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Microscope Field of View Calculator

Field of view, total magnification and resolution — where going from 10x to 40x is four times narrower but sixteen times less area to look at.

The field number is stamped on the eyepiece, usually after the magnification — "10×/22" means a 22 mm field number. The resolution figure is the Abbe limit at 550 nm, which is a theoretical ceiling: real lenses fall short of it, and condenser aperture, mounting medium and stain all matter. Treat it as the best the objective could do rather than what your slide will show. Nothing is uploaded.

Four times the magnification is sixteen times less to look at

Field of view is the field number over the objective, so the diameter falls in step — 10× to 40× is exactly 4× narrower. But you are looking at a circle, and its area falls with the square: 16× less. Across the whole turret, 4× to 100× is 25× narrower and 625× less area, which is why finding a thing at low power and then climbing is the only workable order.

ObjectiveNATotal (10× eyepiece)FieldAreaResolution
4× 0.10 40× 5.500 mm 23.7583 mm² 2.750 µm
10× 0.25 100× 2.200 mm 3.8013 mm² 1.100 µm
20× 0.40 200× 1.100 mm 0.9503 mm² 0.688 µm
40× 0.65 400× 0.550 mm 0.2376 mm² 0.423 µm
60× 0.80 600× 0.367 mm 0.1056 mm² 0.344 µm
100× 1.25 1000× 0.220 mm 0.0380 mm² 0.220 µm

The eyepiece matters as much as the objective here, and it is easy to forget: a field number of 26.5 shows more than twice the area of an 18, through the very same objective.

Field numberField at 40×Area
18 0.450 mm 0.1590 mm²
20 0.500 mm 0.1963 mm²
22 0.550 mm 0.2376 mm²
26.5 0.662 mm 0.3447 mm²

Magnification is not resolution, in both directions

Resolution comes from the numerical aperture alone — an eyepiece adds magnification and no detail at all. Past roughly 1000 × NA the image is simply bigger, which is what "empty magnification" means: a 25× eyepiece on a 40×/0.65 objective reaches 1000× against a ceiling of 650×. The half that goes unsaid is the other end — the 4× and 10× objectives sit BELOW their useful range with a standard 10× eyepiece, so they resolve more than that magnification lets the eye collect.

EyepieceTotal on a 40×/0.65Verdict
5× 200× still carrying detail
10× 400× still carrying detail
15× 600× still carrying detail
20× 800× empty magnification
25× 1000× empty magnification
40× 1600× empty magnification

An oil objective at NA 1.25 reaches about 0.22 µm, while the 4× at NA 0.1 manages 2.75 µm — a difference of more than tenfold that magnification alone can never make up. Useful magnification runs from about 500 to 1000 times the numerical aperture; the useful range is what the objective earns, not what the eyepiece asks for.

How to use

  1. Enter the objective magnification.
  2. Enter the eyepiece magnification and its field number.
  3. Enter the numerical aperture of the objective.
  4. Read the field of view, area, resolution and whether the magnification is useful.

Frequently asked questions

How do I calculate field of view?

The eyepiece field number divided by the objective magnification. A field number of 22 through a 40x objective gives 0.55 mm across, and through a 4x it gives 5.5 mm.

How much less do I see at higher magnification?

Far more than the numbers suggest. Going from 10x to 40x is four times narrower but sixteen times less AREA, because you are looking at a circle. Across the whole turret, 4x to 100x is 25 times narrower and 625 times less area.

What is the field number?

The diameter in millimetres of the field stop in the eyepiece, usually stamped after the magnification — "10x/22" means 22. It matters as much as the objective: a 26.5 shows more than twice the area of an 18 through the same objective.

What is empty magnification?

Magnification past what the optics can resolve, which is roughly 1000 times the numerical aperture. A 25x eyepiece on a 40x/0.65 objective reaches 1000x against a ceiling of 650 — bigger, with nothing new in it.

Does a stronger eyepiece show more detail?

Only up to a point, and only at the high-power end. Resolution comes from the numerical aperture alone, so an eyepiece adds magnification and no detail. Past 1000 times NA the extra magnification is empty.

Can magnification be too LOW as well?

Yes, and it is rarely mentioned. With a standard 10x eyepiece the 4x and 10x objectives sit below their useful range of 500 to 1000 times NA, so they resolve more than that magnification lets the eye collect.

How fine a detail can an objective resolve?

The Abbe limit is half a wavelength divided by the numerical aperture. At 550 nm an oil objective at NA 1.25 reaches about 0.22 micrometres, while a 4x at NA 0.10 manages only 2.75 — more than a tenfold difference.

Does this send anything anywhere?

No. Every figure is computed in your browser, and nothing is uploaded or stored.

🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.