Lens aperture is found on the camera lens, and it determines the amount of light that passes through the lens opening and onto the digital sensor or photographic film. This is done by changing the size of the aperture opening on the lens, much like the eye pupils contract and expand to let more or less light in.

When you’re out shopping for a new lens, you’re most concerned with the lens’ focal length and the minimum aperture value. A 50mm 1.8 seen on a lens would mean that the lens’ focal length is 50mm, and the maximum aperture opening (i.e. minimum aperture value) on that lens is f/1.8.

Now, a less aperture value translates to a wider lens opening, and a greater aperture value refers to a narrower lens opening. Why you might ask!

Well, it’s not critical to your photography that you know the logic behind that, but I’m gonna explain it anyway for those folks who like to dig in deep and learn the reasoning behind everything like yours truly here =)

You see, f/1.8 means exactly what it looks like: the lens’ focal length divided by its aperture opening. Let’s say you have a lens with a 50mm focal length. F/1.8 is equal to 50/1.8, which is obviously greater than 50/22. This is why a lens aperture value of 1.8 refers to an opening larger than that of 22.

Now, let’s assume you have a 50mm lens with a minimum aperture value of f/2. Remember, the f is the focal length. So, doing the math you’ll have 50/2 which is a 25mm wide aperture opening. Right? Cool!

Now let’s assume you have another lens with a 100mm focal length and a minimum aperture value of f/2 as well. Doing the math, you’ll have 100/2 which is a 50mm wide aperture opening.

You see, an f/2 aperture opening on a 50mm lens is narrower than that on a 100mm lens. The ratios are the same, but the actual width of the two openings is different. This is why on lenses, the minimum aperture value reads as a ratio, for example 1:2 or 1:1.8 or 1:3.5 and so on…

Lens aperture explained

On a zoom lens, like let’s say a 50-100mm lens with a minimum aperture value of f/2, the ratio changes as the lens zooms in and out. So the ratio will vary. With the lens zoomed all the way out, the focal length is 50mm. 50/2 is 25mm aperture opening. However, with the lens zoomed all the way in, the focal length is 100. 100/2 is 50mm aperture opening. This is why it is common to see the ratio on a zoom lens something like 1:1.4-2.8, or 1:2-4, or 1:3.5-5.6 and so on…

Hope you learnt something new today, and enjoyed reading this article =)

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9 Comments

  1. big thanks, I found exactly what I want )

  2. Hi,
    Quick question: If the relationship is focal range divided by f-stop number to get the physical opening of the aperture in mm, for example 55/2= 25mm.

    How the heck can one explain a 24mm lens with a f1.4 number?

    The maximum opening of this lens would then be 24/1.4= 171mm

    Seems a bit wide huh?

  3. LOL, me and math! Sorry bout that Diana.

  4. but to calculate your f/stop couldn’t you divide the Dept of the Aperture by the focal length?

  5. Wow, thank you so much for posting this! I’ve learned a lot! 🙂

  6. I’ve undertood how to get the f1.4, f2.8 etc but the 1:…. how do you get it?

  7. perhaps my question is not clear. I know that f1.4 is bigger than say f2.8. But when a lens is marked 1:1.4, how do you get the 1: ?

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