmented stops as T stops or not.
>
> Jerry
>
> There were motion picture cameras with the same arrangement, i.e.,
> a pellicle mirror. I don't know how those were arranged for exposure
> either. Presumably the loss through the mirror would be constant so
> that a single correction factor would do for all lenses.
I never actually tried a Canon Pellix, but it was approximately
contemporary with the Canon FT, which had (stop down) though the lens
metering, and looked quite similar, so I assume it had through the lens
metering as well.
I believe the semi-transparency was achieved through partial silvering,
which would make the reflective surface relatively insensitive to
variations in colour, polarisation and incidence angle - in other words
it should work in a similar manner regardless of focal length.
However, my main worry would be the substrate. It has to be extremely
thin in order to avoid any optical effects, placing a plane-parallel
transparent medium at 45 degrees in a converging wavefront opens up a
whole pandora's box of possible aberrations if it is more than a few
wavelengths thick. A pellicle mirror is thus normally made from a very
thin foil kept under high tension to keep it flat. That makes it _very_
susceptible to physical damage. Secondly, I would also have some concern
about polarisation effects - not from the mirror surface, but from the
underlying substrate.
Those are some of the reasons why optical engineers tend to prefer prism
beamsplitters whenever possible, since the paths through glass can be
equalized for both beams, and the optical axis can be kept normal to the
air-glass interfaces. They are also much more mechanically stable - but
there is a size and weight penalty. Also, the lenses need to be
specially designed to include the glass thickness of the prism beamsplitter.
If I were to design a camera from scratch using a pellicle mirror, I
would prefer to place the sensor in the reflected path rather than the
transmitted - in that way I would have to worry less about the optical
properties of the substrate. Unfortunately, this won't fit many existing
camera designs.
- Helge Nareid
=============================================================================================================
To unsubscribe from this list, go to www.freelists.org and logon to your
account (the same e-mail address and password you set-up when you subscribed,)
and unsubscribe from there.
Other related posts:
