Over and Under Exposing Film ISO(ASA)
When I first started shooting film, I read how a lot of color negative film can retain data even when overexposed. In fact, a lot of articles I read advised to purposefully overexpose films such as Portra 400 by shooting them at 200 ISO(ASA) instead of 400.
First, let me say that ISO and ASA are both measurements of how sensitive a film is to light. The higher the number the more sensitive and the “faster” the film is considered. A higher ISO/ASA film is called “fast” because it allows you to use a faster shutter speed while maintaining the proper exposure. For the purpose of this article ASA and ISO represent the same thing: how sensitive a film is to light.
As a digital photographer who was exploring film, overexposing film confused me. In a digital camera, if I wanted to overexpose a picture by using ISO, I would increase the ISO from 400 to 800. However, the opposite is true for film. To overexpose an ISO 400 film a photographer would need to shoot at an ISO of 200 not 800. This is because of the way the camera’s light meter functions.
A camera’s light meter (if your camera has one) uses three different inputs: ISO, shutter speed, and f-stop (aperture). Adjusting the camera’s shutter speed will obviously tell the camera how fast or slow to fire the shutter and the f-stop will tell the lens how open or closed the aperture ring should be. Setting the ISO on your camera, however, doesn’t directly affect the amount of light hitting the film during a capture, instead is telling your light meeter how light-sensitive the film is. The ISO information allows your camera to then use the shutter speed and f-stop information to determine the correct exposure.
All that being said, if you are overexposing an ISO 400 film by setting your camera’s ISO setting to 200 you are essentially tricking your camera’s light meter into thinking you are using a lower ISO, or less light-sensitive, film. When your camera thinks you are using an ISO 200 film instead of an ISO 400 film it will make you use slower shutter speeds and lower f-stops to get what it thinks is a “correct exposure”. This will allow more light to hit your film than if your camera thought you were using an ISO 400 film and will result in an “overexposed” image.
I’m a visual learner so here is an example. The four images below were shot on Portra 400. The two images on the left were exposed for an ISO of 1600 while the two images on the right were exposed to an ISO of 200.
The beautiful thing about a lot of color film, is that it retains information in the highlights even if the image is overexposed by a few stops. On the other hand, a film image will quickly get “muddy” when it is underexposed, and less data will be recoverable. I have overexposed all of my color film by at least one stop (except Ektar 100) and have been happy with the results in most cases.
I hope this helps any photographers who are jumping into the world of film, I know I didn’t fully understand this stuff at first. If you don’t know something feel free to ask, I may not have all the answers but I am more than happy to point you in the right direction. As always, If anyone has any questions (or tips) I’d love to hear them!
- DK
PS: here are some of my favorite shots that were all shot by overexposing 35mm film ISO.
