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Microscope magnification calculator: total magnification and real size

This microscope magnification calculator multiplies the magnification of the objective by that of the eyepiece – the overall magnification of a compound microscope, as OpenStax College Physics explains. Example: a 40× objective with a 10× eyepiece magnifies 400 times.

Example: Objective magnification 40 × · Eyepiece magnification 10 × → Magnification: Total magnification 400×. Source: OpenStax College Physics 2e, 26.4 Microscopes: overall magnification m = m_o · m_e (“the product of the individual magnifications”), objectives typically 5× to 100×, normal optical microscopes up to 1500× with a theoretical resolution of about 0.2 μm, worked example (−30.0)(8.33) = −250 (read 09.10.2026). Updated: .

×
engraved on the objective, e.g. 4, 10, 40, 100
×
usually 10
mm
measured length; 0 = off
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Result

Magnification
Total magnification 400×
Details
QuantityValue
Objective40×
Eyepiece10×
Total magnification40 × 10 = 400×
Calculated from the nominal magnifications. A camera adapter or intermediate lens multiplies the result again.

How it is calculated

Total magnification = objective magnification × eyepiece magnification. A compound microscope has two lens systems: the objective forms an enlarged image, and the eyepiece magnifies that image again. OpenStax: “The overall magnification of a multiple-element system is the product of the individual magnifications of its elements.” Objectives typically range from 5× to 100×.

Common combinations

ObjectiveEyepieceTotal magnification
4×10×40×
10×10×100×
40×10×400×
100×10×1000×
40×15×600×

In the OpenStax worked example, an objective with focal length 6.00 mm and an object 6.20 mm away gives m_o = −30.0; the eyepiece gives m_e = 8.33; together (−30.0)(8.33) = −250. The minus sign only means the image is upside down.

Real size from the image

Magnification is image size divided by object size, so the real size is the image size divided by the magnification. A cell drawn 20 mm long at 400× is 20 ÷ 400 = 0.05 mm = 50 µm long; a 5 mm detail at 1000× is 5 µm. This holds only if the image really is shown at that magnification, for example a photo printed to scale; for a free-hand sketch, measure against a scale bar instead. Focal lengths and image distances of single lenses come from the lens equation calculator; the magnification of a telescope (focal length ÷ eyepiece focal length) from the telescope magnification calculator.

Frequently asked questions

How do you calculate total magnification of a microscope?

Multiply the objective magnification by the eyepiece magnification: 40 × 10 = 400×.

What is the magnification with a 100× objective?

With the usual 10× eyepiece it is 1000×. OpenStax puts the limit of normal light microscopes at about 1500×.

How do I find the real size of a cell?

Divide the measured image size by the total magnification: 20 mm ÷ 400 = 0.05 mm, which is 50 µm.

Why is the magnification sometimes negative?

A negative sign means the image is inverted. In the OpenStax example (−30.0)(8.33) = −250: 250 times, upside down.

Does a camera adapter change the result?

Yes. Any extra lens multiplies the magnification again; enter its factor as part of the eyepiece value.

Is more magnification always better?

No. OpenStax gives normal light microscopes up to about 1500× with a theoretical resolution of about 0.2 µm; beyond that the image gets bigger but not sharper. The calculated magnification says nothing about resolution.

Sources and legal basis

As of: · Editorial policy

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