RED SURVIVAL GUIDE - Manual (Page 31)

60 RED SURVIVAL GUIDE | REV. D REV.D | RED SURVIVAL GUIDE 61
R E D | S U R V I VA L G U I D E R E D | S U R V I VA L G U I D E
BACKGROUND
In a nutshell, an optimal exposure records enough
light to minimize noise, but not so much that important
highlights have the potential to become clipped.
The key is to be able to quickly assess whether the
exposure goals are being achieved. With RED, several
in-camera tools are available for both exposure and the
appearance of on-screen previews.
The first important tool is the histogram, which is
displayed in real-time on the bottom left of the LCD
preview screen. A histogram depicts the relative
fraction of an image (vertically) that comprises each
tonal level (horizontally). Further to the left represents
deeper shadows, and similarly, further right represents
brighter highlights:
A HISTOGRAM DEPICTS THE RELATIVE FRACTION OF AN
IMAGE (VERTICALLY) THAT COMPRISES EACH TONAL
LEVEL (HORIZONTALLY).
RED cameras show separate overlaid histograms for
each of the red, green and blue color channels. All
three histograms are useful for assessing clipping risk
in those channels, but the green histogram can also
give a rough approximation of overall image brightness.
In general though, RGB histograms only truly represent
brightness when all three histograms correspond;
as specific regions of these histograms diverge, the
more saturated and colorful those tones will often
appear, and the less those regions of the histogram will
represent brightness.
SHADOWS MIDTONES HIGHLIGHTS
HISTOGRAM & TONES
Images usually appear correct whenever the tonal
distribution within the histogram reflects the tones
which are perceived in the scene. For standard scenes
with a predominance of midtones, this usually means
the histogram will have a “mountain in the middle” with
a decline toward the sides. If the histogram piles up
toward the left or right, the image will often appear too
dark or bright.
However, always aiming for a central histogram would
be a mistake—especially with high and low-key
scenes. For example, a central histogram would cause
a snow scene and a nighttime cityscape to appear
too dark and bright, respectively. In other words,
histograms cannot be treated as the equivalent of a
digital light meter since they represent reflected, not
incident light.
Regardless, the key is that histograms alone don’t
indicate proper exposure—just how the on-screen
preview will appear using the current ISO and look
settings. An optimal exposure could easily have a sub-
optimal histogram, and vice versa. Trying to achieve a
central histogram at the lowest ISO speed could still
lead to overexposure, for example—even if the on-
screen preview appears correct—because highlight
clipping will become much more likely. Ultimately,
the raw image data is what determines whether an
exposure is optimal. To assess this, we’ll need to use
the histogram in conjunction with tools in the next two
sections.
LIKELY TOO DARK
LOW KEY HISTOGRAM HIGH KEY HISTOGRAM
LIKELY TOO LIGHT
GOAL POSTS
In order to quickly balance the competing trade-offs of
noise and highlight protection, RED cameras also have
indicators at the far left and right of their histogram.
Unlike the histogram though, these are not affected by
the ISO speed or look setting, and instead represent
raw image data. The indicators are depicted as vertical
bars to each side of the histogram, and are often
referred to as the “goal posts,” since the aim is usually
to achieve a histogram which doesn’t adversely hit
either side.
The height of each goal post reflects the fraction of
overall pixels that have become either clipped (on the
right), or near the capabilities of the camera to discern
real texture from noise (on the left). The full scale
for each goal post represents a quarter of all image
pixels. In general, the left goal post can be pushed up
to about 50% height and still give acceptable noise,
but even a small amount on the right goal post can
be unacceptable, depending on where this clipping
appears in the image.
UNDEREXPOSED
OVEREXPOSED
BALANCED EXPOSURE
TRAFFIC LIGHTS
DISCUSSION
Sometimes the goal posts alone don’t provide enough
information about how an image is clipped. RED
cameras therefore also indicate which color channels
have become clipped (to the right of the histogram).
These indicators appear as a red, green and blue
dot for each color channel – and are therefore often
referred to as the “traffic lights.”
Some advocate a strategy called “expose to the
right” (ETTR), whose central principle is to record as
much light as possible without clipping—causing the
histogram to appear shifted to the far right. While this
approach works well with stills photography, it greatly
increases the likelihood of clipped highlights with
video footage, since lighting conditions are often more
dynamic.
Furthermore, ETTR can be misleading since the live
histogram doesn’t represent raw image data. An ETTR
strategy that doesn’t also pay attention to the goal
posts therefore runs a high risk of overexposure—
and increasingly so at lower ISO settings. Instead
of recording as much light as possible, we instead
encourage only recording as much light as necessary
to meet one’s goals for image noise, but not
necessarily any more than that—all while paying
attention to the goal posts and traffic lights in addition
to the histogram.
When about 2% of the
image pixels for a particular
color channel have become
clipped, the corresponding
traffic light will turn on. This
can be particularly helpful
in situations where just the
red channel has become
clipped within a skin tone,
for example. In that case, the right goal post would be
much lower than it would appear otherwise, since all
three channels haven’t become clipped.
TO LEARN MORE, PLEASE VISIT THE "LEARN" SECTION
ON RED.COM FOR ADDITIONAL TUTORIALS ON EXPOSURE
WITH RED CAMERAS.