Microscope Magnification
How big is that cell really? Work out the actual size from a drawing or photo, the image size from the actual size, or the magnification, with the unit conversions shown and a size ladder from viruses to sand grains.
Magnification and Units
A microscope’s total magnification is the eyepiece times the objective: a 10× eyepiece with a 40× objective gives 400×. Magnification only makes things look bigger; resolution decides how much detail you can see. A light microscope cannot separate details closer than about 0.2 µm, however much you magnify.
Cells are measured in micrometres (µm): 1 mm = 1000 µm and 1 µm = 1000 nm. Always convert the image size and the actual size to the same unit before dividing.
From Image to Real Size, Step by Step
AKey Relations
Convert both sizes to the same unit first.
A scale bar keeps working when a picture is enlarged or shrunk; a stated magnification does not.
Four times the magnification shows a field a quarter as wide. Divide the field of view by the cell size to count how many fit across.
Sources & Verification
- B. Alberts et al., Molecular Biology of the Cell, 6th ed., Garland Science, 2014 — Ch. 9 (Visualizing Cells): resolution limit of light microscopy (about 0.2 µm) and typical cell sizes.
- R. Milo & R. Phillips, Cell Biology by the Numbers, Garland Science, 2015 — sizes of viruses, bacteria and cells used in the size ladder (rounded).
Frequently Asked Questions
How do you calculate the actual size of a cell?
Measure the image size with a ruler and divide by the magnification: actual size = image size ÷ magnification. A cell that measures 3.2 mm in a drawing at ×400 is 3.2 ÷ 400 = 0.008 mm across. Multiply by 1000 to get micrometres: 8 µm.
What does I AM stand for in biology?
Image = Actual × Magnification, the memory aid for I = A × M. Cover the quantity you want: A = I ÷ M, M = I ÷ A, I = A × M. Always put the image size and the actual size in the same unit first.
How do you convert millimetres to micrometres?
Multiply by 1000: 1 mm = 1000 µm, and 1 µm = 1000 nm. So 0.0075 mm is 7.5 µm, the size of a red blood cell, and 0.0001 mm is 0.1 µm = 100 nm, the size of a flu virus.
How do you calculate the magnification of a microscope?
Multiply the eyepiece by the objective: a 10× eyepiece with a 40× objective gives 400×. From a photo, divide the measured size by the real size: a 25 µm cell that is 50 mm wide in the photo is magnified 50 000 ÷ 25 = 2000 times.
How do you use a scale bar?
Measure the bar with a ruler and divide by the length written on it: that is the magnification. A bar labelled 100 µm that measures 40 mm gives 40 000 ÷ 100 = ×400. Unlike a stated magnification, a scale bar stays correct when the picture is enlarged or printed smaller.
What is the smallest thing a light microscope can show?
About 0.2 µm (200 nm): details closer together than that blur into one, whatever the magnification, because of the wavelength of light. Bacteria (about 2 µm) are easy to see; viruses (around 0.1 µm) need an electron microscope.