[gourneau]

For my use it is just arbitrary. SimpleCV does have the ability to calibrate cameras, which should give you what you want.

[nightski]

You can calibrate the camera, estimating the intrinsic params such as focal length. But for size you would need two images and to know the location each were taken relative to each other (extrinsic params). This is not impossible, just more difficult than a simple calibration and photo snap.

On top of that, you will have to be able to identify the object you are trying to measure (unless it is a manually driven process).

[lost_my_pwd]

What about follicle width? There is variation from person to person, and even on a single person, but I am sure you could find an average that is suitable.