Hello & welcome, David!
I have not yet found how to "freeze" the aperture, like an AV setting. The ND filter state icon says Av, but that function does not seem to have any effect on this camera.
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1. During multiple interval pictures, "freeze" the aperture at a set value (say f5.6) and freeze the shutter speed as well (say at 1/125).
2. During multiple interval pictures, "freeze" the aperture and allow the automatic exposure to vary the exposure time.
The A2000 does not have a real aperture (i.e. an iris diaphragm), just an ND filter as a kind of emulation.
With inactive ND filter you'll get the lowest F value (e.g. F/2.8 ), with active ND filter the highest one (e.g. F/8.0). Zooming also changes the optics, this way the "aperture" value changes, too...
Use a fixed shutter speed, also use a fixed ND filter setting (IN means: always active, OUT means: never active)
BTW - not sure whether the ND filter control works on this camera; you'll see a big difference in the brightness of the images taken with the same exposure time...
On Cameras with more manual control & a real aperture, case 1) is called "
Manual mode", case 2) is called "
Av Mode" - the A2000 do not have these modes, like the Ixus-series cameras...