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[GaryK]

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[Turtle91]

Hi Mike,
Welcome to the forum!

In no way do I mean to put myself forward as an expert - I'm just the unemployed (unemployable??) guy that is building my first scanner and have been reading these forums A LOT. There is sooo much here that is sooo fascinating... And I'm lucky enough to be the first one to read your post!!!

From what I've read you want to have the optical axis of the camera centered on the center of your page and use the optical zoom to fill the camera lens with just the page you're scanning. I think that has to do with not wasting megapixels taking pictures of stuff you don't need. But I also think it saves time in pre-processing when you don't have to crop out extra stuff. Having the optical axis centered and normal to the page minimizes a lot of the errors inherant in scanning (keystoning, skew, etc.)

The camera position is limited by your scanner design. You don't want the camera in front of your lights because it will cast shadows, and you don't want it to be in the way of a second camera's view. I don't know enough about optical zoom to know if using it sacrifices pixels??? But I would think - in general - you want your camera as close as possible to the page without creating shadows or interfering with the other camera. There are other issues of camera placement you can think about - like wether you want it on a tripod or other exterior mount or if you want to mount the camera to the platen itself so you can have a fixed focal range and no shifting of the image throughout the book scan. Fun stuff!

As far as taking both pictures simultaneously - I haven't read where someone said that was required or desired. I can make a guess though: Software that some people use to remotely control their camera is called SDM (stereo data maker). The SDM software is a sub-set of the CHDK (canon hack developers kit). It takes 2 pictures simultaneously of the same object from different angles to get 3-dimensional data about the object. In book scanning we aren't too worried about the 3-D aspect (unless you get into the process of automatically detecting the surface shape for correction purposes - pretty interesting topic found here http://www.diybookscanner.org/forum/viewtopic.php?f=1&t=788). They are mostly interested in using the software to remotely trigger the cameras.

I think the biggest reason for triggering both cameras at the same time is to save time. If you have to trigger each camera seperately then that doubles the amount of time between changing pages - significantly extending the amount of time it takes to scan your project.

I am like you - I like to get everything together in my head first. You don't happen to be an engineer do you?? There are a lot of different aspects to this project and you CAN get caught up in thinking about each of them sooo much that the project itself never gets started! I am in the middle of my first scanner and I have noticed that tendency. There are just so many interesting things that take your research off in new directions...

Cheers!
Dion

[daniel_reetz]

Hey Miket161, as Turtle91 said - welcome. Glad to have you. Turtle's answers are great. I'll just add a little.

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.

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?

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.

[GaryK]

xxx

[daniel_reetz]

Questions are always welcome around here, and in case there is any doubt, I always welcome our resident "non-experts" (ha, what does that even mean on a project where you're building the tech from scratch?) to chime in and answer.