title: Is there a way to find mm per pixel value for a camera?
body: <p>I need to implement dimension inspection of an object with a tolerance of 20 microns using image processing. To measure the dimension in mm, i need the mm per pixel value for pixel to mm conversion.</p>
  
  <p>Camera and lens Specifications:
  5 MP Matrix vision camera (2592 x 1944)
  25 mm lens</p>
  
  <p><strong>How i tried to do it:</strong> 
  I used a 30 cm ruler to get the actual field of view in mm covered by the camera.I got a plot of the image using Matplotlib function in OpenCV as shown in the fig.
  <a href="https://i.stack.imgur.com/BnbFF.png" rel="nofollow noreferrer">Image for scaling</a></p>
  
  <p>From the image i got 31 mm as the actual width covered by the camera and the camera resolution is 2592 x 1944. So i obtained mm/pixel = 31/2952 = 0.011959876.
  But i want to know if it is the correct way to find the mm/pixel value using a centimeter scale specially when tolerance of 20 micron is needed in dimension inspection. If this is not the correct way, then a solution procedure for finding mm/pixel value would be really helpful.</p>