What “field of view” means
In microscopy, the field of view (FOV) is the diameter of the circular region visible at the
specimen plane. When you increase magnification, the visible circle typically becomes smaller.
Important: This calculator reports
FOV diameter (and optionally the
radius).
If you need an
area, you can compute it from the radius:
\[
A=\pi r^2
\]
Units: mm and µm
Microscope specifications often use millimeters at the eyepiece and micrometers at the specimen.
The conversion is:
\[
1\ \text{mm}=1000\ \mu\text{m}
\qquad\text{and}\qquad
1\ \mu\text{m}=0.001\ \text{mm}
\]
Approach A: Using the eyepiece Field Number (FN)
Many microscopes specify an eyepiece Field Number (FN) in millimeters (mm).
You might see it printed on the eyepiece, for example:
WF10×/20 which indicates FN = 20 mm.
Under the common assumption that the objective magnification dominates the scaling at the specimen plane,
the FOV diameter at the specimen is approximated by:
\[
\text{FOV diameter at specimen}=\frac{FN}{M_{\text{objective}}}
\]
Interpretation: If FN is fixed, a larger objective magnification \(M_{\text{objective}}\)
produces a smaller specimen-plane FOV.
Approach B: Scaling between objectives (ratio method)
If you already know the FOV at one magnification (from a calibration slide, a manufacturer table, or prior measurement),
you can scale to another magnification using:
\[
\text{FOV}_2=\text{FOV}_1\cdot\frac{M_1}{M_2}
\]
This expresses the idea that FOV is approximately inversely proportional to magnification.
Worked examples
Example 1: FN method
Suppose the eyepiece field number is \(FN=20\ \text{mm}\) and you use a \(40\times\) objective.
\[
\text{FOV}=\frac{20}{40}=0.50\ \text{mm}=0.50\times 1000=500\ \mu\text{m}
\]
Radius (optional) is \(r=\frac{0.50}{2}=0.25\ \text{mm}=250\ \mu\text{m}\).
Example 2: Scaling method
Suppose \(\text{FOV}_1=2.0\ \text{mm}\) at \(M_1=10\times\). Find \(\text{FOV}_2\) at \(M_2=40\times\).
\[
\text{FOV}_2=2.0\cdot\frac{10}{40}=2.0\cdot 0.25=0.50\ \text{mm}=500\ \mu\text{m}
\]
Typical ranges and common “sanity checks”
-
Field Number (FN): many teaching/lab microscopes are commonly around 18–26 mm.
Values far outside that range may indicate a mistaken input or units.
-
FOV diameter: microscope specimen-plane FOVs are commonly from a few mm (low power)
down to hundreds of µm (higher power), depending on optics.
-
If your computed FOV is extremely large (for example > 10 mm) or extremely tiny (for example < 20 µm),
re-check whether you entered FN in mm and magnification as a pure number (e.g., 40 for 40×).
Why the diagrams are useful
-
The single-circle diagram helps you “feel” the diameter at the specimen plane and shows a scale bar.
-
The two-circle comparison visualizes how the FOV shrinks when magnification increases.
-
The common magnifications table lets you quickly estimate FOV across 4×, 10×, 20×, 40×, 60×, 100×.
Practical tip: For more accurate measurements, calibrate with a stage micrometer (or an image scale bar)
and then use the scaling method across objectives.
