About this tool
The Microscope Magnification & Field of View Calculator is a free, in-browser tool for light microscopy. Enter the objective magnification, the eyepiece (ocular) magnification, and the eyepiece field number, and it returns the total magnification and the diameter of the field of view in both millimetres and micrometres. It can also estimate a specimen's true size from a measured image dimension. All maths runs locally in your browser.
Total magnification is simply objective × eyepiece, so a 40× objective with a 10× eyepiece gives 400×. The field of view diameter at the specimen is the field number divided by the objective magnification: FOV = field number ÷ objective. With a field number of 20 mm and a 40× objective, the field of view is 20 ÷ 40 = 0.5 mm, i.e. 500 µm.
The field number is a property of the eyepiece (commonly 18–22 mm) and is printed on it, which is why the field of view uses the objective — not the total — magnification. If you enter a measured size seen down the scope, the true specimen size is that measurement divided by the total magnification. This is optical microscope field of view, distinct from a camera sensor's field of view.