ISO progression at very low speeds?

Alan9940 · May 20, 2021

I've recently starting shooting dry plates and, as anybody who shoots these knows, they're REAL slow; in the ISO 0.5 to 3 range. I know the general rule of doubling or half the film speed to get the next number in sequence--so, 100, 50, 25, 12, etc--but what's the proper sequence below ISO 3? Is one full stop less than ISO 3 = ISO 1.5?

Thanks!

Bill Burk · May 20, 2021

Weston Master II goes down in third stops: 3, 2.5, 2, 1.5, 1.2, 1, .7, .6, .5, .35, .3

Alan9940 · May 21, 2021

Bill Burk said:
Weston Master II goes down in third stops: 3, 2.5, 2, 1.5, 1.2, 1, .7, .6, .5, .35, .3

Thank you, Bill. I was hoping someone would be able to simply list the progression down from 3.

Chan Tran · May 21, 2021

While it's not mathematically correct (because the ISO number for normal and high speed are not mathematically correct either) I think it should be 3.2, 2.5, 2.0, 1.6, 1.3, 1, 0.8, 0.6, 0.5, 0.4, 0.32.

Bill Burk · May 22, 2021

I just ran out the numbers based on log mcs going down by .1 each interval... getting the arithmetic value... getting its reciprocal and then multiply by .8

The ASA series simply repeats moving the decimal point as you go.

So it would be :

10, 8, 6, 5, 4, 3.2, 2.5, 2, 1.6, 1, .8, .64, .5, .4, .32, .25, .2, .16, .12, .1, .08, .064, .05, .04, .032, .025, .02

That’s as far as I did it but it’s obvious by definition the series repeats.

removed account4 · May 22, 2021

wow. you guys are hard core! .. I usually guesstimate and over expose and develop by inspection .. I'm a slacker

gone · May 22, 2021

Bill Burk said:
I just ran out the numbers based on log mcs going down by .1 each interval... getting the arithmetic value... getting its reciprocal and then multiply by .8

Bill, you're a wiser man than I when it comes to math. If it weren't for the computer, I'd be lost just trying to figure out developer dilutions. Thankfully I shoot only 35mm these days, so the only numbers I need to know are 24 and 26.

nerologic · Jun 1, 2021

jnantz said:
wow. you guys are hard core! .. I usually guesstimate and over expose and develop by inspection .. I'm a slacker

Nah, sound more like a film chef. “Cook it til it’s done. Season to taste”.

grat · Jun 2, 2021

Develop until golden brown and delicious?

Warm Winter · Jun 16, 2021

Short answer is yes, the ISO standard is an arithmetic scale. Half the number to half the speed, double the number to double the speed. If you're using the logarithmic scale then it's subtract 3 or add 3 for a doubling, so 21° is half as fast as 24°
It goes down to 0.8/0° for the bottom end
there is a different standard for negative (ISO 5800:2001), monochrome (ISO 6:1993) and slide film (ISO 2240:2003) and they're only about 38CHF each if you really want to dig into trying to figure out an exact value for the plates
otherwise, the basic rough math should work fine since at the low end, even small changes will be proportionally tiny in their effect vs for a fast film

ISO progression at very low speeds?

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