The internal representation of exposure values in Canon firmware is in 1/8 EV increments.
Some shutter speed measurements from movie mode: https://www.magiclantern.fm/forum/index.php?topic=16814.msg164062#msg164062
However, timings in photo mode are very likely to be different.
On most models, setting shutter speed in 1/8 EV increments from software should give either exact value or a difference of +/- 1/8 EV (api_test.lua checks for that). However, this is only what Canon firmware reports back.
How this translates to mechanical shutter timings is unknown. This table
contains some sensor timings
, but sensor exposure is a little longer than mechanical shutter opening.
If you have a light source that does not flicker and does not change in intensity, you can try to take pictures of a blank wall at various shutter settings, and record the median (not average) value (e.g. with deflick.mo
) so we can cross-check the table referenced above.
BTW, ISO is likely to change (slightly) the sensor response (both gain and linearity). Changing aperture does not always give an ideal change in brightness (lens-dependent) and will also affect the vignetting (also lens-dependent and affected by focus distance). All these are going to affect an EXIF-based deflicker algorithm.