I'm doing project which involve **analyzing objects width** based on taken photo with given precision about 0.1 mm. I got to the point where i have to choose which camera should i buy in order to obtain desired **photo precision**.

Let's say i found 5MP camera. My question is how can i determine what is the "real" value that coresponds to camera's resolution? I read about **DPI** which has application in printers area and **PPI** which has application in displays. 

  [1]: http://www.elpcctv.com/mi5100-5mp-usb-camera-module-usb20-aptina-125inch-color-cmos-sensor-100degree-lens-p-221.html