Figuring out exposure when the meter ISO setting doesn't go low enough

[LFLarry]

I am not sure if this is the right place for this question, so redirect me if needed.

I am working with some vintage dry plates that probably have an ISO rating of 1, or maybe even less. My meter only goes down to ISO 3, so this got me to thinking that I may want to develop some type of conversion that I can use. Since 100 is a simple multiple of 1, I thought it may be a place to start by creating a formula to convert my ISO rating to ISO 1.

If you have any ideas, please let me know.

Thank you.

[paulbarden]

What I do when dealing with this very same problem is: set the meter at 4 ASA, and simply open the aperture two stops more than what the meter says (or alternatively, select a shutter speed two speeds slower than indicated for 4ASA) That will give you an exposure for 1ASA.

[Eric Woodbury]

Stops are factors of 2x (not base 10). It will be simpler if you give your ISO another +1/3 stop and count stops back from ISO 128 -- 64, 32, 16, 8, 4, 2, 1 -- or seven stops.

I have no idea about reciprocity failure with such slow product.

[LFLarry]

That's a great idea Paul. Thank you.

What I do when dealing with this very same problem is: set the meter at 4 ASA, and simply open the aperture two stops more than what the meter says (or alternatively, select a shutter speed two speeds slower than indicated for 4ASA) That will give you an exposure for 1ASA.

[Two23]

I've been doing this lately. I have a Minolta IVf meter that only goes to ISO 3. So, I set it to ISO 4 and take a reading. I then add x2 stops to shutter speed. I.e., shutter speed at ISO 4 is 1/2s, I hold out two fingers and count, "1 second, 2 second." I use 2s as my exposure time. The alternative is to use Sunny 16 if it's sunny out. That works too in the right conditions.


Kent in SD

[LFLarry]

Thanks Kent. That is simple and straight forward.

Sounds like you have some experience with slower emulsions (dry plates). Do you have any idea if reciprocity applies or not? I have been playing around with some liquid emulsions, and got some plates from Jason Lane, and also just picked up some vintage plates. The concept of reciprocity just surfaced in my mind and I thought I better ask to see what the deal is.

I've been doing this lately. I have a Minolta IVf meter that only goes to ISO 3. So, I set it to ISO 4 and take a reading. I then add x2 stops to shutter speed. I.e., shutter speed at ISO 4 is 1/2s, I hold out two fingers and count, "1 second, 2 second." I use 2s as my exposure time. The alternative is to use Sunny 16 if it's sunny out. That works too in the right conditions.


Kent in SD

[Two23]

Thanks Kent. That is simple and straight forward.

Sounds like you have some experience with slower emulsions (dry plates). Do you have any idea if reciprocity applies or not? I have been playing around with some liquid emulsions, and got some plates from Jason Lane, and also just picked up some vintage plates. The concept of reciprocity just surfaced in my mind and I thought I better ask to see what the deal is.

Since they were designed for exposures of a few seconds, I've had no trouble at those speeds. When it comes to exposures of 1 minute or more, yes reciprocity failure is setting in.


Kent in SD

[Nodda Duma]

Larry the rule of thumb I’ve used for my plates is add 50% time beyond 45 seconds and double the time above 2 minutes.

To be honest Kent probably has better working knowledge nowadays with my plates than I do and can tell you better... Between work, coating and testing plates, kids, and shooting slide film for the fall colors, I haven’t found a lot of time to shoot my own plates outside of characterization exposures..and my exposure times are typically only a few seconds where reciprocity isn’t an issue.

Cheers,
Jason

[LabRat]

If you do this a lot, the Gossen Luna Pro series of meter goes down to very low ISO settings, so a good meter for that range...

Note that the needle models used an obsolete battery, but conversions are available...

Steve K

[Mark Sampson]

Luckily, the longer your exposures get, the less critical your exposure time becomes.
Remember that the old-timers developed by inspection, so they could compensate (to a point) for inaccurate exposure. That's how it was done before exposure meters or ISO numbers... it worked for Edward Weston, and thousands of photographers before him.
With the advent of faster, panchromatic emulsions, roll and 35mm film, and exposure meters, the 'time-temperature' method gained acceptance. Helped along by Archer, Adams, and White with the Zone System, but that's another story.
In the words of my old teacher David Vestal, "Don't underexpose, don't overdevelop."