Dynamic Range vs. Latitude

I’d like to talk about a couple terms that often get used interchangeably, but they’re really pretty different, “dynamic range” and “latitude.”

Dynamic Range (DR) is a measure of how much light a sensor or film stock can handle. If you have a scene where the shadows are just on the edge of being completely black, and the highlights are just on the edge of being completely white, and you measure the difference between the light coming into the camera from the shadows vs. the light coming into the camera from the highlights, that measurement is the camera’s (well technically, the film stock’s or sensor’s) DR. It’s measured in terms of “stops,” a difference of a stop being a doubling of light. (It doesn’t matter how much light is there, two candles are one stop brighter than one, and two photons are one stop brighter than one.) DSLRs like the 5D or the gh4 generally have about 10-11 stops of DR, while more expensive cameras like the C300 have 12, and top-of-the-line cameras like the Alexa and Red Dragon have 14 and 16 respectively. That means the Alexa can process four times as much light as the C300, and the Red Dragon can handle four times as much light as that.

How is this useful to us? If you’re shooting a foggy ocean on an overcast day, it isn’t. The dynamic range of the scene is maybe 2 or 3 stops, so any camera can capture everything. But let’s say you’re shooting a scene with a noonday sun. You might have a good 16 stops difference between the darkest dark in the scene and the lightest light. The Red Dragon will capture the whole scene, and it’ll look pretty close to how it looks in real life. The Alexa will clip more of the highlights — a cloud might be all flat white, for instance, rather than lightly modeled. And the C300 will clip even more. The clouds might start to look “video-y.” With a DSLR, you might even start clipping the sky. When a blue sky in real life is white on screen, that’s not good. Now, you can of course bring down the exposure in camera so the sky and clouds aren’t clipping, but now your shadows are much darker. And if you’ve got an actor with a hat on, you really need those shadows to not be too dark.

There are other factors that make up overall image quality, like resolution and color science, but to me, DR is one of the most important.

*NB: DR measurement is not an exact science. It’s not important that the Alexa has 14 or 14.5 or whatever stops of DR. Just think, for instance, it’s got a stop or two more than the C300, so it’ll outperform the C300 in that respect by that much.

Latitude, on the other hand, is much different. Latitude is basically how much you can mess up exposing a shot and still have it look good. Latitude on digital cameras is pretty much non-existent. Rotating the iris on the lens has a pretty linear effect on the image on your monitor. Not so with film. Look at this: https://www.flickr.com/photos/gaffatejp/2887892359

The +1 overexposed, correct exposure, and -1 underexposed shots all look pretty much correctly exposed to me. the +/- 2 shots even look pretty good. He’s shooting Porta 160, a negative film. I’d say that Porta 160 has a latitude of about 3 or 4 stops. Its DR, however, is much, much higher.