I've been experimenting with OCR, an iPod touch and lenses. Objective was being able to capture text, up to 4 characters at a time (chengyu), over as wide a range of character sizes as possible.
There is a lot of leeway in how much magnification to use. More magnification shifts OCR use towards smaller point sizes. However, at higher magnification you will need a (very) steady hand to focus (smaller depth of field). As an experiment, you might try holding a small magnifier, eye loupe or linen tester in front of the ipod touch camera.
In general, the best lens is the lens with the lowest magnification which still does the job. Good results were obtained with lenses with a magnification of x3 to x4. A magnification of x3 to x4 corresponds to a diopter of +12 to +16, or a focal distance of 8.3 to 6.25 cm. In the end, I settled upon a lens with a focal distance of 7.5 cm (magnification x3.33). (
Achromat f = 75mm) This allows OCR on characters between 6 pt and 24 pt, with the "sweet spot" being about 12 pt. To get meaningful results for 6pt text, I need to select the "camera icon" and zoom in; for 24pt text I need to select the "camera icon" and zoom out.
To focus the image, adjust the distance between iPod and book. You'll get a sharp image when the distance is the focal distance. Once focused, it helps if the iPod rests on something: if you hold the iPod in your hand the image can be jittery, especially at small point sizes. But in the end, there is only so much one can do. iPod Touch camera performance is profoundly unexciting.
Summarizing: if you have an iPod OCR can be made to work by adding a lens, but the user experience is not as intuitive as with an iPhone.
