[LargeFormat] lens formulas for G claron

[Clive Warren]

At 10:25 am -0800 22/1/02, Les Newcomer wrote:
>clive,
>  Wile I'm not suggesting you change your methods that work, this formula is
>only accruate when the iris is infront of the lens.  Since the front element
>is refracting the light--converging it. The amount of light stopped by an
>iris in front of the lens and an iris in back of the lens will be different.
>
>How much error depends on the lens.  When I was working on an fstop scale
>for my casket set, I set up a point source light (old B&L microscope
>illuminator) one focal lenght away from the lens (used a cardboard box with
>the lens mounted on a shipping tube, so I could cut it down as I went from
>focal lenght to focal length.
>
>I then used a piece of onion skin laid over the front of lens. This allowed
>me to measure the "effective aperture" My experiences with the casket set
>was depending on the focal lenght the effective aperture varied from the
>measured diameter from 10% to 1/3 of a stop.  Still within tolerances for a
>boler shutter.
>
>If you have a light meter and an enlarger, you can measure the light at wide
>open and know that's f9, then mark the scale for every stop below that. 2/3
>down is f11, or you could continue in whole stops with 12.5 18 25 36 and be
>the only photographer on the continent with a G claron in continental stops!
>
>This will give you T-stops and takes into consideration any loss of light in
>the lens. But DOF charts should be made using the previous method as it's
>the position of the iris that controls depth of field.
snip

Les,

Thank you for that excellent explanation.

Have come across another twist for measuring the effective diameter 
of the diaphragm that is similar to your method. Focus the lens at 
infinity, use a sheet of opaque paper instead of the focusing screen. 
Make a pinhole in the paper opposite the centre of the lens. Place a 
piece of printing paper emulsion side towards the lens with a bright 
light behind the pinhole, long enough to produce a black disc on the 
paper after development. The diameter of the exposed disc is the 
effective diameter of the stop! Starting wide open that would give 
the reference point for f9.

Now have a whole range of methods to use when the G-Claron arrives  - 
should probably check out the other lens that I calibrated using the 
more simplistic method mentioned earlier, however this time using 
these more accurate approaches.....

Thanks again.

Cheers,
        Clive