Copy film ISO 4

hey shooting some copy at night since its tunsten rated but can't find any chart that goes past ISO 12

Need to get down to ISO 4

2 options: open your aperture 1.5 stops wider than your chart suggests; i.e. read for f8 shoot at f4.5 (the suggested time for ISO 12). Or dive into math. (admittedly not my field!) - I suppose multiplying the suggested exposure times by 2.5 should get you somewhere, but it is probably not "exact". <-FTR: At night "nice" things like reciprocity failure and similar are ready to spit into your soup and no exposure guidance table can replace metering &/ experience (AKA trial & error AKA bracketing). Math for exposure is never really exact. - film ratings come in 1/3 f-stops, while a lot of lenses click in half f-stops and conventional shutters only in entire f-stops...

Best of luck and bring a Kindle or similar to pass your endless exposure times!

(If somebody good at math jumps in and corrects me; I'm curious & grateful!)

The ISO sequence in 1/3 f-stop increments is:

3 - 4 - 5 - 6 - 8 - 10 - 12 - 16 etc.

Thus the difference between 12 ISO and 4 ISO is 1 2/3 f-stops. You can adjust the aperture. If your light meter or table give the aperture for ISO 12, then if ISO 4 material is used, open up 1 2/3 f-stop. An alterative would be to slow the shutter speed to compensate. The multiplying factor for 1 2/3 f-stops is 3.16. To slow the shutter speed by this factor we find the receptacle = 1/3.16 = 0.32. To use, multiply the shutter speed given for ISO 12 by the factor. Suppose the light meter or table gives 1/100 of a second for ISO 12. Now the shutter speed adjustment is 1/100 X 1/ 0.32 = 1/32 of a second (round to 1/30 of a second).

If they're night shots, the chances are you'll be using a long exposure. So just multiply the shutter speed or time given at 12 ISO by 3.

I.e. if your meter says 1 second, give 3 seconds. If it says 10 seconds @ ISO 12, then give 30 seconds for ISO 4.

I don't see why this needs to be at all complicated.

"I suppose multiplying the suggestedexposure times by 2.5 should get you somewhere, but it is probably not "exact"."

- The arithmetic ISO scale is perfectly linear, therefore ISO 4 needs exactly 3 times more exposure than ISO 12.

Of course the copy film might have peculiar reciprocity characteristics, but that's an entirely other can of worms to open.

. . . just multiply the shutter speed or time given at 12 ISO by 3.

I concur.

Even if it is a short shutter speed, still multiply it by 3: as per the example of 1/100s, simply multiply that by three you get 3/100s, which is rounded to 1/30s

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I don't see why this needs to be at all complicated.

It is often good and/or interesting to know the relationship in "stops" and that is a bonus and can be relevant in the way some people learn or grasp the subject matter: but yes, I knowing that stuff, would still simply still multiply the shutter speed given at ISO12 by three.

And it's also good to mention reciprocity.

WW