5D II - Intervalometer/Ramping

[ap-photo]

OK, I'm new to this timelapse thing but since the new ML, I've been trying them out....But I've run into an issue.
I'm sure it's user error (ie. my own fault/settings etc) but I'd love some help.

I've been shooting sunsets, and the exposure is normally fine until it gets resonably dark, then the camera calculates massive exposure times. Up to 150seconds f/13 @ ISO800. From what I can tell, it's trying to turn my night scene, into a day scene...to match exposure that it was metered from in the original image.
Would that be correct? If so, what should I be doing?
I meter off the midtones @ 50% - is this my problem?

Thanks, any help would be great!
(and sorry if it's a basic questiuon...)

Ant

[a1ex]

Set manual ramping to some small negative value (for example, at the end of the timelapse should be 3 EV darker than at the start). Leave auto ramping enabled, so it can compensate for brightness change.

I usually choose something around 50 or 75% (that's the tone range that should be kept at constant - or constantly ramped - brightness).

You should end up with something like this: https://vimeo.com/49101374

Or, expose to the right (meter at 90%), deflicker with this script ( http://www.magiclantern.fm/forum/index.php?topic=2553 ) and then adjust the brightness in your video editor.