[vanakaru]

From the manual:
Fixed=The movie is recorded at the aperture value set before movie recording starts. (default setting)
Auto=The aperture is controlled automatically.

Whatever option I use I get aperture correction when I move from light to dark and back. Or is there something else that compensates like ISO?
I need the aperture stay fixed. Is it possible?

[deltoidjohn]

Switch it to fixed and it will stay at whatever aperture setting you tell it to.

The ISO and shutterspeed are still on auto though, so you need to use AE lock to stop if from adjusting those parameters. You have to make sure the AF/AE button is set to AE lock first.

[vanakaru]

Ah! Yes.