Exposing Frames


Q I’ve moved to using cameras that have a more cinema background, but I feel a little insecure about setting up for recording. I have a pretty good handle on how I want to set my exposure, but setting the shutter has been a bit confusing. At first, I thought I understood about changing the shutter setting and getting my brain around shutter angle, but then something happened. I was playing around with a camera I had never used and saw that I could change the frame rate. But, when I put it into record, the frame rate wasn’t really the recording frame rate. Now, I’m struggling to get my understanding back to solid ground. Help!
Jim H.
Via email

A In this era of constantly changing technology—HD, Quad HD, Ultra HD, 2K, 4K, 8K, 3D, HFR, 16, Super 16, 35, 4/3, APS-C, Full Frame, Linear, Log, 1D, 3D (LUT, that is), 8-bit, 10-bit, 12-bit, 16-bit, Stereo, 5.1, 7.1, Atmos™—it’s not surprising that solid ground can be hard to find.

You mentioned that you have a handle on how you want to set exposure. I assume you’re talking about how you’re setting the aperture on the lens. It’s a good idea to remember that aperture is all about the lens, and how much light passes through and exits out the back of the lens.

In still photography, the next part of the exposure equation is the shutter speed, or how long the image sensor is exposed to the light from the lens. With cinema-style cameras, we can’t think only about shutter speed because we’re making a series of exposures.

Because we (normally) need to shoot at a standard frame rate (e.g., 24 frames per second), we don’t have the luxury of picking from every shutter speed we like, as we would be able to do on a still camera. Why? We have to capture a full image every 24th of a second, so our shutter speed must be 1/24 of a second or faster.

If you were shooting film, the shutter rate would have to be even faster. That’s because you need time when the shutter is closed, so that the next frame of film can be moved into the film gate.

This is where shutter angle comes into play. A still film camera, like an SLR, uses a shutter—often made out of fabric curtains—that travels across the frame. It then needs to be reset after each exposure, which is the shutter sound you hear when the shutter button is pressed.

Shutters made out of fabric work well for still cameras, but they don’t have the durability needed for the millions of exposures needed to make a movie. In addition, the reset noise would make sound recording very difficult. Instead, motion-picture cameras use a spinning disc between the lens and the unexposed film. Part of the disc is opaque and part is transparent. By varying the ratio of opaque to transparent, you can change the length of time each film frame is exposed.