Updated: 6 days ago
In every book I have read about tintypes there is very little use for exposure meter in collodion photography. There are at least two obvious reasons supporting this claim. Wet plates have a peak of sensitivity in the blue part of visible light and in invisible UV light, exposure meters are optimized for the visible spectrum of the light. The second problem is the instability of the iodized collodion when the aging collodion is losing sensitivity.
When you are working in changing light conditions, especially with artificial light, test exposure can't solve the problem definitively. The natural light intensity can in a short time period change more, then is narrow error margin you have exposed wet plates.
In the picture above is a beautiful Hurter & Driffield's Actiongraph from 1888. You can see on the chart how dramatically is light intensity changes during the day or how much it is depending on the time of the year. (Light intensity at 11 a.m. in December is the same as light intensity at 6 a.m. in June) What's more, this Actiongraph is optimized for 52 degrees of North Latitude, so if you are working in a different latitude, you need to have a different chart. And the weather, from very bright to very dull, can change the exposure in five STOP range.
So, what now? Do you have to use for setting exposure time of wet plates just your experience, judgment, and a series of trial and error steps? I don't think so.
I'm using an exposure meter for every wet plate photo session and, from my point of view, it is a very useful instrument. Of course, the use of an exposure meter in wet plate photography is far from reliable it has with gelatine films. But it can be a valuable part of the process and it is not expensive to try one. I'm using the Pocket Light Meter app on my iPod (frankly, it is the only reason I still have this small and very old gem) Frankly, I was looking for an application for my Android phone, but Pocket Light Meter was the only one I found with the option to set ISO 1.
For the setting of the exposure base, I'm using a light meter targeting my palm (in a clean day, shooting outdoor, you can use the darkest part of the blue sky, above the horizon at a 90-degree angle to the sun). When you are using the measure on your palm you should double the measured exposure time. We can say that the standard coefficient for palm measure is two. When I'm at the beginning of a new shooting session, the first task is to find the right exposure coefficient (multiplier) for the calculation of exposure. It depends on a lot of things - visible versus UV light ratio, collodion sensitivity, subject tonal scale, quality of glass used in windows if you are working indoors. Once I know that coefficient, I can take it as reliable for the next few hours. I will try to explain it with an example.
All four pictures of white tulips were made in my studio with North light, aperture set to F11, exposure meter was set to ISO 1. The light meter suggests for the first picture exposure time 26 seconds. Because I'm inside, behind a window, I'm starting with coefficient 3 and exposure for 80 seconds. The first picture is underexposed and I guess this scene needs around one STOP of exposure more. For the next picture, I increase the coefficient to 5. The exposure meter suggests 20 seconds, so I expose for 100 seconds. The second picture is much better exposed, but it is still on the dark side of things. For the third picture, I increase the coefficient to 6, adding in this way just 1/5 of STOP to exposure. The third picture is exactly what I'm looking for. The fourth picture I'm making just from curiosity, to see how much it would change. A bit more than I'm able to show on a scan.
Without a light meter, I would have very little chance to notice or guess small changes in light level. If I would continue that day with a different subject or composition, I could rely on the exposure meter with a pretty high level of confidence. With an exposure meter, you can check your exposure just before the shoot, very valuable on cloudy days, when the light level can change in minutes in more than one STOP.
The last picture was made with an exposure time of 100 seconds using the exposure meter time of 16 seconds and coefficient 6.