Nailing the right exposure can make a tremendous improvement in the quality and flexibility of digital footage. However, this also requires carefully balancing several potentially competing factors. In this article, we'll discuss the strategy and settings for customizing this balance.
Optimal exposure starts with a deceivingly simple strategy: record as much light as necessary, but not so much that important highlights lose all texture. This is based on two fundamental properties of digital image capture:
Noise. As less light is received, image noise increases. This happens throughout an image with less exposure, but also within darker regions of the same image for a given exposure.
Highlight Clipping. If too much light is received, otherwise continuous tones hit a digital wall and become solid white. Alternatively, this might happen in just one of the individual color channels, which can cause inaccurate coloration. Unlike image noise, which increases gradually with less light, highlight clipping appears abruptly once the clipping threshold has been surpassed.
The goal is to strike an optimal balance between the disadvantages of too little and too much light and therefore between noise and clipping. In general, minor underexposure is acceptable and recoverable, whereas minor overexposure sometimes is not. Many therefore err on the side of less light in order to protect against highlight clipping.
HIGHLIGHT PROTECTION & ISO SETTING
With RED® cameras, the best way to shift the balance between noise and clipping protection is with the ISO setting. Although ISO doesn't actually change the raw image data, it does change how the image is displayed, because it shifts which portions of the scene are mapped to middle gray. The following diagram shows how this works for a 2-stop change in ISO speed:
Note how the tonal range remains identical for both ISO settings, but that in each case the number of stops above and below the midtones has been redistributed. A higher ISO setting allocates a greater fraction of the camera's tonal range to the highlights, thereby providing more clipping protection. Furthermore, even when highlights become clipped, these often appear smoother at a higher ISO since the roll-off from highlights to clipping occurs over a broader tonal range. On the other hand, using a higher ISO also causes darker and therefore noisier portions of the scene to become brighter, thereby increasing visible noise:
For most scenes and uses, exposing with an ISO of 640 to 2000 strikes the best balance between highlight protection and low image noise. If one ventures outside this range, exposure needs to be much more precisely controlled, and has much less margin for error. When in doubt, ISO 800 is a good start, but the optimal setting also depends on image content. For example, lower contrast scenes generally don't need as much highlight protection, and may therefore benefit more from ISO settings as low as ISO 320.
COMPLICATIONS & DISCUSSION
Sometimes violating the above recommendations is unavoidable, depending on the lighting and brightness variation within a scene. For example, with direct reflections and other specular highlights, some clipping is often unavoidable. Alternatively, with high contrast, low-key lighting, some regions will often remain virtually solid black. Fast motion or rapidly-changing subject matter might also require more highlight protection than usual.
Be careful when using the LCD preview image; this can often be misleading under changing ambient light, or if the brightness is set too high or low for natural viewing. Leaving the LCD at its maximum brightness can often trick one intro underexposing the image, for example. Try to rely on objective tools such as the histogram goal posts and exposure false color mode.
The key is to expose with post production in mind. Is an otherwise dark-toned region intended to be substantially brightened afterwards? Is minimal noise critical for visual effects? In any case, it's often better to get an image technically correct during capture and then looking "right" in post production than the other way around.
- Also see part 2 of this tutorial: Exposure with RED Cameras: In-Camera Histogram Tools
- Also see part 3 of this tutorial: Exposure with RED Cameras: False Color & Zebra Tools