Need Advice: 480mm cells in a 240mm shutter

[Richard Raymond]

Folks,
I bought a Rodenstock 480mm f/9 Ronar in a Copal 3 shutter. The shutter slow speeds drag and I have sent the shutter off to Grimes for a CLA. While the shutter is away I would like to use the 480mm cells in a shutter that is marked for a 240mm lens. I will swap out the cells when I want to change focal length. Anyway, is there a simple offset that I can use on the shutter aperture setting that will get me close to an appropriate setting? For example, can I use the f/8 setting as an f/16 setting with the 480mm cells? (I will be using Illford HP4 plus film.)
Thanks for any advice.
Ric

[E. von Hoegh]

What would the film have to do with it??

Yes, f5.6 will equal f11 and so on.

[Richard Raymond]

Thanks for the reply. I mentioned the film only because it has a little wider exposure latitude than if I were shooting some E-6 work and therefore the exact exposure is not quite as critical.
Thanks again for your help.
Ric

[Dan Fromm]

What would the film have to do with it??

Yes, f5.6 will equal f11 and so on.

Unfortunately not guaranteed.

Richard, put the 480 cells in the shutter, open the aperture wide, stop down until the diaphragm blades are just visible from the front, stop down until they vanish. Where they vanish is f/9. Count down from that point; 1/2 marked stop down is f/11, and so on.

This will work is the shutter's diaphragm will open to f/9 for a 480 (entrance pupil's diameter has to be at least 480/9 = 53.3 mm). If the doesn't open wide enough, then measure (crudely is good enough, hold the lens at arm's length, put a ruler against the front cell and there you are) the entrance pupil's diameter with the diaphragm just vanished and there will be the maximum aperture (smaller than f/9). From there, one marked stop down is one stop down, ...

Why guess when you can measure?

[E. von Hoegh]

Thanks for the reply. I mentioned the film only because it has a little wider exposure latitude than if I were shooting some E-6 work and therefore the exact exposure is not quite as critical.
Thanks again for your help.
Ric

OOps. Didn't occur to me, I was looking for a more complex reason...

[E. von Hoegh]

Unfortunately not guaranteed.

Richard, put the 480 cells in the shutter, open the aperture wide, stop down until the diaphragm blades are just visible from the front, stop down until they vanish. Where they vanish is f/9. Count down from that point; 1/2 marked stop down is f/11, and so on.

This will work is the shutter's diaphragm will open to f/9 for a 480 (entrance pupil's diameter has to be at least 480/9 = 53.3 mm). If the doesn't open wide enough, then measure (crudely is good enough, hold the lens at arm's length, put a ruler against the front cell and there you are) the entrance pupil's diameter with the diaphragm just vanished and there will be the maximum aperture (smaller than f/9). From there, one marked stop down is one stop down, ...

Why guess when you can measure?

Dan's right. I forgot about pupillary magnification.

[Richard Raymond]

Thanks to you both. I appreciate your willingness to help.
Best regards,
Ric

[Cor]

Richard,

this how I would approach it.

Set your camera up, with the 240mm lens mounted.

Use a evenly lit surface as your target (ie a white wall), focus on infinity.

Take your spotmeter (you have one, right?) if possible mount it on a tripod, directly touching the ground glass. Wrap it up with a dark cloth, so no extra light can hit the ground glass and or the meter.

Take readings (in EV) at the different F stops of the 240 mm lens/shutter (to be precise, twice or three times at each F stop)

Mount the 480mm cells, and move the aperture and take measurements again.
It should than be obvious what the F stop scale of the 480 will be in relation to the values of the 240mm F stop scale.

good luck,

best,

Cor