[LargeFormat] 9 1/2" Dagor

[LNphoto]

I didn't have Richard's expertise when I needed a scale, so I came up 
with what may be easier depending on your situation.

I mounted the lens via some gaffer tape to my enlarger. Added some 
diffusion material just above the neg holder.

I don't know if I really needed to focus the enlarger or not, but I did.

Knowing that my lens wide open was 4.5, I simply stopped down the lens 
and used my digital light meter (minolta IIIf) to measure the changes. 
When I got to a half stop loss, I made a mark.  1 stop another mark on 
the masking tape on the shutter.

When I was done I had a T stop scale.. they look like an f stop scale, 
but are more accurate in terms of light transmission. A bit off when 
calculating depth of field, though.  Since I always did DOF by ground 
glass, it wasn't a problem.
On Friday, October 17, 2003, at 04:14  AM, Richard Knoppow wrote:

>
> ----- 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
>
>
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