Microscope basics
Microscope field of view calculator
Work out how wide the circle you see in the eyepiece is, carry a measured field from low power to high power, find the area a camera captures, and estimate the size of a cell from how many fit across the field.
The field of view formula
FOV diameter = field number ÷ (objective magnification × auxiliary lens magnification)
The field number (FN) is the diameter, in mm, of the eyepiece's field stop, and is often engraved on the eyepiece after the magnification (for example "10×/22"). MicroscopyU gives 6 to 28 mm as the overall range and 16 to 18 mm as typical for a 10× eyepiece; research microscopes often use 22 mm or more. The eyepiece magnification itself does not appear in the formula, because the field number already describes what the eyepiece shows.
Worked example: FN 22 with a 40× objective: 22 ÷ 40 = 0.55 mm, or 550 µm. With a 10× objective: 2.2 mm.
Field of view at high power from low power
In a class lab you usually measure the field once at low power, with a transparent ruler or a stage micrometer, and calculate the rest. The field diameter scales inversely with the total magnification:
FOV(high) = FOV(low) × magnification(low) ÷ magnification(high)
Worked example: 4.5 mm (4500 µm) at 40× total. At 400× total: 4500 × 40 ÷ 400 = 450 µm. If about 10 cells fit across that field, each cell is roughly 45 µm.
Field of view of a microscope camera
A camera sees a rectangle, not the eyepiece circle. Its size at the sample is the sensor size divided by the magnification between sample and sensor (objective × camera adapter):
field width = pixels across × camera pixel size ÷ (objective × adapter)
Worked example: a 2048 × 2048 sCMOS camera with 6.5 µm pixels on a 20× objective and a 1× adapter: 2048 × 6.5 ÷ 20 = 666 µm on each side, at 0.325 µm per pixel. A file from the microscope usually records this pixel size in its metadata, which is how viewers draw a correct scale bar.
Questions
How do you calculate the field of view of a microscope?
Divide the eyepiece field number (mm) by the objective magnification. A field number of 20 with a 10× objective gives a 2 mm field of view; with a 40× objective, 0.5 mm.
Where do I find the field number?
It is engraved on the eyepiece after the magnification, for example "10×/20" or "WF10×/22". If it is missing, measure the field at low power with a stage micrometer or a clear ruler.
How do you find the field of view at high power?
Multiply the low-power field diameter by the low-power magnification and divide by the high-power magnification. 4.5 mm at 40× becomes 4.5 × 40 ÷ 400 = 0.45 mm at 400×.
How do I estimate cell size from the field of view?
Count how many cells would fit across the field diameter and divide: a 450 µm field that holds about 10 cells end to end means each cell is about 45 µm.
Is this calculator free?
Yes. It runs in your browser and sends nothing anywhere. SlideScope, the desktop microscopy app made by the same team, is a paid app with a free preview.
Sources
Results are theoretical or arithmetic; check them against your instrument's documentation before relying on them for clinical or regulated work.
Measure on your real images
SlideScope opens 34 microscope and slide-scanner formats, CZI, ND2, SVS and LIF among them, with their own calibration, so lengths, areas, cell counts and scale bars come out in µm without setting anything up. Open a few of your own images with the free preview. Already decided? Subscribe now.