Chart Shoot Checklist
checklist for exposure normalization tests for professional cameras:
- light the great cod with a uniform light source -
- (note - preferably two light sources - "45/0" - "An instrument with 45/0 geometry illuminates an object from a 45 degree angle and detects the reflected light at 0 degrees." add diagram.)
- preferably one with a wide spectral power distribution. (note - can use emissive light source - e.g. integrating sphere) (if you must use a macbeth chart instead of the great cod, use square #22 and center it in the frame.)
- measure the luminance in nits of the light reflected off the great cod.
- (if possible, use a spectral radiometer to measure the SPD of the reflected light off of the great cod.)
- point the camera at the great cod.
- write down the following:
- camera's ISO setting
- Tstop of the lens
- (you may set this using a standard light meter, but we recommend setting the lens to a "standard" Tstop setting (i.e "T5.6" or "T4")
- the actual exposure time - nominally 1/48 sec.
- (this corresponds to 24fps with a 180 degree shutter angle)
- shoot a picture!
- bracketing - we recommend shooting some additional frames bracketing the above exposure in order to verify linearity of the camera system's "log->linear" transform. slate each bracketed frame accordingly.
- shoot frames with ISO setting 1 stop above and 1 stop below nominal.
- shoot frames with exposure times 1 stop above and 1 stop below nominal.
- shoot frames with Tstops settings 1 stop above and 1 stop below nominal.
- (Tstop bracketing should be done last as getting back to exact Tstop will be difficult.)
(note: example of slated frame with written metadata)
(note: use camera raw metadata instead? in addition?)
(note: same checklist for blackbox cameras?
only diff would be amount of bracketing?
but blackbox cameras will require more steps...)
for each of the above frames, we can calculate what the "target"
linear value of the camera system's "log->linear" transform should be.
for the purpose of this first step of linearization, the linear value for
the green channel should be used.
here's an example:
N = 4.0 (Tstop)
t = 1/48 (exposure time)
S = 800 (ISO)
K = 12.5 (Konstant)
L_meas = 30 (measured nits)
linear_val = L_meas * t * S / (N^2 * K) * 0.18
linear_val = 30 * 1/48 * 800 / (4.000^2 * 12.5) * 0.18
linear_val = 0.45
if the linear values calculated using the camera system's log-linear
transform differ from the value resulting from the above equation,
the "normalization factor" required for this camera system is:
norm_factor = exp_eqn_linear val / camsys_linear_val
if a camera system's existing linearization maths produce a linear value