As noted above a manual lens is best, a Canon digital one set to MF isn't the same altho you can do the lens twist to trick the camera into thinking the lens is manual.

Shooting wide open helps along with a fixed white balance. Also leave your LCD screen switched on so its displaying the live view image between shots. Philip Bloom taught me that last year on his workshop.

If the scene is day to night or changing exposure then its AV is the best option if you are not willing to sit there changing the manual settings yourself and keep your shutter speeds under 1/100th sec.

this video explains the lens twist method

Top advice. Cheers.

I can think of two ways to change exposure over time and they both involve interalometers and servo motors:

1) connect interalometer to your geared and declicked aperture ring (easier with a cinema lens) and animate the iris over time. Downiside: change in DOF over time, but may be moot if lens is at infinity.

2) use a fader ND and rig a servo and interalometer to it. Downside: an extra filter and at least 2 stops light loss, but not a problem for long exposures.

Using AE modes is possible, but yields unpredictable results and you don't have much control. If you're at aperture priority, it will adjust shutter speed or ISO, but these are in third stop increments at best and will not be totally smooth in the transitions (but might be close enough for government work).

Never thought of the fader option! But again the length of time is usually the biggest factor, so for me personally, I'll stick to what I was doing, but now I'll use the lens twist! Really great info from the above video.. (Deflicker in post has served me well for AV only stuff.).Love it.. Learnt something valuable here, Much apprieciated.