Calibration day

A beautiful sunny day without a cloud in the sky. A good time to do some metering work, and work on exposure times. I set up the big Kodak, found a suitable subject, and began my test.

Unlike conventional film which is sensitive to a broad spectrum of light, and can be metered rather easily, the wet plate process falls into the IV spectrum, and a rather narrow portion of or too. A standard light meter is of little use, but I managed to get ahold of a UV meter that is showing promising results.

Decades ago, when I first start shooting film, I learned about the sunny f16 rule. It is a rather brilliant method of figuring out your exposure without a meter. let us say it is a bright sunny day. You have loaded your camera with 400 iso film, but the battery in your meter is dead. No worries if you know the sunny f16 rule. Set your shutter speed to match your film speed, and set your f stop to 16. Now fire away. Clouds roll in, but still sunny? Open up to f11. More clouds no shadows? f 8.

This is all possible because you can see the light that your film is sensitive to. Not the case with the wet plate colloidian process. You can not see UV light.

So, I have a meter that does, but exposure times can be long, and what if clouds roll in during an exposure? Well, this meter gives you dosage. So, if your plate needs x amount of UV light to be properly exposed then all you need to do is keep an eye on the dose of UV the meter is registering, and presto you should have a perfect exposure.

Variables though. That’s the hitch. As colloidian ages it’s sensitivity is altered. Brands differ some too. So, one needs to go through a calibration process.

The images below show the results of today’s test. Starting in the upper left the exposure time was 5 seconds, and a dose of 10. Upper right was a few seconds longer, and a dose of 15. Lower left was a dose of 20, and lower right was 30.

The development time was kept the same at 20 seconds. Though all of the tintypes are decent, I like the looks of the third one. Tomorrow I shall set the camera up again run some more tests and see how it goes.