Microscope magnification formula
A compound light microscope magnifies in two stages, so the total magnification = objective magnification × eyepiece (ocular) magnification. Each lens is marked with its own power, such as 10×, 40× or 100×, and you simply multiply the two together.
The stages multiply rather than add because the eyepiece enlarges the image the objective has already produced. Magnification is a pure ratio, so the answer has no units — it is written with an × sign instead. Magnification is a ratio of image size to real size, so the ratio calculator handles the same arithmetic in general form.
How to use this calculator
Enter the magnification printed on the objective lens you are using and the magnification of the eyepiece. The tool multiplies them to give total magnification. To estimate true specimen size, divide a measured image size by this total magnification.
The usual slip is entering the wrong objective. Only the lens currently clicked into the light path counts, so check the turret before reading off a number. Also remember that the marked value belongs to the lens, not the microscope as a whole. Specimen sizes run into micrometres and nanometres, which are far easier to handle in the form given by the scientific notation calculator.
Worked example
A high-power objective of 40× used with a standard 10× eyepiece gives a total magnification of 40 × 10 = 400×. Switching to a 100× oil-immersion objective would give 1,000×.
At 400×, a structure appearing 4 mm wide in the field of view is really 4 ÷ 400 = 0.01 mm, or 10 micrometres, across — about the size of a plant cell. Past roughly 1,000× a light microscope stops revealing new detail because the wavelength of visible light sets a hard resolution limit, and further enlargement only spreads the same blur over a bigger area. Scale bars work by setting up an equality between two ratios, exactly the setup solved by the proportion calculator.
Frequently asked questions
- How do I find total magnification?
- Multiply the objective lens power by the eyepiece power. For example, a 40× objective and 10× eyepiece give 400×.
- What are typical objective powers?
- Common objectives are 4× (scanning), 10× (low power), 40× (high power), and 100× (oil immersion).
- What is the eyepiece magnification?
- Most light microscopes have a 10× eyepiece, though 5× and 15× eyepieces also exist. It is printed on the ocular lens.
- Does higher magnification always mean a better image?
- Not necessarily. Beyond the resolution limit of the optics, higher magnification gives a larger but blurrier image (empty magnification).
- Why does the image get darker at higher magnification?
- A high-power objective collects light from a much smaller area, so less light reaches your eye. Open the condenser diaphragm or raise the illumination when you switch up, otherwise a correctly focused specimen can look faint or empty.