ARRI ALEXA 35 Body & PL Mount Set (LBUS) - Manual (Page 39)

Main Parameters 39
9.4 Exposure Index
The exposure index selects which sensor signal becomes mid gray in the LogC4 image and
consequently in the monitor image. Mid gray is encoded as 28% in the LogC4 signal. When the output
is set to Rec. 709 and no look file is loaded that modifies the signal, mid gray becomes 40% in the
video signal.
The LogC4 signal is exposure normalized. That means, when one closes (opens) the aperture and
increases (decreases) the exposure index by an equivalent amount, mid gray stays at the same value.
Neither varying the exposure nor the exposure index changes the dynamic range of the sensor. By
exposing the sensor differently, the distribution of the tonal range below and above mid gray changes.
This is shown in the figure below.
The LogC4 encoding maintains a fixed relation between signal values and the relative exposure stops
in the scene. For example, a signal value of 10% represents an exposure of 1.5 stops below mid gray
(which is at a signal level of 28%). A signal value of 40% represents 2 stops above mid gray. These
relations stay constant regardless of the exposure index. Because the number of stops above mid gray
that can be captured increases with decreasing exposure, the maximum of the LogC4 function will
change with exposure index.