# What is microscope magnification?

Finds a compound microscope’s total magnification from the eyepiece and objective, the field of view from the field number, the field of view at another magnification, a specimen’s actual size from its image, or the magnification of an image.

- Page: https://www.acalculator.org/biology/microscope-magnification-calculator
- JSON spec: https://www.acalculator.org/biology/microscope-magnification-calculator.json
- Version: 1b5568c517c4

## Default answer

Example with the default inputs (Find Total magnification, Eyepiece (ocular) power (×) 10, Objective power (×) 40, Eyepiece field number (mm) 20): 400×; field of view 0.5 mm (500 µm).

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| find | Find | Total magnification, field of view at another magnification, actual size, or an image’s magnification. |
| e | Eyepiece (ocular) power (×) | The eyepiece lens magnification, usually 10×. |
| o | Objective power (×) | The objective lens in use: 4×, 10×, 40× or 100× on most microscopes. |
| fn | Eyepiece field number (mm) | The field number printed on the eyepiece, such as 18, 20 or 22 mm; leave empty to skip the field of view. |
| f1 | Field of view you know | The field of view measured at one magnification. |
| m1 | Its total magnification (×) | The total magnification at which you know the field of view. |
| m2 | New total magnification (×) | The total magnification for the new field of view, with the same eyepiece. |
| i | Image size | The size of the image: measured on a drawing or photo, or on the screen. |
| m | Magnification (×) | The magnification of the image. |
| a | Actual size | The real size of the specimen. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| result | Answer | The magnification, field of view or actual size worked out. |
| magnification | Magnification (×) | The total magnification, or the image’s magnification. |
| fovMm | Field of view (mm) | The diameter of the circle you see, in millimeters. |
| fovUm | Field of view (µm) | The diameter of the circle you see, in micrometers. |
| sizeUm | Actual size (µm) | The specimen’s real size in micrometers. |
| sizeMm | Actual size (mm) | The specimen’s real size in millimeters. |

## Method

Total magnification = eyepiece × objective; field of view = field number ÷ objective; FOV₂ = FOV₁ × M₁ ÷ M₂; actual size = image size ÷ magnification; magnification = image size ÷ actual size.

## Assumptions

- A compound light microscope with no extra magnification between the objective and the eyepiece (no zoom or magnification changer).
- The field of view scales with the objective when the eyepiece stays the same, so total magnifications compare the same way.

## Worked examples

1. find = total, e = 10, o = 40, fn = 20 gives magnification = 400, fovMm = 0.5, fovUm = 500, result = 400×; field of view 0.5 mm (500 µm). Source: OpenStax, Microbiology, §2.3 Instruments of Microscopy (total magnification = ocular × objective; 10× ocular with a 40× objective is 400×), https://openstax.org/books/microbiology/pages/2-3-instruments-of-microscopy; Nikon, Glossary of Microscopy Terms: field of view (field number ÷ objective magnification, in mm), https://www.microscope.healthcare.nikon.com/resources/glossary/field-of-view: 20 ÷ 40 = 0.5 mm.
2. find = total, e = 10, o = 100 gives magnification = 1,000, result = 1,000×. Source: OpenStax, Microbiology, §2.3 Instruments of Microscopy (total magnification = ocular × objective; 10× ocular with a 40× objective is 400×), https://openstax.org/books/microbiology/pages/2-3-instruments-of-microscopy: objectives from 4× to 100×, oculars usually 10×.
3. find = fov, f1 = 0.0045, m1 = 40, m2 = 400 gives fovMm = 0.45, fovUm = 450. Source: Nikon, Glossary of Microscopy Terms: field of view (field number ÷ objective magnification, in mm), https://www.microscope.healthcare.nikon.com/resources/glossary/field-of-view: the field of view is the field number ÷ the objective, so it scales as 1 ÷ magnification.
4. find = actual, i = 0.05, m = 2,000 gives sizeUm = 25, sizeMm = 0.025, result = Actual size 25 µm (0.025 mm). Source: OpenStax, University Physics Volume 3, §2.4 Thin Lenses (m = h_i ÷ h_o, equation 2.22), https://openstax.org/books/university-physics-volume-3/pages/2-4-thin-lenses: object size = image size ÷ m.
5. find = mag, i = 0.05, a = 0.000025 gives magnification = 2,000, result = Magnification 2,000×. Source: OpenStax, University Physics Volume 3, §2.4 Thin Lenses (m = h_i ÷ h_o, equation 2.22), https://openstax.org/books/university-physics-volume-3/pages/2-4-thin-lenses.

## FAQ

### How do you find the total magnification of a microscope?

Multiply the eyepiece (ocular) power by the objective power. A 10× eyepiece with a 40× objective magnifies 10 × 40 = 400 times; with the 100× oil-immersion objective it is 1,000×.

### How do I work out the field of view?

Divide the eyepiece’s field number by the objective power. The field number, in millimeters, is printed on the eyepiece: a field number of 20 with a 40× objective gives a field of view 20 ÷ 40 = 0.5 mm (500 µm) across.

### How does the field of view change at higher magnification?

It shrinks in the same proportion. A field 4.5 mm across at 40× is 4.5 × 40 ÷ 400 = 0.45 mm across at 400×, with the same eyepiece.

### How do I calculate the actual size of a specimen?

Divide the image size by the magnification, in the same units. A cell 50 mm across in a drawing at 2,000× is 50 ÷ 2,000 = 0.025 mm, or 25 µm, across.

### How do I calculate magnification from a drawing or photo?

Divide the image size by the actual size, after putting both in the same unit. An image 50 mm long of a 25 µm cell: 50 mm = 50,000 µm, and 50,000 ÷ 25 = 2,000×.

### Does a higher magnification always show more detail?

No. Detail depends on resolution, which a light microscope reaches at about 1,000×. Beyond that the image gets bigger but not sharper (OpenStax Microbiology).

## Sources

- OpenStax, Microbiology, §2.3 Instruments of Microscopy: total magnification is the ocular magnification times the objective magnification (40× objective with a 10× ocular: 400×); oculars usually 10×, objectives 4× to 100×; brightfield images sharp up to about 1,000×. https://openstax.org/books/microbiology/pages/2-3-instruments-of-microscopy (retrieved 2026-10-19)
- Nikon, Glossary of Microscopy Terms: field of view, the diameter of the area seen, found by dividing the field number by the magnification of the objective (mm). https://www.microscope.healthcare.nikon.com/resources/glossary/field-of-view (retrieved 2026-10-19)
- OpenStax, University Physics Volume 3, §2.4 Thin Lenses: magnification m = h_i ÷ h_o (equation 2.22). https://openstax.org/books/university-physics-volume-3/pages/2-4-thin-lenses (retrieved 2026-10-19)
- OpenStax, Biology 2e, §4.1 Studying Cells: light microscopes magnify up to about 400 times, about 1,000 times with oil immersion. https://openstax.org/books/biology-2e/pages/4-1-studying-cells (retrieved 2026-10-19)