When does autofocus not focus? Related: Automatic Exposure

[dalex]

A question about the autofocus feature (this is also about the auto exposure feature--see below).

If Silverfast is set to autofocus, is it supposed to attempt to find the optimum focus each time a new frame is scanned?

When I scan several frames, the scanner focuses on the first frame, but doesn't on the successive frames. Some other scanner software I've tried refocuses for each frame.

I'm interested because I'm also trying to understand what happens when the 'Set Automatic Exposure' feature is turned on or off. When it's turned on, the scanner has to make an initial short pass to figure out what exposure to use, but the strange thing is that when I turn this off every scan always makes a short pass before the main scan pass as well.

Through e-mail, Silverfast support suggested the initial pass could be for autofocus, and there's the issue: except for the first frame, autofocusing is not happening during that first short pass, but the first short pass is always happening

So I'm confused as to what the scanner is actually doing, whether it's not focusing when it should, and whether it's using auto exposure when it shouldn't.

I'm using Silverfast AI version 6.4.4r2 with a Minolta Scan Elite 5400 on Mac OS 10.3.

Thanks!

[dalex]

I think I've answered my own question about the focusing.

If I always do a prescan before the main scan, then the focusing occurs for every frame. If I only do a scan, but no prescan, the focusing does not occur except for the first frame.