Deciding on Exposure Indexes for Film Photography
I now have been shooting film again for some time, and I was not always getting the results I wanted. This made it necessary to adjust the scans in Lightroom. Something I actually wanted to prevent by shooting film! So what was wrong? My first thought: exposure!
As per the advice from Richard Photo Lab I decided to shoot several test rolls to see how changing exposure would impact the final result, and to decide how I want to expose film in the future. Since I mostly use one of four specific films the choices for the test were easy: Ilford Delta 100 and Kodak Tri-X 400 for black and white, and Kodak Ektar 100 and Kodak Portra 400 for color.
I took 10 pictures with each film on my trusted Mamiya RZ67 Pro-II with 110mm lens. All exposures have been incident metered with a Sekonic Flashmate L-308S.
The examples on this page show for each film the frame under exposed with 1.5 stop, the frame exposed as metered, and the frame over exposed with 3.5 stops.
The results make it quite clear that the "right" exposure for each film is different. That is why in stead of using the ISO value for each film as shown on the box (the box speed) you should use the Exposure Index for that particular film. The Exposure Index can be calculated by taking the exposure from your test roll that looks best for the look you want to achieve, and applying the under or over exposure value to the box speed. The resulting value, the Exposure Index (EI), then should be used to set your light meter.
For example: if the box speed of a film is ISO 400, and your test images show that an over exposure of 2 stops leads to the best results, you should expose this film as if it was a ISO 100 film. This means that you will use this film with an EI of 100. If the best result was achieved by under exposing this film with 2 stops, then for you it would have an EI of 1600. The film then should be processed as per the box speed (no push or pull processing).
The tests gave me a clear indication of how to expose these four films going forward. I encourage you also to shoot some test rolls and see how the results will help you to improve your film photography.