Enter the field number (FN) and the objective magnification into the calculator to determine the field of view.

Field of View Calculator
Microscope field diameter is the main mode. A telescope true field of view tab is included for secondary search intent.
Microscope FOV
Telescope FOV
FN 18
FN 20
FN 22
Common microscope eyepiece field numbers are 18, 20, and 22.
4x
10x
40x
100x
Use the printed objective value such as 4x, 10x, 40x, or 100x.
0.5x
1x
1.5x
2x
Leave this at 1 if there is no reducer or auxiliary factor in the optical path.
50°
68°
82°
This is the eyepiece apparent field of view, usually listed by the eyepiece maker.
Use the telescope focal length in millimeters.
Use the eyepiece focal length in millimeters.
0.63x
1x
2x
Use 1 for no Barlow or reducer. A 2x Barlow doubles effective focal length.
Results
Metric 1
–
Metric 2
–
Metric 3
–
Metric 4
–

Field of View Formula

The following formula is used to calculate the linear field of view for a microscope (field diameter at the specimen plane) from the eyepiece field number and the objective magnification.

FOV = FN / M
  • Where FN is the eyepiece field number (a length, typically in mm; often printed like “FN 20”)
  • M is the objective magnification (e.g., 10 for 10×)
  • FOV is the linear field of view at the specimen plane (same length unit as FN)

To calculate microscope field of view, divide the field number (FN) by the objective magnification.

Field of View Definition

Field of view (FOV) is the extent of the observable area seen through an optical system. In microscopy it is often expressed as a linear size (e.g., the diameter of the visible circle at the specimen plane). In telescopes and cameras it is often specified as an angular field of view (degrees), which is calculated differently.

Field of View Example

How to calculate a field of view?

  1. First, determine the field number (FN).

    Find the eyepiece field number (often labeled “FN” and given in mm).

  2. Next, determine the objective magnification.

    Use the objective magnification (e.g., 10×, 40×).

  3. Finally, calculate the field of view.

    Calculate the field of view using the equation above.

FAQ

What is a field of view?

Field of view (FOV) is the extent of the observable area. For microscopes it is commonly the linear diameter/width of the visible area at the specimen plane; for many other optical systems it may be given as an angular field of view (degrees).