The chart is created with real-world data from the Timelapse+, and uses the same algorithm for determining exposure. This makes for an easy way to test changes to the algorithm tuning without a several hour actual test. Higher values on the chart represent a brighter exposure/darker sky. The y scale is in stops, relative to the start of the bulb ramp.
Light sensor reading
This is the (filtered and averaged) value read from the light sensor on board the Timelapse+. It has a limit of how little light it can detect, so part way through the chart it levels off even though the target exposure continues. At a preset threshold, the algorithm stops basing the values off of the current reading but instead locks on the the last rate of change before the threshold was reached. (This can be adjusted in
This value is an estimate of the actual brightness change of the sky and was calculated from the in-camera exposure senquence and the averaged luminance value of the photos (to correct for exposure inaccuracies). This line on the chart is for tuning reference and is not a value accessible in the field.
The is the exposure value as calculated by the Timelapse+, as based on light sensor reading and the tuning values.
The Timelapse+ controls the exposure value rate, not the exposure directly. This makes for smooth transitions and minimizes flicker in response to momentary changes in light. The Timelapse+ continuously "steers" the exposure to keep it on track. The following parameters determine how it controls the rate.
(P) Proportional [response to PRESENT]
This represents the response to the immediate error (error as define as the delta between the current exposure and the current light reading).
Settings->Time-lapse->Bramp Tuning->P Tune
(I) Integral [response to PAST]
This determines the response to the average of the immediate error over the last 10 readings. The represents the accumulated error and can help keep the value close to the target but can cause oscillation if set too high.
Settings->Time-lapse->Bramp Tuning->I Tune
(D) Derivative [response to FUTURE]
This is rate of change of the current light reading and can help respond to changes before they happen (rather than responding after the change like the other two values). Setting this value too high can cause instability.
Settings->Time-lapse->Bramp Tuning->D Tune
Code snippet from auto bulb ramp portion of shutter.cpp
This code can be edited for testing -- press update below the chart above to test changes to the algorithm.