The Stills Approach:
If you are a still photographer, you have a great deal of flexibility in choosing your F-Stop, Shutter Speed, and ISO in whatever way you’d like to get an image.
You will also typically select an automation mode that favors manual control of either shutter speed (to manually control the ability to stop or blur action) or aperture (to manually control which items are in focus and which are blurred.
Some cameras also allow you to set both manually and automate the ISO to get an appropriate exposure, although many stills photographers prefer to keep ISO fixed at it’s nominal setting to minimize noise (typically around 100ISO)
The Cinematic Approach:
If you are a cinematographer, you are much more constrained in your choices.
Shutter speed is normally fixed at twice the frame rate. So at 24FPS, you would shoot at 1/50 of a second. Better yet, on a cinema camera, use shutter angle instead of shutter speed and set it to 180. The removes shutter speed from your ability to control exposure. It becomes a tool to manage the impression of natural motion through motion blur.
For most shots, you also will have a specific and fixed preference for aperture. Most likely you will select an aperture that is small enough to cover your subject (and any small motions they make) and open enough to separate your subjects from the background.
If you have a focus puller (1stAC), you might be able to choose an aperture from a wider range and you might also have the 1stAC manually control aperture.
However, as for shutter speed, aperture is not readily, or primarily, available for exposure control.
ISO is also more restricted. The cinematographer will likely want to maximize the ability to record dynamic range (DR) either as an overall range or biased to record highlights or shadows and blacks.
To do this most cinema cameras record their widest DR at an ISO around 800. Once again, this setting is made less available for exposure control. Also, on hybrid cameras, ISO changes in larger steps than on cinema cameras and AUTO-ISO can be quite noticeable in a shot
So, How DO you manage exposure for digital cinema?
It might be possible to select some fixed combination of aperture and ISO that works for a shot. The most common way to manage exposure is to use neutral density filters (NDs)
Hybrid camera shooters will sometimes use variable NDs. These create issues. The setting is not repeatable and polarization is always included in the light path. Also, it can get expensive to buy multiple variable NDs to fit a number of lenses.
This is one reason for the use of matte boxes: to hold a single set of fixed ND filters in front of multiple lenses and to make switching filters faster and easier than lens-attached filters.
The best approach is to use internal ND filters (preferably electronically switched). Many cinema cameras include this feature in the camera body.
Stills photographers wishing to transition to digital cinema need to develop their own response to getting an exposure while also achieving the artistic result they want. They should also include the cost, complexity, and time requirements of dealing with ND filters when making decisions about adding equipment (like matte boxes and the rigs required to use them), or deciding to switch to a cinema camera which has features such as built in NDs
Parquin Cinematography 