Field of View of the Hacked QuickCam 2500 from Logitech
The imprint on a petri dish serves as reference to measure the field of view. Small patterns as this are of good help to find correct focus and magnification too.
The 4.5 mm dimension was measured using a caliper. From there the other dimensions can be calculated by measuring the size of the letters in pixels.
This picture shows the detail using the original lens of the cam, just put more far away from the CCD Chip.
You can get stonger magnification by inverting the lens.
So once you know the size of the field of view you can determine the dimensions on your pictures.
This is a nematode captures with the Inverted-Lense Camera: