Hey Miket161, as Turtle91 said - welcome. Glad to have you. Turtle's answers are great. I'll just add a little.
miket161 wrote:I know that you want to get the camera normal to the surface of the page and in the center of it.
Yes, with one caveat. If you zoom, you want to be slightly below the center of the page, toward the "crease" of the platen. That is because, if you zoom, you will zoom in on the center of the platen glass - cutting off the gutter of the book. Try it and you'll see what I mean.
miket161 wrote:My question is do you use the (optical) zoom function of the camera to capture just the page of the book,
or do you move the camera to do it, or lastly do you just capture everything and use software to take care of it?
People have done both. In the past, I've used zoom for different reasons:
1.Sometimes to put the cameras further away from the glass because the scanner design required it.
2. Sometimes because the middle of the zoom range is usually optically best (least distortion).
3. Sometimes because I wanted a higher-resolution capture of a smaller object.
As you've found, you basically have two choices. Zoom or don't. If you set the camera up so that it is capturing your entire platen area, that means that you might be "wasting" a lot of pixels taking pictures of the space around the book. Does this bother you? Then consider zooming.
I like the simplicity of not-zooming. Here is an example from the scanner I'm working on now that illustrates a way to NOT use zoom at all. I have a 9x12 inch scanning area. I want a minimum of 300DPI across the entire thing, but I don't really care about more than that. Basically, that implies that I need at least (9*300 X 12*300) = 2700*3600 image. Now of course you really more pixels than that - long story - but if you're imaging at 300 DPI across the entire platen it really doesn't matter if you zoom or not, because you're always capturing all pages at the same resolution all the time, whether they are big or small. Does that make sense?
miket161 wrote:Another question: I have read here, that when using two cameras, you want to take both pictures at the same time.
What is the reason behind this?
If you capture two pages at once you go twice as fast. The cameras don't need to fire at the exact same time - just to get close. It's a speed thing.