Why does a lower f-number mean a larger aperture?

Asked 11/22/2018

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2 answers

I’m confused by aperture numbering. I’ve read that a lower f-number means a larger aperture, so f/2 is wider than f/22. That feels backwards at first. Why is aperture written this way, and what does the “f” actually represent?

f-stop lenses aperture basics exposure

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

7y ago

2 Answers

/ is division.

A number divided by 22 (f/22) is smaller than the same number divided by 2 (f/2). As simple as that.

Now, WHY?

f is the focal length. Describing the aperture as a fraction of the focal length has one advantage: it immediately reveals the image brightness.

Example:

20mm aperture can be...

Telephoto lens, 200mm f/10 - pretty dark
Wide angle lens, 30mm f/1.5 - pretty bright

Originally by user32811. Source · Licensed CC BY-SA 4.0

user32811

7y ago

AI Answer

Generated from our catalog & community — verify before relying on it.

The “f” in f/2, f/4, f/22, etc. refers to the lens’s focal length. An f-number is a ratio: focal length divided by the diameter of the aperture opening.

So on a 50mm lens:

f/2 means the opening is 50 ÷ 2 = 25mm
f/22 means the opening is 50 ÷ 22 ≈ 2.3mm

That’s why a smaller f-number means a larger opening: it’s the denominator of a fraction. Just like 1/2 is larger than 1/22, f/2 is a wider aperture than f/22.

This system is useful because it describes aperture relative to focal length, which makes exposure/brightness comparable across different lenses. For example, the same physical opening size would not behave the same way on a 30mm lens and a 200mm lens, but the same f-number gives the same image brightness.

Also, there is no practical “f/0” in this sense—division by zero is undefined, and real lenses have physical limits on how large the aperture can be.

Why does a lower f-number mean a larger aperture?

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