Biology · Introductory biology · Concept
Magnification and scale bars in microscopy
Magnification tells how many times larger an image is than the real object: magnification = image size ÷ actual size, with both lengths in the same unit. A scale bar is a line drawn on the image and labeled with the real length it represents, so measuring it gives the magnification, and comparing any feature with it gives that feature’s actual size, even after the image is resized.
Image size, actual size and magnification
Measure a feature on the image, then divide by its real size. Rearranged, actual size = image size ÷ magnification. Convert both lengths to one unit first: 1 mm = 1,000 µm and 1 µm = 1,000 nm.
Reading a scale bar
Measure the bar on the image and divide by the length printed on it. A 50 µm bar that measures 20 mm, or 20,000 µm, shows a magnification of 400. A feature twice the bar’s length on the image is 100 µm long.
Why figures use scale bars
A stated magnification, such as ×1,000, is true only at the size the image was prepared. Printed smaller, projected larger or zoomed on a screen, the image changes size but its label does not. A scale bar changes size with the image, so it stays correct.
Magnification is not resolution
Resolution is the smallest distance at which two points can still be told apart: about 200 nm for a light microscope, and far less for an electron microscope. Enlarging an image past its resolution makes it bigger but reveals no more detail.
Total magnification of a light microscope
Multiply the objective lens’s magnification by the eyepiece’s: a 40× objective with a 10× eyepiece gives 400×. Higher magnification shows a smaller field of view.
Common mistakes
- Dividing lengths in different units: convert mm to µm before dividing.
- Using a stated magnification after the image has been resized.
- Confusing magnification with resolution: enlarging a blurred image does not sharpen it.
- Dividing the wrong way: magnification is image size over actual size, usually much larger than 1.
Key terms
- Magnification
- How many times larger an image is than the real specimen: image size ÷ actual size. Enlarging an image doesn’t add detail that wasn’t captured.
- Microscopy resolution
- The smallest distance at which two nearby points can still be told apart. It differs from magnification: a blurry image can be enlarged without getting any sharper.
- Scale bar
- A bar of known length drawn on an image, such as 10 μm, for measuring the specimen. It stays accurate only if it is resized and cropped along with the image.
- Physical field width
- The real distance across the whole image, often in micrometres (1 μm = 10⁻⁶ m). Dividing it by the image’s width in pixels gives the distance each pixel covers.
Work through an example
On a printed micrograph, a 10 µm scale bar measures 25 mm, and a cell measures 60 mm across. Find the magnification and the cell’s actual diameter.
Find a cell’s actual size from a scale bar →Sources and scope
Authored study material. Tool results depend on the stated inputs and model assumptions.
Try in the workspace
Open the example inputs, change a value and keep a useful result on your board.
Open the scale-bar tool Open worked example on a board Microscopy referenceYour existing work stays on this device. Examples open as editable copies.