Bulb/Focus Ramping

Bulb Ramping allows the capture of a timelapse that gradually changes exposure, compensating for the transition from day to night.

Auto exposure ramping: this option will adjust shutter and ISO automatically, by looking at image brightness of previous shots.
Manual exposure ramping: this option will adjust shutter and ISO to follow a fixed exposure ramp (a fixed amount of EV change per shot). Can be used as standalone or combined with auto ramping.
Manual focus ramping: this lets you adjust focus gradually while shooting the timelapse. It requires a lens with autofocus and it can only work in LiveView.

Mode: sunset / sunrise / auto. In sunset mode, the exposure will always increase. In sunrise mode, the exposure will always decrease. This idea was suggested by Tom Lowe on RedUser.net forum.
Maximum ramping speed: this parameter is used for computing the optimal smoothness factor. A lower value will reduce flicker, but if the value is twice as low as the real rate of brightness change, ML will lose the ability to ramp correctly. For example, if you set a maximum ramping speed of 0.1 EV / shot, and the scene requires a ramping of 0.2 EV / shot, ML will start flickering heavily.

1. Take a picture of your scene. You will use it to say: I want my timelapse to be exposed like this picture.

2. Enable Bulb Ramping and Intervalometer.

3. Leave the camera still while ML runs a calibration step:

Make sure you have a static and well-lit scene (any static scene which does not require long exposure should be fine).
After calibration, you should get a nice S-curve on the screen.

4. Now you will have to say what tone range to meter for (i.e. highlights, midtones…). Follow the wizard:

Use arrow keys to select your reference picture (which you just took).
Use the main dial to select the tone range to meter for. You can't perfectly match two images just by varying one parameter (exposure), so you have to choose what's important for you in this picture.
For lowest flicker, meter for midtones (choose the 50th percentile, i.e. median, because it's a robust estimation, unlike simple averaging). Leave some headroom for highlights (underexpose a bit).
If highlights are important, meter for them (choose 80th percentile for example). You will have to shoot RAW and remove flicker when you develop the RAW files.
The algorithm works best when brightness is close to 50% (try not to choose extreme values for it).
When you are ready to start, press SET.

5. Sit back and relax :)

ISO is chosen between 100 and maximum auto ISO value from Canon menu.
Shutter speed is chosen between 1/8000 (lower limit) and the delay between two shots minus two seconds (upper limit). Example: for 10-second intervals, shutter speed will be between 1/8000 and 8 seconds.
Aperture is fixed (you can change it manually).

Don't adjust ISO and shutter before the timelapse, they are fully automatic.
Use a ND filter to reduce flicker during daylight.
Reduce flicker in post. We recommend VirtualDub with MSU Deflicker plugin (free, works with Windows and Wine). See also Timelapse workflow using free software tutorial.

Exposure is metered using a condition like this (for example): 70% of pixels should be below 50% brightness.
Exposure for every shot is computed from previous shots, using a feedback controller algorithm with a smoothing factor.
ISO is chosen using the 180 degree rule, so the resulting shutter speed stays between 90 and 270 degrees (that is, between 1/4 and 3/4 of the delay between two shots).
Only full-stop ISOs (100, 200, 400 etc) are used (because you are supposed to shoot RAW).
Shutter speed can be adjusted with a resolution of 10ms.
Frames with fast shutter speeds (less than 1 second in Rebel cameras, less 0.1 seconds in 60D) are taken in Manual mode. You will get flicker.
It can go from 1/8000s @ ISO 100 (daylight) to several minutes of exposure time @ ISO 6400 (complete darkness).
Exposure algorithm is a feedback controller designed with pole placement - the closed loop response will have two real poles placed at the smoothing factor value. Smoothing factor is computed in such a way that, when scene ramping speed matches the speed selected in menu, ramp is followed at exactly 1 EV behind it.
If the lighting changes suddenly a few stops between two shots (for example, you change the ND filters or the aperture), the algorithm should recover completely after 2 or 3 shots. A sudden exposure change is considered when the exposure difference is greater than 2 stops.

When you use bulb ramping, Magic Lantern will save a log file with the exposure parameters, metered values and so on. Please send this file to developers.
These log files can be used to see how well the ramping went and to fine-tune the algorithm.

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