[LargeFormat] lens formulas for G claron

[Les Newcomer]

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.

Les


> The equation I use is:
> 
> diameter of iris opening =  Focal Length/f Number
> 

> Cheers,
> Clive