Wednesday, 13 August 2008

Epson flatbed filmscanners

I've got a couple of these scanners (4870 and 3200) which I use for scanning 4x5 film and more recently 6x9 and 6x12 120 film. While they're quite good up to (somewhere around) 2400 dpi, I don't think there would be many people argue that they go beyond this the wise (in my opinion) are content with just 2400 or less.

A common pursuit is to adjust the height of the film holders to obtain better focus, and I've certainly tried that too. In fact its from making many investigations myself that I've come to the (more or less final) conclusion that while there may be gains in focus to be had, that there will be other problems introduced by this.

My theory is that the location of the film plane is a combination of two separate functions. One is of course focus, the other is to do with the physical location of the sensors.

the problem is created by the fact that film being scanned is not the same width as the sensor and that the light from the light source of the scanner is therefore always at 90 degrees to the sensor.

The figure at left here attempts to put this graphically. Here you can see that the scan head is not the same width as the film being scanned (when scanning at the full width of the scanners film capacity). Essentially the scanner is making some optical reduction of the film image onto the scanner 'head' surface. Since the array of receptors on the scan head (a bar that travels along the of the film and creates an image by sampling the line in a series of steps) is very dense this 'reduction' doesn't really matter.

Its a method of allowing a shorter sensor to scan a wider area. So, what does this mean?

Well, it means that

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