What is an F-Stop?

So what exactly is an f-stop? Well, to put it in the very simplest terms, it is the opening that lets light into your camera. And so the numbers on the f-stop relate to the size of the opening that is letting light into your camera. F-stops are measured by a scale, and this is known as the f-stop scale. If you are not familiar with a camera, the f-stop numbers can be very confusing, as they do not seem to make any sense. F-stops are actually a measurement of the diameter of the aperture. Logically, they should be expressed as a fraction and this number would tell you the diameter, in millimeters, as a fraction of the actual focal length of the lens. So if you had a zoom set at 40mm with an aperture of f/8, the diameter of the aperture opening is 5mm (40 divided by 8).

Adding to this confusion, the numbers that correspond to different f-stops seem backwards, because an aperture of f/8 is actually smaller than an aperture of f/4. So, the larger the number, the smaller the opening. And the smaller the number, the larger the opening.

And then you get into the fact that most f-stop numbers are not a full number. They are an f/5.6, or an f/19, or an f/6.3, or an f/1.4. If you are not familiar with f-stops and aperture openings, none of this makes any sense. These numbers depict half stops and third stops, as well as full f-stops.

The typical range for f-stops on a camera, progressing from a wide setting to a small setting is f/4, f/5.6, f/8, f/11, and f/16. Some lenses will have a wider range and may offer half stops and/or third stops.

Another funny thing about f-stops, is if you halve the number of the f-stop, the aperture lets in a quarter amount of light, because it is it two stop decrease. If you are thinking about it logically, you would naturally assume that if you took and halved your f-stop that you would be letting in half the light. But that is not true because with each f-stop decrease you are halving the amount of light, therefore with two f-stops, you would only have one quarter of the light.

If you take your f-stop scale, and add your half stops, the scale is f/4, f/4.8, and f/5.6. If you then do your third stop range. It would go f/4, f/4.5, f/4.8 (for your half), f/5 and f/5.6.

The key thing to remember about f-stops is that it is a measure of the amount of light that is being let in through your lens. You can think of it as having a paper towel roll, and looking through the roll at the light. If you took a piece of tinfoil and put it over the end of the paper towel roll and poked a pinhole in it, you would have a high f-stop or small aperture opening. The bigger you made the hole, the smaller your f-stop number would become and the more light you would be letting in. Using your aperture control with your shutter control on your camera will give you the proper exposure. The best way to figure out what is happening with the different settings, is just to play with it. Especially if you have a digital camera, you can just delete whatever doesn’t turn out well.

Don’t worry if you don’t know what all the controls on your camera mean. You can learn as little or as much as you want. After all, you can always just set it on auto and your camera will take the picture for you, and chances are it will turn out great. The manual controls are for those shots your camera has a hard time taking, or just for some creative control. Maybe you don’t want your shot to look like reality. I post information that you may find helpful in learning how to use your camera. And if it is time for you to buy a new camera, I can help with that too.

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8 responses to “What is an F-Stop?”

  1. David A says:

    You mean 40/8 right? Not 40/40.

  2. Son of a Photographer says:

    1) I second David A.

    2) Also, a comment….I believe you contradict yourself; if you follow the logic of taking the focal length divided by the effective aperture to get the f-stop, then it’s not confusing at all that a larger aperture makes a smaller f-stop. That’s the foundation of division!

  3. Wolf says:

    Your ‘errors’ have already be pointed out by others, but I believe that “f-Stops” must have been invented by Brits, as they seem to complicate everything and make everything as awkward as possible. Thus the smaller the F.#, the larger the Aperture. I rest my case!

  4. vince says:

    Well i have to say that this article left me way more confused about f-stops than before i read it.

  5. Irene coates says:

    I have aPanasonic Lumix FZ70 and the f-stop goes to f8 only- yet I noticed on other people’s cameras it goes as hight f16 – does it matter?

    Thanks Irene

  6. Jeff Lucas says:

    Hi every body,
    I have a 6 inch catadioptric telescope which I know is 150/1400. Now I know that the 150 equates to six inches, thus the diameter of my main mirror and the focal length is 1400mm. My question is:
    what is 1400mm expressed as an f stop?

    If anyone could give me the answer together with the formula to work it out, I would be most grateful.

  7. Carol Doucet says:

    I thought it was explained very well, especially after trying to understand what others explained.

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