What is the term F stop and how to understand it

When talking about photography, what comes to our minds are shutter speed, aperture, and ISO quantity, but as soon as the term F stop is mentioned we get confused, in this article I will try to clarify the idea and term of F-stop as simple as possible, the idea behind the F stop, what it is and how to understand it while shooting.

What is the term F stop

As we talked in a previous article about what the aperture is, the more the aperture value increases (we raised the number), the more we narrow the aperture and reduce the amount of light entering through the camera sensor, and vice versa. Whenever we increase the aperture value (we raise the number), the depth-of-field expands and vice versa.

But where did the numbers next to the letter F come from? and why do I have to switch to ready-made numbers instead of choosing my own?

The term F-stops stands for F-number, i.e. the aperture number. Most of the modern lenses contain a set of numbers on its axis, these numbers are mathematically consecutive equations within a geometric sequence of the Sqrt sequence of number 2:


The value of the Sqrt of the number 2 = f / 1.4, so if we want to calculate the following result within this geometric sequence, the Sqrt of the number 2 will be raised to the power of 2, meaning:

Then the result will be f / 2, while the Sqrt of the number 2 to the power of 0 = 1, apertures less than f / 1 are raised to the negative power.

Every time we double the output, we have added a step or step to the amount of light that we control through the aperture. Every single step means that we are doubling the amount of light that enters through the aperture.

But why the Sqrt of 2? What is its relationship to the aperture

To reach a jump or one step in lighting, which means twice the amount of light, we change the aperture of the lens to an aperture with a larger diameter in a fixed ratio, and this ratio relates to two things:

  • The first is the area of ​​the circle that the aperture will form, and thus the pathway through which the light will pass toward the sensor.
  • The second is the diameter of the aperture, which is double the amount of light that should pass through.

And from it, if we have a lens aperture (and here the numbers will be just numbers for examples, nothing more), it will allow light to pass through a certain amount within a certain diameter of the aperture, if we want to get a jump or a step (twice the light), then we have to calculate the diameter of the aperture that would let the light pass through.

In fact, if we want to multiply the light by the diameter of the aperture to reach the next diameter measurement, we have to multiply the diameter of the aperture by the Sqrt of 2.

In addition, if we want to reduce the lighting in half, and go from the aperture diameter to a step smaller, we need to divide the aperture diameter by the Sqrt of 2.

Why does everything seem complicated?

In fact, thank God, you do not need to do these mathematical equations to deduce the aperture value, the numbers shown on the camera screen show them directly, the reason behind this article is to try to explain what F-Stops are and how this term relates to the aperture and the amount of light.

In this article I have summarized the relationship of the aperture to the focal length and the size of the lens itself. If you would like to learn more about the topic, see the references section at the bottom of this article, I will explain the link in a separate article at a later time.

Well now I think the term F-stop has become somewhat clearer, remember that in photographic cameras, you will find these serial numbers in a list of the aperture value. You do not need to calculate these equations, but you must know how the aperture mechanism works, remember also that you move from number to number The next one means that it allows twice the amount of light to pass through to the camera sensor, and changing the number when it is lower means that you reduce the amount of light passing through the sensor by half.

reviewed by Daniel Davis