How to measure an object's real size on the image?

Question

I have a picture of a melanoma. I need to calculate its real size (eg mm^2). Any ideas how to this? Or, how many pixels are in 1 mm^2?

Answer 1

You need to know the distance from the camera to the object, the focal length of the lens, and the pixel spacing of the imaging sensor (or the size of the sensor in both mm and pixels). Or you could put a calibration measure (such as a ruler) next to the object at the same distance.

If you don't have those, a lady bug and an elephant could take up the exact same number of pixels, so the answer could be almost anything.