Once you can use a microscope, you can measure with it. The bright circle you see is the field of view. If you know how wide it is, you can estimate the size of a cell by how many fit across. Cells are small, so we measure them in micrometers (µm), using powers of 10.
Key idea
1 mm = 1,000 µm. The field of view shrinks as power grows: at 4 times the power it is a quarter as wide. Cell size ≈ field of view ÷ number of cells that fit across.
Why it works
Field width × magnification stays the same. If a field 4.5 mm wide at 40× is viewed at 400× (10 times the power), everything looks 10 times wider, so only $4.5 \div 10 = 0.45$ mm of the slide fits in the circle.
| Total magnification | Field of view (mm) | Field of view (µm) |
|---|---|---|
| 40× | 4.5 | 4,500 |
| 100× | 1.8 | 1,800 |
| 400× | 0.45 | 450 |
Worked examples
Worked example: New power, new field
At 40× the field is 4.5 mm. How wide is it at 100×?
- The power grows $100 \div 40 = 2.5$ times.
- The field shrinks 2.5 times: $4.5 \div 2.5 = 1.8$ mm.
Answer: 1.8 mm
Worked example: Estimate a cell
At 100× (field 1.8 mm), about 6 cells fit end to end across the field. How long is one cell?
- Change to micrometers: $1.8 \times 1000 = 1800$ µm.
- Share among 6 cells: $1800 \div 6 = 300$ µm.
Answer: About 300 µm.
Worked example: Change units
A red blood cell is about 0.008 mm across. How many micrometers is that?
- Multiply by 1,000: $0.008 \times 1000 = 8$.
Answer: About 8 µm.
Common mistake
Forgetting that the field shrinks at higher power, or mixing millimeters and micrometers by a factor of 1,000. Check: a cell you need a microscope to see should come out between about 1 and a few hundred µm.