Microscope Field of View
The diameter of the field of an optical light microscope is the field number, which is the diameter of the field view in mm measured at the intermediate image plane. The field number is typically listed on the microscope eyepiece. The microscope field of view is found with the following formula:
Field of View = Field Number (FN) ÷ Objective Magnification
If an auxiliary lens is being used on a stereo microscope, the magnification factor of this lens should also be employed in the equation by multiplying this with the objective magnification as shown below:
Field of View = FN ÷ (Objective Magnification x Auxiliary Lens Magnification)
Typically the lower the magnification of the eyepiece, the higher the FN is. So for example, a 5x eyepiece might have a FN of 26mm, while a 10x eyepiece may be 22mm and a 30x eyepiece may only have a FN of 7mm.
The higher your magnification, the smaller the microscope field of view will be. If you think of looking at the above aphid through the microscope, if you were to zoom in to view only the leg of the aphid, your field of view would definitely be smaller, while the magnification is increased.