Exposure and Development

Judging exposure time is always difficult in astrophotography. However, feeling difficult doesn't mean doing nothing. One thing to do is to gain experience until you can have a good feeling about roughly how much density you will get on the film given the sky condition and exposure time. Most people stop just here. What's the next reasonable step?

Exposure and development are tightly connected. They both contribute to the final image. This has been known for ~100 years by photographers. The simplest idea is, determine your exposure based on the shadow and your development based on the highlight. The ultimate extension of such an idea is the Zone System developed by Ansel Adams.

It is almost impossible to apply the Zone System to astrophotography. Perhaps the only exception is lunar photography. However, I still never heard of anybody who applies (or claims to apply) Zone System to lunar photography. For deep-sky photography, it is difficult to apply because there is no way to accrurately measure the brightness of celestial objects and there is no way to accurately predict the strength of reciprocity failure.

Dispite this, I believe that the Zone System is still a good thing to learn for astrophotographers. We don't need to actually apply it to astrohpotography. What we want to know is the concept behind the System, such as concepts of tone reproduction, and how exposure and development affect the shadow and highlight on the film. This is especially helpful for people using color reversals. For people who play with negatives or computer post-process, they may not care too much about what's on the film. They can alter the images on prints or in digital dark rooms. However, if you care about what's on your sildes, you have to care about exposure and development.

Now when I take slides, what I'm thinking about is not just the correct exposure, but the correct exposure-development combination, for each different celetial scene. This helps me to get a better control over both the shadows and hightlights on my slides.

Read Ansel Adams.