This is my workup on effective exposure practices for cinematography. It’s an attempt to provide a fairly effective set of practices for ensuring the best consistent image.
To keep a consistent feel between shots (and overall), there are a few factors which should generally stay the same between shots:
- ISO Sensitivity. This generally should remain at the “native ISO” level of your camera’s sensor, which is the level at which the sensor exhibits the maximum amount of dynamic range (levels of difference in luma between lightest and darkest areas representable). I’ve been pretty good about shooting most of my footage at 400 ASA, which matches light levels used with the most common traditional film, but this may vary, depending on the camera body you’re using.
- Shutter Speed. It’s an amateur mistake to unwittingly vary the shutter speed / exposure time between shots, because the way we perceive motion in film is very particular. The majority of film work tends to use a 180 degree “shutter angle”, which corresponds to
1 / 2 x fps– which means that with a frame rate of 24 fps, your exposure time / shutter speed would be 1/48th of a second to correspond to a 180 degree shutter angle. (There are obviously exceptions to this basic rule, but it tends to be consistent within a scene.)
- Aperture. This may not have to remain constant between shots, but you don’t want your aperture settings to be dictated by the amount of light available to you, alone. It’s better to be able to compose a shot artistically, then deal with the amount of light coming into your lens separately. Not to mention that far too many “independent film” cinematographers have gotten a reputation for jacking the aperture all the way open and leaving it there.
Zones, and the Zone System
This is an interesting working understanding of light, and how to properly expose, put together by Ansel Adams (more information on Adams' work is available here) and Fred Archer. I’m not going to go into the specifics of the Zone System, but it is important to understand the basic concepts to properly understand the rest of this article.
I can’t recommend a traditional light meter enough. It’s completely worth the cost, and will save hours of wasted work and blown shots – and generally will tend to be superior, in terms of accuracy, to cellphone apps.
(My light meter is an old manual light meter, which my father gave to me – just showing that it’s not the bells and whistles, but rather the basics which are important to have.)
ND filters (vari and otherwise)
So, if you can’t adjust the ISO sensitivity or shutter speed, and shouldn’t be adjusting via the aperture, how do we control our exposure levels?
Enter the neutral density filter; sunglasses for your lens. Depending on your rig (and funding levels), you could either use a “vari ND filter” (which is essentially two circular polarizing filters, mounted in a single assembly), or a series of single ND filter sheets of varying ratings (which can be combined to form particular exposure reduction values). Most indie cinematographers tend not to have 4x4 filter slots, so it may be easier, with the notable exception of lens changes, to go with vari ND filters.
I highly recommend the Tiffen 77VND filter for vari ND work, assuming a 77mm filter thread – which is common amongst a fair amount of cine lenses. (If you’re working with Cooke primes or similar glass, you’re probably not going to be be worried about any of my recommendations, but you’d also probably have to spring for vari ND filters with a larger filter thread size.)
One caveat about ND filters – they have the potential for introducing slight color casts into the picture, which makes grey cards or other white/color balancing equipment even more necessary.
The trick with ND filters is using them to adjust for your Zone V reading, which means understanding how to reliably adjust them to cut down a certain number of stops of light. This is an entire set of tutorials, and another topic, which is best covered elsewhere.
Lighting for standard exposure
Medium-to-light skinned people generally are supposed to be exposed for Zone VI (+1 EV), with darker skinned people exposed for Zone V (0 EV). Zone V is pretty easy to expose for, since it essentially means setting exposure for the metering value, but Zone VI needs to be exposed at +1 EV, which means that it should be twice as bright (or a full stop brighter) than the metering value. Exposing this way may leave certain things blown out or underexposed, based on your lighting setup, but it will produce “ideal” skin lighting for a standard well-balanced exposure.
Exposing to the left, right …
ETTR maximizes dynamic range by exposing for highlights, and ETTL maximizes dynamic range by exposing for shadows. The former prevents blown highlights, and the latter prevents details from being lost in the shadows. If you decide to use one of these techniques, it should depend on which portions of your image you’re willing to lose details in.
ETTR is very popular, partly due to its ability to hide one of the primary deficiencies inherent in most DSLR and inexpensive digital sensors – the way that highlights immediately clip, rather than gradually rolling off (as they do with film and more expensive sensors).
There are some mitigating techniques which can be used to avoid over-exposure (blowing highlights). Some of the more common ones are:
- Gobos - A simple “go between”, which can be used to block light. This can be made of any opaque material, and only needs to be clamped or secured between the light source and the target object or person.
- Scrims - A screen used to soften light. This can disperse a hard light source in to a softer one, and is secured much like a gobo. As it does not decrease the amount of light appreciably, but instead spreads it out, this may not be effective for heavy over-exposure.
- ND Film - Windows can be covered with neutral density film, effectively reducing their light-transmitting capacity. This technique has been used with great effectiveness in a number of bigger budget productions, but can be a little pricey.
- Fill light - Matched light temperature fill light or bounced fill light can be used to raise the overall exposure level, which will bring higher zones back to a more manageable level. The disadvantage with this technique is that digital sensors tend to like a certain amount of light level variance, or they may produce a seemingly “flat” image.
These techniques are designed to try to produce the widest dynamic range possible for your camera, as well as the widest possible latitude in post-processing for retaining details and image quality. There are a number of circumstances where the “standard” way of doing things could be eschewed in favor of a particular dynamic or idiom. An example would be intentionally losing the background in black to hide a lack of depth or intentionally blowing highlights around an actor to produce an ethereal quality.
Much like other parts of cinematography, there’s no one right way to do things. You don’t have to maintain ISO, aperture, and shutter angle / exposure time – but it can help, depending on what you’re trying to accomplish.