Scanning Traditional Art
Intro
If you wish to scan art larger than your flatbed scanner was originally made to handle, this tutorial will show you how to get the best results. The technique covered in this tutorial will work for flatbed scanners of all sizes but for now I will focus on an A4 (and letter-sized) flatbed scanner as they are quite common. Let's get to it.
The Setup
For this little art project I have my trusty Canon A4/Letter-sized scanner and an unfinished A3 acrylics painting (A3 is twice the size of A4 and obviously doesn't fit in the scanner all at once).
The Setup
The Scanning
So I need to scan the art work in 3 steps: top, middle and bottom as shown in the picture below.
The 3 scanning steps

Once I have scanned all three parts and imported them into my image editing program, I need to assemble the three digitally scanned parts. For this I make a new document large enough to fit all three scanned parts. Next, in my image application, I need to remove parts of the three scanned images. I use the select tool to select and delete the un-numbered parts from approximately where the red stippled lines are displayed in the image above. The digital parts I wish to keep are the ones represented by the areas numbered 1, 2 and 3. The rest I will delete since they haven't been in full contact with the scanner surface while I scanned. Therefore these parts will be differently lighted and possibly blurred also.
Assemble The Parts
After I have removed the unwanted parts described above, I need to put the three remaining parts together. For this I create 3 new layers in the new document I created earlier, one for each scanned part. Next I have to manually move each scanned part into place so they fit together. It's a bit like a puzzle and requires a little work. Once that is done I can still see, just barely, where the parts come together. To eliminate these 'seams' I adjust a few parameters as described in the next part.
Adjustments
I need to adjust the brightness/contrast levels until the seams are fully gone. I zoom in real close before I adjust to get the best results. Once that is done I merge the layers and crop the image. The job is done.
Wait, there's more!
Before I end this art lesson remember that the technique described in this tutorial works on even larger sized art also. The key is to scan the art in separate parts, delete unwanted sections of these parts, assemble the rest and finally adjust the contrast and brightness levels. That's it. I hope you found this short art tutorial useful and thanks for reading.