What does “1:3.5–5.6” mean on a Canon EF-S 18-55mm lens?

Asked 7/11/2021

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On the front of my Canon EF-S 18-55mm IS II lens, the markings include “1:3.5–5.6.” I understand that 3.5–5.6 refers to the lens’s maximum aperture at 18mm and 55mm, but what does the “1:” mean? Why is this usually written online as “f/3.5–5.6” instead?

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

Photography Stack Exchange contributor

4y ago

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It means exactly the same as f/. 1: is just an older syntax for denoting a ratio. You might also occasionally see 1/. Since the f-stop is essentially a ratio of focal length to entrance pupil diameter (the apparent size of the aperture when looking into the front of the lens), one of these notations is used to indicate, in this case, that at the 18mm end, your apparent entrance pupil size is 18/3.5 = 5.14mm, and at the 55mm end it's 55/5.6 = 9.82mm.

Originally by user68706. Source · Licensed CC BY-SA 4.0

user68706

4y ago

AI Answer

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“1:3.5–5.6” means the same thing as “f/3.5–5.6.” The “1:” is an older way of writing the aperture ratio.

An f-number is a ratio of the lens’s focal length to the diameter of the entrance pupil (the apparent size of the aperture seen through the front of the lens). So:

at 18mm, f/3.5 means 18 ÷ 3.5 ≈ 5.1mm
at 55mm, f/5.6 means 55 ÷ 5.6 ≈ 9.8mm

That’s why the lens shows a range: as you zoom from 18mm to 55mm, the widest available aperture changes from f/3.5 to f/5.6.

So the short answer is: “1:” and “f/” are just different notations for the same aperture specification.

What does “1:3.5–5.6” mean on a Canon EF-S 18-55mm lens?

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