ARRI ALEXA SUP 7.0, Pocket Manual, Page: 38 / 100

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Legal and Extended Range
An image encoded in 10 bit legal range has a code
value range from 64 to 940 (876 code values), and
a 10 bit extended range signal has a code value
range from 4 to 1019 (1015 code values). Contrary
to popular belief, extended range encoding does
not provide a higher dynamic range, not does legal
range encoding limit the dynamic range that can be
captured. It is only the quantization (the number of
lightness steps between the darkest and brightest
image parts) that is increased by a marginal amount
(about 0.2 bits).
The concept of legal/extended range can be applied
to data in 8, 10, or 12 bit. All ProRes/DNxHD materi-
als generated by the ALEXA camera are in legal
range, meaning that the minimum values are en-
coded by the number 64 (in 10 bit) or 256 (in 12 bit).
The maximum value is 940, or 3760, respectively.
All known systems, however, will automatically
rescale the data to the more customary value range
in computer graphics, which goes from zero to the
maximum value allowed by the number of bits used
in the system (e.g. 255, 1023, or 4095). FCP will
display values outside the legal range (“superblack”
and “superwhite”) but as soon as you apply a RGB
filter layer, those values are clipped. This handling is
the reason why the ALEXA camera does not allow
recording extended range in ProRes.