Converting Polaroid Scale for G-Claron

[G Benaim]

Hi all,

I'm mounting a barrel 240/9 g-claron onto a polaroid shutter, and wanted to know what the conversion is like. I have the 127mm tominon copal shutter, goes from 4.5 to 45, and remember seeing that 5.6 on the tominon scale equals f9 on the claron scale, and you just transform from there, e.g. 5.6 1/2=11, 8 1/2=16, etc. I just wanted to check with others who've done this to make sure, at least as a good starting point. Thanks,

GB

[Dan Fromm]

As one cheapskate to another, here's a trick that works for me:

Put the front cell in the shutter. Lock the shutter open. Open the aperture wide, then close down very slowly while looking through the front of the lens. When the aperture just becomes visible on stopping down/invisible on opening is wide open, i.e., f/9. One marked stop down from there is one stop down, i.e., f/11.

Jim Galli had some misadventures with remounting 210 and 240 G-Clarons in ex-Polaroid Copal Press Shutters that had held Tominons shorter than 127 mm. It seems that those shutters have restricted maximum apertures.

Good luck, have fun,

Dan

[erie patsellis]

Dan,
as an aside, many of the Copal/Polaroid shutters have a stop pin and a bunch of holes on the back, once you determing the max aperture position, there's likely to be a threaded hole really close that you can move the stop to.

erie

[Dan Fromm]

Thanks, Erie. There've been a number of discussions about how to get the wretched things to open wider. Not my problem, and not the OP's either.

[Struan Gray]

I ran my 240 Germinar-W in a shutter from a 127mm Tominon until I bought the 240 Apo-ronar I referred to in another thread today. If you just add two stops to the marked values, you'll be close enough for government work.

127*2 = 254, which is close enough to 240 for the film not to care. If you double the focal length of the glass around the aperture hole, you double the f-number, and get two stops less exposure.

There are subtleties and gotchas to do with the design of the lens, but in this case they were negligable. If you're the sort of person who brackets colour reversal film in 1/10th stop intervals you need to care, but then you wouldn't be piffling about with scavanged shutters anyway.

[G Benaim]

Struan, are you sure it's 2 stops more, or is it 1 stop/twice the exposure?

[Struan Gray]

It's two times the f-number, which is two stops less exposure.

f-number = focal length / hole diameter

If you double the f-number you have halved the relative hole diameter and reduced the relative *area* of the hole by four. Thus the light intensity at the focal plane will be one quarter of what it was, and that's two stops less exposure.

Quick check: is f16 one stop or two stops less than f8? QED.