[pure-silver] Re: correction for bellows extension
- From: Jean-David Beyer <jeandavid8@xxxxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Wed, 04 Mar 2015 21:24:35 -0500
On 03/04/2015 09:08 PM, `Richard Knoppow wrote:
>
[snip]
> Well, we can look at it this way, we have two quantities that are
> square law, one is the amount of light changing as to the inverse of the
> square of the distance and the other is the amount of light varying
> directly as to the square of the radius of the aperture. So, as the
> distance gets greater the aperture must also become greater both by a
> square of a quantity. Since the f/stop is proportional to the amount of
> light transmitted there is a direct relationship between the distance of
> the lens from the film and the required diameter of the aperture. As one
> becomes larger the other must also. Note that f/stops are the ratio of
> the aperture diameter to the focal length so has an inverse relationship
> to the amount of light transmitted. To be trivial f/16 transmitts twice
> the light that f/22 does. There have been attempts in the past to
> calibrate the aperture in terms of ratio of light transmission but none
> has been successful. Note that "T" stops, which were popular for
> motion picture lenses around the 1950s was not such a system, it was
> just a correction for the light loss through the lens but still used the
> f/stop arrangement. I don't remember the history exactly but think Zeiss
> had a numerical system of this sort where the stops with higher numbers
> transmitted more light. Its been too long since I thought about this and
> am not coming up with all the data I once had at my fingertips.
>
My Wisner convertible Plasmat Set has the aperture dial engraved in
millimeters because there are 11 different focal lengths you can get by
using one or two of the cells. There are 5 cells.
2 are 250mm f/13
1 is 350mm f/13
1 is 400mm f/13
1 is 450mm f/13.
If you use two at a time, the aperture is greater.
2 250mm come out to f/9 with 152mm focal length.
the 400 and 450 together come out to f/9.6 with 258mm focal length.
Anyhow, there is no way Ron Wisner could have engraved all that on his
modified Copal #1 shutter.
--
.~. Jean-David Beyer Registered Linux User 85642.
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