F-Stop

What is an F-stop, what is its significance, and why is it essential in filmmaking? To understand the F-stop, let’s first grasp the concept of aperture.

Show the Iris, Aperture, and F-stop.

The aperture in a camera lens is a circular opening that controls the amount of light entering the camera and reaching the image sensor or film (in the case of IMAX).

The larger the aperture, the bigger the opening, the bigger the diameter, and the more light it allows through. Similarly, a smaller aperture and smaller opening mean a smaller diameter and less light coming through.

Here, we understand how light comes through, but how do we determine the amount of light that enters the camera? It is through F-stop. An F-stop is the measure of the amount of light that enters through the camera lens.

F-stops have the standard range 1, 1.4, 2, 2.8, 4, 5.6, and so on. Each of these numbers represents a different amount of light. That means if we change the F-stop number, we can change the light, thereby changing the aperture. Right?

But how do we know how to change the f-stop to achieve the desired result? Let’s see how F-stop is calculated:

F-stop = focal length of the lens/diameter of the aperture

In this formula, we see that the F-stop is the fraction of the diameter. And what is the diameter? It is the aperture size that we discussed earlier in this video. The larger the aperture, the bigger the diameter. So, if we increase the diameter, the F-stop value will decrease. We can say that the larger the aperture’s diameter, the lower the F-stop number. A larger aperture diameter means a larger aperture, which means more light it allows through. Therefore, a lower F-stop number allows more light. And increasing the F-stop reduces the light coming through the lens.

That means, with every increment in the standard range of 1, 1.4, 2, 2.8, 4, 5.6, the light is decreasing. But how much light is reduced with every increment? Well, every single increment from 1 to 1.4 to 2 to 2.8 is reducing the light by half, and every single decrement is doubling the light. This means that moving from 1 to 1.4 or 2.8 to 2, the amount of light decreases by half, whereas moving from 1.4 to 1 or from 4 to 2.8, the light doubles. Each of these numbers is called a stop or full stop. Additionally, there are half stops as well as one-third stops, which provide more range and flexibility in achieving the desired exposure.

Now we know how to tweak exposure in our image using F-stop, but another important function of F-stop is to determine the depth of field of the image.

To understand the depth of field in greater detail, you can refer to my video, specifically dedicated to this topic.

For this video, just know that with a smaller F-stop number, you will have a shallower depth of field, giving a bokeh effect in the image and blurring everything apart from your subject. When you increase the F-stop number, thereby decreasing the aperture size and the amount of light entering the camera, you can reduce the blurriness in the image, creating a wider depth of field and keeping much of the image in focus.

• Now you might wonder why, at every stop, the light is doubling or getting halved when the increment is not linear. Well, it is because it is based on mathematical calculations involving the square root of 2. We won’t get into the formulas, but it is important to know why we have the standard range as it is; every square root value, such as 1.4, is doubling or halving the light.

Let’s look at the formula to clear any doubt.

F-stop = focal length of the lens/diameter of the aperture.

Let’s take 1.4 and 2 (Ideally, it should double the light, and we know that it is evaluated through the use of square root 2)

For f/1.4:

 1.4 = 50 / Diameter

Diameter = 50 / 1.4 = 35.7143

For f/2:

2 = 50 / Diameter

Diameter = 50 / 2 = 25

 Now if we multiply 25 * 1.4 = 35, The result is 35, hence proving the light is doubling if we move from 2 to 1.4.

• You may also wonder, what is the point of the formula if F-stops are pre-standardized? They help us identify the aperture size and how much light it can let in. This information becomes useful while choosing the camera for your shoot.

For example, a 50mm lens with f/1.4 will cost you four times the price of a 50mm with f/1.8. Well, of course, the f/1.4 lens has a better depth of field and allows more light, which can be very helpful in low-lighting conditions.

To identify the amount of light a camera can allow, use the formula:

F-stop = focal length of the lens/diameter of the aperture

For f/1.4:

1.4 = 50 / Diameter

Diameter = 50 / 1.4 = 35.7143

For f/1.8:

1.8 = 50 / Diameter

Diameter = 50 / 1.8 = 27.7778

So, a diameter of 35.71 allows more light than 27.7, hence the f/1.4 lens is more expensive and is suitable for low-lighting conditions.

Summary

Look at the following chart.

Pro-Tip

If someone ever asks you about the speed of your camera lens, the answer would be the smallest F-stop value of your lens; that is because the speed of the lens is referred to as the maximum amount of light that enters through the lens, which as we know is measured in F-stop. So the lowest F-stop value will open the aperture to its max size, allowing the maximum light possible for your lens. The lens with the lowest f stop numbers is often called a fast lens for its capability to offer widest aperture thus allowing more light in for the sensor.

I believe F-stop should be clear to you. If you still have any questions, feel free to put them in the comment section. I will try my best to respond to your queries. And don’t forget to follow the channel for more informative videos like this.