Sunset from our meadow on 2010 December 18

Whenever I setup a scene to do a time lapse I hope that something interesting will occur within the camera's field of view.  On this Saturday evening, I made a gamble and started shooting a time lapse to the north at 6 frames per minute (fpm) from our meadow.  After an hour, the cloud cover to my west was turning a fantastic array of orange and red.  Hopefully those colors would make it into my scene to the north.  But in order to do so, a northwest - southeast band of slightly lower clouds would have to shift to the east.  After a few more minutes, I realized the band wasn't sliding east and it would just sit west of my scene intercepting increasingly brilliant orange sunlight. So my scene looked like this at the end.

I finally bailed on my first time lapse and shifted the camera west to grab another time lapse at 12 fpm looking at this scene.

I usually leave autoexposure on during my time lapses and so I did with the second time lapse I started.   At the beginning the camera was shooting 1/125th of a second at about F4.5.  But the shutter started to slow down to gather enough light to make a perfectly exposed image for each frame.

Shutter speed (sec) vs. time (CST) of the time lapse above.  The downward jog is when I adjusted the autoexposure to -.33 f-stops.  The time increments below match each time the shutter changed speed.  Thus the increments are not always the same, especially from 17:22 to 17:26 CST.

The result is something that isn't quite real.  At first, the sun is directly illuminating the clouds and there is plenty of light.  Later on, the scene goes gray for awhile as the clouds lose direct sunlight.  But then color begins to reappear and intensifies right up to the end.  The scene at the end is really a lot darker than the beginning but the cloud bases must be receiving secondary illumination from clouds further west that became directly illuminated by the setting sun as you see below.