I analyzed a bunch of microphotographs recently and used "% of image area" to describe the area of structures (chorionic villi, had to show their density in placenta). However my professor suggested I should have used mm2. There is no problem in conversion, but I suspect different microscopes may have different areas of field of view for the same scale. Thus the conversion will make the results just more difficult to interpret.
So does the absolute area of microphotographs of the same scale and size differ between microscopes/cameras? Or is it somehow standartized?