Flatbed manufacturers like to make us feel good about our purchase. They encourage us to believe that the $200 scanner we just bought can infact scan at 9600ppi. We need to be aware of the substantial difference between the 'optical' and 'interpolated' capability of our scanner.. The quality of the resulting scans are worlds apart.

Look at a box that a scanner comes in. Somewhere on it is a claim that reads 'scans at 9600ppi' or similar. So what do they really mean? and what about those other figures? 300x600? on the same box?

Say we run a scan at 9600ppi on one of these flatbeds. The flatbed will carry out the scan at it's highest optical resolution and 'interpolate' the rest. Interpolation is a fancy word for calculated guessing! It's an amazing idea but it is not your friend. The scanning software is only using a very primitive means of guessing - the resulting image does not contain true pixel information - just made up stuff. It won't look too good!

The true optical resolution will be the lowest number (and the smallest print) you can see on the box or in the book that hopefully came with the scanner. Typically it is 300, 600 or 1200ppi.  So our $200 flatbed is really only scanning at 300ppi and inflating the resolution to 9600ppi by guessing the pixels that might have been there and adding them in!

The main point here is that there is no point at scanning at a higher resolution than the true optical capability of the flatbed.

If you really do need to increase the resolution above the optical resolution of the scanner, it is better to do it in photoshop - at least you can choose which interpolation mode it will use to resample the image as well as having the tools to repair the inevitable loss of image quality.

Reflected Light Problems

Flatbeds are great for turning anything from body parts to art work into digital files we can then hack at our leisure, but they suffer from a fatal flaw - reflected light!

Imagine you are in a church looking up at a stained glass window that has light streaming through it. The sun is transmitting light through the window and your eyes are the CCD capturing the light. The window looks great! the colours are saturated, the edges are sharp, it's alive!

By chance at home later on you come across a picture in a magazine of the same window. You hardly recognize it! The ambient light is falling on the page, some of it absorbed into it, some reflecting towards your eye.. hmm, it looks so flat, unsharp and lifeless. Not too good compared to the real thing!

This is how a flatbed works so straight away we know that we will more than likely have to deal with these flaws - poor tonal range, flat colours and soft images.. maybe that's how they called them 'flat'beds!