[LargeFormat] 9 1/2" Dagor

[Richard Knoppow]

----- Original Message ----- 
From: "philip lambert" <philip.lambert@ntlworld.com>
To: <largeformat@f32.net>
Sent: Thursday, October 16, 2003 11:44 PM
Subject: Re: [LargeFormat] 9 1/2" Dagor


> Presumably when your lens is wide open the iris has an
opening of 1.4
> inches; this is 9.5 divided by 6.8.  On this basis f16
would be 0.6 inches
> diameter. You can work out the size for f22 and f32.
Apply a cardboard
> strip to the shutter alongside the iris lever and mark the
scale position at
> these various apertures. Maybe you could hen varnish the
surface of the card
> with clear lacquer.  If you can work with a strip of alloy
and a cheap
> engraver tool it might look less home-made.  I used a Dymo
tape on a
> government lens/shutter I bought 30 years ago that had
left its aperture
> scale behind on the instrument it had been fitted to.
Anybody who is into
> graphic art could draw a scale on a computer and print it
(not me then).
> Experiment with exposure after this.  You might care to
get the shutter
> speeds calibrated. I use a Calumet device and often find
shutters running
> half a stop slow (except Copals).  The point I am making
is a slight
> inaccuracy in the aperture calibration is likely to be
insignificant
> compared to speed inaccuracy.
> Incidentally as I said a few times before, a lot of sleepy
shutters can be
> improved by taking the glasses out of the shutter and
standing the shutter
> in a closed jar on a layer of pennies into which some
lighter petrol has
> been poured. Keep the shutter above the petrol line. One
of my small
> SynchroCompur shutters had lost all its slow speeds and
they were restored
> by this means.  Philip
> ----- Original Message ----- 
> From: "Larry J Schaaf" <ljs@verizon.net>
> To: <largeformat@f32.net>
> Sent: Friday, October 17, 2003 12:56 AM
> Subject: [LargeFormat] 9 1/2" Dagor
>
  This is not quite accurate. The lens in front of the
aperture changes its effective size. The image of the stop
as seen from the front is called the entrance pupil. It is
the size of the entrance pupil which determines the amount
of light which gets through the lens.
  Fortunately, the entrance pupil is easy to measure. In
fact, its sometimes easier to measure it than the physical
diameter of the iris.
  To measure the size of the entrance pupil you need:
A large card (maybe 8x10) with a white surface, a piece of
mounting board is perfect. Make a small hole at the center
of the card. It does not have to be a pin hole but should be
small.
  A light to place behind the hole (flashlight will do).
A flat mirror which will cover the front of the lens (a
shaving mirror will do).
A translcent screen large enough to cover the front of the
lens (bond paper or drafting velum will do).
  The lens can be in a view camera.
  First, focus the lens exactly at infinity. To do this we
will autocollimate the lens.
Place the light behind the card. Put the flat mirror over
the front of the lens (with the shutter open please).
  The light from the hole will be reflected back to the
card. Adjust the lens so that the spot is close to the hole.
Adjust the lens for the sharpest image of the hole. The lens
is now set _exactly_ at infinity.
  Now to measure the size of the entrance pupil leave the
card and lamp where they are and place the translucent
screen over the lens. The diameter of the circle of light is
the size of the entrance pupil. Because the light coming
from the lens is collimated the distance between the screen
and lens is not critical. If the hole in the card is not too
large the edges of the circle of light will be reasonably
sharp.
  To calibrate the diaphragm calculate the required diameter
of the stop for the desired f/stops by deviding them into
the focal length. Set the circle of light to that size and
mark the indicator. That's all there is to it.
  Keep in mind that the marked stops on most lenses are
rounded off. For instance, if one begins a series at f/1 and
goes up by half stops we will find that f/5.6 is actually
5.657, f/11 is f/11.31, etc. The full stops will be even
multiples, i.e. f/2, f/4, f/8, not a big deal.
  Once you have the stop scale marked you can scratch the
marks in or have it engraved if you want to get fancy.
  This method is a good way to check the calibration of
lenses with stop scales. Some shutters, Ilex is an example,
can have the stop indicator bent a little, enough to throw
off the calibrations.
  I would not get too fussy because the lens will have an
unknown amount of transmission loss, probably larger than
small scale errors. Making the scale is easy, finding the
tiny screws to mount it may be more difficult.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com