Measure Anything with a Camera and Software
Kevin C. Tofel writes "Using a simple concept, iPhotoMEASURE software can measure any objects you can take a picture of. Include a printout of a 7.5- or 15-inch square in the photo and the software can measure any distance or object in the pic to within 99.5% accuracy. Although geared towards contractors, there's any number of consumer usage scenarios as well. Enough to justify a $99 price tag? Jury's still out on that."
All it does is count pixels. Take the picture with the known size block in it, then count how many pixels the block takes up: that's your dots per inch -- for objects the same distance away as the known size image. I read somewhere that the Japanese (I think) used to do this at musical instrument trade shows, by wearing a 1 cm square tie tack and taking photos of each other holding instruments. They could get the dimensions of the instrument from the photos that way, and make great cheap knock-offs.