Avoid blurry corners

[geaplanet]

I have taken photos with several cameras and always have the same problem: the center appears perfectly but corners are always unsharp. I've also changed the lighting and still have the same problem.
Is there a trick to get clarity on the whole page?
The problem is in the used cameras?
samsung s760
canon powershot a410
nikon coolpix p100

I'm going to buy a new camera (350€-400€ max), does anyone could recommend one with not having trouble getting perfect scanned pages?

Thanks in advance.

PD. Data: some books are in A3.

[dpc]

You may want to start here.

[duerig]

In my personal experience, it is much easier to make sure everything is in focus when you have a DSLR. The advantage here of a DSLR is that you can change their aperture size. Smaller aperture means a darker picture (you need more light), but a larger depth of view (easier to keep everything in focus). The cheap point and shoot cameras do not have this capability. So perhaps you can find a refurbished DSLR in your price range, verify that it is compatible with gphoto2, and use that.

[geaplanet]

Thanks for your answers.

Why is important gphoto2 compatibility?

[duerig]

gphoto2 is a project that lets you control your cameras from a computer. This includes remote capture but also automatically downloading the resulting pictures. The former is important because touching the camera each time you take a photo means more opportunities to accidentally move it out of alignment. The latter is very convenient and allows you to automatically interleave left and right pages and such depending on what program you use.

For the higher-end cameras (like DSLRs), gphoto2 is the computer interface. For cheaper Canon point and shoot cameras, the same thing is done with CHDK.

[rkomar]

If you're using CHDK, and have the camera fixed at some distance from the page, then you can take some pictures at slightly different focal lengths, and see which gives the best result over the whole page. Then use that good focal length for all of the rest of the shots rather than let the camera refocus on each shot. With a point and shoot camera, you may have to sacrifice some sharpness at the center of the image to gain some at the edges.

[dpc]

There are a number of tethered camera control options for DSLRs. I use a pair of Canon DSLRs and Canon offers a free PC app that allows you to control pretty much every camera option via a USB connection, including image preview and file transfer to the PC. Canon also offers a free SDK (free to US residents) that has an C/C++ API that allows you to write an app that offers the same sort of camera control. I've written software to control my T2i cameras and it works quite well.

Here's a link that I found that describes a number of these "tethering" programs: Best Tethering Photography Software for DSLR Cameras

[powercircuits]

A camera can only focus over a certain range of distances at once. This is known as the 'depth of field'. Since the center of the page will be closer to the lens than the corners, you need enough depth of field to allow both to be in focus at the same time.

In general, a lens with a smaller aperture will have a larger depth of field (with a pinhole camera, everything is in focus regardless of distance). On the other hand, a smaller aperture means less light is collected so you either need brighter lighting or longer exposures to make up for it.