Biology · Introductory biology · Worked example
Draw a scale bar of the right length
A digital micrograph is 1,200 pixels wide and shows a field 80 µm across. Choose a scale bar length and find how many pixels long to draw it.
Find pixels per micrometre
1,200 pixels span 80 µm.
Choose a bar length
A common choice is a round length, 1, 2 or 5 times a power of ten, between a tenth and a quarter of the field width: here 8 to 20 µm. Both 10 µm and 20 µm qualify; take 20 µm.
Convert to pixels
Multiply the bar length by the pixels per micrometre.
Resize the image
Shrunk to 600 pixels wide, the image still shows 80 µm, and the bar shrinks to 150 pixels: still a quarter of the width, still 20 µm.
Result
At 15 pixels per micrometre, a 20 µm scale bar is 300 pixels long.
Your turn
An image 2,000 pixels wide spans 0.5 mm. How many pixels long is a 50 µm scale bar?
Show the answer and explanation
200 pixels.
0.5 mm = 500 µm, so there are 2,000/500 = 4 pixels per micrometre, and 50 µm is 200 pixels.
Keep exploring
The scale-bar tool opens in field-width mode with 1,200 pixels across 80 µm and a 20 µm bar of 300 pixels. It suggests 10 µm and 20 µm as readable bar lengths.
Return to the concept →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.
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