Low ISO metering

[guyatou]

Hello all --

I have a quick and fairly basic question. I've searched high and low, and can't come up with a definitive answer.

My light meter has settings for ISOs down to ISO 3. I've been shooting Rockland dry plate tintypes, which are about ISO 1. How many full stops are in between? Basically, how many stops do I need to add in order to meter at ISO 3, but get the correct amount of light for ISO 1?

Does the scale (in full stops) go:
ISO 1
ISO 3
ISO 6
ISO 12
ISO 25
ISO 50
ISO 100 etc.

Or am I missing a stop, like ISO 1.5?

The tintypes I shot were all a bit underexposed, but I was also having to add for bellows factor, etc., so my calculations could have been off anywhere along the line.

Does anybody know the answer off the top of their head?

[N Dhananjay]

My understanding is that ISO is an arithmetic scale - halve the ISO and you need twice the exposure. So you would require about 1.5 stops, going from ISO 3 to ISO 1. You will probably also need to accommodate reciprocity failure - once your exposure time exceeds 1 sec, you need to give quite a bit more exposure. The general rule of thumb for traditional style emulsions is in the 1-10 seconds range, double exposure time (so, if it says 3 secs, give 6); in the 10-100 second range, give 2 stops more exposure; greater than 100 secs, give 3 stops more exposure. The failure of the reciprocity law also implies an increase in contrast, so you will need to develop for a shorter time.
Cheers, DJ

[guyatou]

Thanks for the help!

That's the other factor I don't know -- the reciprocity failure of Rockland AG+. I would assume it wouldn't be too much of a factor, as it's really designed for use under an enlarger, with multiple second exposures.

[Michael R]

I believe a stop below ISO 3 is 1.6, followed by 0.8.


Hello all --

I have a quick and fairly basic question. I've searched high and low, and can't come up with a definitive answer.

My light meter has settings for ISOs down to ISO 3. I've been shooting Rockland dry plate tintypes, which are about ISO 1. How many full stops are in between? Basically, how many stops do I need to add in order to meter at ISO 3, but get the correct amount of light for ISO 1?

Does the scale (in full stops) go:
ISO 1
ISO 3
ISO 6
ISO 12
ISO 25
ISO 50
ISO 100 etc.

Or am I missing a stop, like ISO 1.5?

The tintypes I shot were all a bit underexposed, but I was also having to add for bellows factor, etc., so my calculations could have been off anywhere along the line.

Does anybody know the answer off the top of their head?

[Peter Gomena]

ISO ratings were in 1/3 stop increments, 100, 80, 64, 50, 40, 32, 25, 20, 16, 12, 10, 8, 6, 5, 4, 3, 2.5, 2, 1.5, 1.25, 1. (Approximately with a little rounding.) I suppose you can scale it any way you want.

[guyatou]

OK, according to Peter there's two full stops between 1 and 3. That makes sense. I knew I was missing something. Whole stops would be 1, 3, 6, 12, 25, 50, 100, etc.

[jp]

I would set meter iso to 10 and multiply the estimated exposure by 10.

[Michael R]

OK, according to Peter there's two full stops between 1 and 3. That makes sense. I knew I was missing something. Whole stops would be 1, 3, 6, 12, 25, 50, 100, etc.

0.8, 1.6, 3, 6 etc.

[Andrew O'Neill]

Go here and scroll down to the ISO chart...

http://en.wikipedia.org/wiki/Film_speed

[Doremus Scudder]

OK, according to Peter there's two full stops between 1 and 3. That makes sense. I knew I was missing something. Whole stops would be 1, 3, 6, 12, 25, 50, 100, etc.

Wrong!

One full stop is double or half the starting ISO number, depending on the way you are going. The factor is two: multiply by 2 or divide by two.

Example: ISO 50 x 2 = ISO 100; ISO 50 /2 = ISO 25.

And make things easy on yourself. We all know the common ISO numbers in 1/3-stop intervals: 400, 320, 250, 200, 160, 125, 100, 80, 64, 50.

Now, just knock the last digit off and you have the continuation down: 40, 32, 25, 20, 16, 12, 10, 8, 6, 5 (rounded a bit, of course).

If you need even lower, just continue the process: 4, 3, 2, 1.6, 1.2, 1... Sure, we've rounded a bit, but much less than 1/3 stop.

And, back to your original mistake: two full stops means you have to multiply or divide by two twice:

Example: ISO 50 x 2 = 100, 100 x 2 = 200. There are two full stops between ISO 50 and ISO 200. I'll let you do the division example :)

Best,

Doremus