Low ISO metering (Dry-plate,paper neg)

[Cynan]

Let's say I have some lovingly hand-coated glass plates with an ISO 0f 1/2 to 1. Will a light meter be of any use with such a slow emulsion?

My Gossen meter goes down to ISO 3, so should I just triple exposure times by a factor of three? Is it that straight forward?

[Mark Sampson]

ISO 1 is 1-2/3 stops slower than ISO 3. ISO 0.5 is a stop slower than that. The seqence of speeds should run like this:
ISO 25, 20, 16, 12, 10, 8, 6, 5, 4, 3, 2.5, 2.0, 1.5, 1.25, 1.0, 0.75, 0.625, 0.5, ...
Each decreasing ISO number requiring 1/3 stop more exposure.

[Bob Gentile]

"... Will a light meter be of any use with such a slow emulsion...?"Now that's the question. Your exposures are liable to be beyond any reliable reciprocity information.

[Ron Marshall]

In shade your exposures should be around 16 seconds metered (assuming f16 and average lighting).

As a guess, you should double or quadruple that to account for reciprocity failure.

Bracket a few test exposures and develop to get a handle on your emulsion's reciprocity failure.

[Bob Gentile]

In shade your exposures should be around 16 seconds metered (assuming f16 and average lighting)Wow! You're right! I would've thought it would be a lot longer. A LOT longer. But it's actually quite reasonable.

Thanks for pointing that out.

[Cynan]

!!! Thanks everyone. Guess I won't be needing a meter then.

[Donald Qualls]

My Gossen meter goes down to ISO 3, so should I just triple exposure times by a factor of three? Is it that straight forward?

Since the ASA component of ISO is linear (that is, ISO 400 is four times as senitive as ISO 100), it's that simple (before accounting for reciprocity failure).

Unfortunately, reciprocity failure can only be measured, with the process you're using to make your own plates, not at all readily predicted, but you should be able to make many exposures at f/8 or so, which would (with ISO 0.5) require 1/2 second in Sunny conditions, or a couple seconds in shade; that may help keep reciprocity failure under control. Don't forget, you aren't necessarily after the "everything in focus" look we now think of as traditional for large format; look at portraits from the wet plate era, or at Daguerreotypes (both processes were commonly found in this speed range), and you can easily see the very limited DOF that comes from wide apertures (some Dag lenses were as fast as f/5.6, even in the 1840s).

[Jason Greenberg Motamedi]

A serious consideration is that your plates will probably be either orthochromatic (seeing blue, green, and uv) or color-blind (blue and UV only), so using a lightmeter in a regular way will be difficult. One suggestion, which has worked for me in the past, is to place a blue or blue-green filter in front of your meter. Chose the color depending on the color sensitivity (or lack thereof) of your plates. For color-blind plates use a 47B (tri-color blue) filter and for orthochromatic use a 44A.

[Michael Kadillak]

My Spectra Combi 500 incident meter allows you to read as low as an ASA 0f 0.1 in .04 increments to 0.2 ASA and .1 ASA increments to ASA 1.0 and regular increments from that point forward. It also has a special sensitivity button for low light readings on the side of the meter. About the most accurate and flexible meter I have used.

Cheers