What Field of View Means
The field of view (FOV) is the circular area of the specimen you can see when you look through the eyepiece. It is usually given as the diameter of that circle, in millimeters or micrometers.
Field of view shrinks as magnification increases. At low power you might see several millimeters of the slide; with an oil immersion objective you may see only a fifth of a millimeter or so. Knowing the FOV lets you estimate the size of what you are looking at and keep track of where you are on the slide.
The Field of View Formula
For a compound microscope, the field diameter at the specimen is:
FOV = field number / objective magnification
- The field number (FN) is the diameter, in millimeters, of the opening (field stop) inside the eyepiece.
- The objective magnification is the number on the objective you are using.
Example: an eyepiece with FN 18 and a 10x objective gives 18 / 10 = 1.8 mm, or 1,800 µm.
Notice that the eyepiece’s own magnification does not appear in the formula. The eyepiece magnifies the view for your eye, but the field number is what sets how much of the specimen is included.
Where to find the field number
The field number is often printed on the eyepiece after the magnification, for example WF10x/18 or 10x/22. Common values are 18, 20 and 22 mm. Basic eyepieces sometimes have no field number marked; in that case, you can measure FOV directly, as described below.
If there is an extra magnification factor
If your microscope has an additional magnification factor between the objective and eyepiece, such as a built-in head factor or a magnification changer, include it:
FOV = field number / (objective magnification × additional factor)
For example, FN 20 with a 40x objective and a 1.25x head factor gives 20 / (40 × 1.25) = 0.4 mm. See our guide to calculating total magnification for more on where these factors come from.
Field of View Reference Table
These values use the formula with no additional magnification factor. Real fields can vary slightly from the calculated figure.
| Objective | Total with 10x eyepiece | FN 18 | FN 20 | FN 22 |
|---|---|---|---|---|
| 4x | 40x | 4.5 mm | 5.0 mm | 5.5 mm |
| 10x | 100x | 1.8 mm | 2.0 mm | 2.2 mm |
| 40x | 400x | 0.45 mm (450 µm) | 0.5 mm (500 µm) | 0.55 mm (550 µm) |
| 100x | 1000x | 0.18 mm (180 µm) | 0.2 mm (200 µm) | 0.22 mm (220 µm) |
How to Measure Field of View Yourself
Measuring is useful if your eyepiece has no field number marked, or if you want to confirm the calculated figure.
With a clear ruler at low power
- Place a clear plastic metric ruler on the stage, with the millimeter markings over the light.
- Use the 4x objective and focus on the markings.
- Line up one millimeter mark with the left edge of the circular field.
- Count how many millimeter spaces fit across the full diameter, estimating any fraction at the end.
If you count about 4.5 mm across, the FOV at 4x is 4.5 mm, or 4,500 µm.
A ruler works at 4x and sometimes at 10x. At higher powers the markings are too coarse and the ruler’s thickness can make focusing difficult, so use the proportion method or a stage micrometer instead.
The proportion method for higher powers
Because field diameter is inversely proportional to magnification, you can scale a known measurement:
Unknown FOV = known FOV × (known magnification / new magnification)
Example: the FOV at 40x total is 4.5 mm. At 400x total:
4.5 mm × (40 / 400) = 0.45 mm, or 450 µm.
You can use total magnifications or objective magnifications in this calculation, as long as you are consistent and the eyepiece stays the same.
With a stage micrometer
A stage micrometer is a slide with a precisely engraved scale, often 1 mm divided into 100 divisions of 10 µm each. Focus on the scale and count how many divisions span the field. This works well at 10x and 40x, and gives a more accurate result than a plastic ruler. The same slide is used to calibrate an eyepiece scale, as described in our guide to measuring specimens with a reticle and stage micrometer.
Calculating the Area of the Field
If you need the area of the field, for example when counting cells or particles per unit area, treat the field as a circle:
Area = π × (FOV / 2)²
For a 0.5 mm field: area = 3.1416 × (0.25 mm)² ≈ 0.196 mm².
Because area depends on the square of the diameter, small differences in field number make a noticeable difference to counts. A field with FN 22 covers about 21% more area than one with FN 20 at the same objective. If you compare counts between microscopes, convert to counts per mm² rather than counts per field.
Using Field of View to Estimate Specimen Size
Once you know the FOV, you can make a quick size estimate:
- Look at how many specimens, laid end to end, would fit across the diameter of the field.
- Divide the FOV by that number.
Example: at 400x, the field is 450 µm across. About 6 cells would fit across it. Each cell is roughly 450 / 6 = 75 µm.
This is a rough estimate, useful for comparing specimens or checking whether a size is plausible. For measurements you can rely on, use a calibrated eyepiece reticle.
Field of View on a Camera or Screen
A camera attached to a microscope usually does not capture the same circle you see through the eyepiece. The captured area depends on the size of the camera sensor, the objective, and any reduction or enlargement in the camera adapter.
A rough guide is:
Captured width = sensor width / (objective magnification × adapter factor)
Because sensors are rectangular and often smaller than the eyepiece field, the image on screen is often a cropped rectangle from the center of the eyepiece view. This is also why on-screen “magnification” claims for digital microscopes can be hard to compare.
Common Questions
How do you calculate the diameter of the field of view? Divide the eyepiece field number by the objective magnification (and any extra magnification factor). Or measure it at low power with a ruler and scale it with the proportion method.
What is the field diameter at 40x? If you mean the 4x objective (40x total), it is typically about 4.5 to 5.5 mm with common eyepieces. If you mean the 40x objective (400x total), it is about 0.45 to 0.55 mm.
What is the FOV at 100x magnification? With a 10x objective and 10x eyepiece (100x total), the field is typically about 1.8 to 2.2 mm. With a 100x objective (1000x total), it is about 0.18 to 0.22 mm.
Why does the field get darker as well as smaller at higher power? Higher-power objectives spread light from a smaller area of the specimen over the same eyepiece field, so the image tends to look dimmer. Opening the condenser aperture or increasing the light usually compensates. Higher magnification also brings a shallower depth of field, so less of a thick specimen is in focus at once.