How to Begin Testing Exposures with Wetplate Collodion
Once you determine a good exposure with a specific camera and lens under specific lighting conditions with wet plate collodion, it is wise to write that information down. It is always wise to minimize the number of variables when just starting to learn anything new. If you work in a studio where you control the light, then this will become old hat very quickly and you will dial in your times quickly.
I will tell you that I learned the hard way to wait until my plates were varnished and completely dry before making any types of decisions on exposure adjustments. For example, if you dry your plates and don’t varnish them (not recommended) then they will probably be one or two stops lighter. Long story short, be patient and view the end product before making any fine adjustments to any part of your process, and especially your exposure.
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Strategies for Exposure
In a studio environment when establishing exposure times with new lights and/or new lenses then you can use your dark slide to help you determine the proper exposure. Start by pulling the dark slide fully out and expose the plate for the base time. Depending on the shutter speeds you may need to cover your lens between exposures. For the second exposure slide your dark slide in about 1/4th of the way and expose again for the same as you did on the base time. Then for the third exposure push your dark slide in 1/2 of the way and exposure for two base times and then push the dark slide in 3/4th of the way and expose again for 4 base exposure times. This creates a four stop interval. Remember for each stop you double the light so in this test it would be the base exposure, 2 times, 4 times and 8 times base.
If you are working outside with a vintage lens that doesn’t have a shutter then this test is not very attractive. But if you are just getting started with collodion or dialing in a new lens in a studio environment, then this may be an option for you to consider to establish your base exposure time.
Another approach is to meter the scene after you have a good exposure and find an EI or EV value on your meter that matches your tests. I don’t recommend this approach because I never use a meter, but I mention it for those that may want to work that way. Use this as a baseline so if the light changes by one EI or EV then you know that is equal to one stop of light which is either twice the light or half the light depending on the change in intensity. So, if it is brighter by one stop you would half the light and if it were darker by one stop you would double the exposure time. If you have a lens where you can adjust the aperture then you could go that route as opposed to just changing your exposure time. I’ve used this when I want to play with dialing back my lighting in the studio and in theory it does work as long as you have worked through establishing your exposure times based on experience first.
So much depends on your lens because if you are using vintage lenses then you are likely using them wide open because that is your only choice. The only variable you have on controlling your exposure at this point is the shutter speed (time of the exposure). As a rule of thumb for working outdoors or in a rich source of UV light such as a large window I have found that if you are using an aperture of f/5.6 in the open shade (sunny day, just not in direct sunlight) then a good guess is about 1 second. In the direct sun a good guess would be 3 stops (f/16) and in open conditions with a few clouds a good guess would be 1 stop (f/8). If you can’t adjust the aperture then you will have to adjust your exposure time as discussed above. Keep in mind your plates will probably look different in studio lighting as opposed to natural light outdoors. This is why some collodion portrait photographers use a natural light environment. And, it doesn’t hurt that it is free.
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