Sunday, 10 January 2010
How to Scan 35mm Slides Using a Flat-bed Scanner And an iPod As a Backlight
I had a few old 35mm slides I wanted to scan this weekend. Not wanting to shell out for a dedicated slide scanner, it seemed like I should be able to use my flat-bed scanner instead. It turns out there are a couple of problems with this. First of all, these kind of scanners are designed to scan images that are flat on the glass, and so mounted slides (which are offset slightly from the glass) appear very out of focus. This is easily fixed by removing the slide from the mount and putting on the glass directly. The second, more serious problem is that the scanner is designed to measure light reflected from paper, where as slides are meant to illuminated from the back (transmissive), so they usually turn out much too dark in the scan. Searching for answer, I found this link, where someone has built a simple backlight out of paper that attempts to reflect light from the scanner around the back of the slide. Unfortunately this didn't work for me, probably because my Canon Lide scanner uses LEDs for illumination. So instead I had the idea to use my iPod, displaying a pure white image, as a backlight. This works amazingly well. The iPod screen fits flush on the glass, and you can change the display brightness to adjust the exposure. Sadly it only works in black and white, because the white light interferes with the R/G/B illumination of the scanner, but the results are still pretty good (see above).