What is f-stop in Photography and How to Figure It Out?

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Optimum lighting is essential for clicking good photographs. The exposure triangle formed by the shutter speed, aperture, and the ISO of the camera, allows light to enter the camera and reach the sensors for image formation. However, there needs to be a component of the camera that can measure the light and alter the exposure triangle so that the photographer gets valuable hints on how to adjust the frame. The f-stop, along with metering, is a mechanism to measure how much light should be allowed to enter for a good shot. If you don’t know what is f stop in photography and want to use it for clicking better pictures, follow along.

Back to Basics

To understand the mechanisms like f-stop, you should be aware of the components of a camera. In a digital camera, the front end is where the lenses are attached while the back end has the sensors. Between both of them is a set up which is commonly called the iris of the camera. The iris has a circular shield with a hole at its center. The hole is called the aperture, and it can open or close to allow light to enter.

Now, since you know what the aperture is, let’s discuss the f stop. Simply speaking, the f-stop is only a scale of numbers that indicate the ideal exposure for a particular scene. As per the f-stop scale numbers, the photographer needs to adjust the aperture and allow the required amount of light to come in.

The f-stop scale has several numbers. Each number is a ‘major point’ where either the amount of light that comes into the camera doubles or reduces by half. It depends on whether you are going upwards or downwards through the f-stop scale. Consequently, the aperture size will change, too.

Why Use f-stop Scale Numbers?

Now, you could ask that if ultimately the aperture adjustment is what will decide the amount of incoming light, why complicate things with so many numbers? Why not simply change apertures and check the exposure on the exposure indicator scale? Let’s see.

The problem with depending only on the aperture diameter to judge the exposure is that while you use the same aperture, different amounts of light will enter through different lenses. So, there is no uniformity, and the estimation of light could go wrong.

It happens because every new lens has a different focal length. While you change the aperture size, you cannot change the focal length of the lenses. That’s why using a single aperture size for two different lenses will not help in the proper judgment of lighting.

Instead, if there is a mechanism by which both aperture size and focal length are taken into account then one can arrive at a standard measurement. This can be done by taking ratios of every possible combination of focal length and aperture diameter.

Important Calculations

For calculating the ratio, you need to divide the focal length by the aperture diameter. You will get a value that does not really mean anything in terms of units of distance or units of light. It is just a number. However, it represents a certain amount of light that this combination of aperture size and focal length will let in. More importantly, it is significant when you use a different lens with a different combination of focal length and aperture diameter. If their ratio comes out to be the same, then the same amount of light will enter through the camera as before, irrespective of what type of lens you use.

Let the focal length be f, the aperture diameter be d, and the ratio be x then,
f/d= x

These ratios that you get after dividing the focal lengths by the diameters are the numbers that the f-stop scale uses.

The Applications

Even though we said that these are the numbers that the f-stop scale uses, you generally see f-stop numbers of the form f/16, f/22, etc., written on the cameras.

To understand what they mean, let’s just tweak the formula a little.
As, f/d= x
So, f/x= d

This f/x is the f-stop number written on the cameras. The f-stop number is written in this format because ultimately, the number needs to tell you how wide your aperture (d) should be.

To summarize all of it, we can say that the f-stop numbers on the scale are inversely proportional to the aperture diameters. So, as you move down the scale, which is the direction of increasing f-stop numbers, the aperture diameter keeps decreasing. Therefore, the light entering the camera keeps getting reduced to half. The opposite happens when you move upwards through the scale.

Additional Information

When people talk about f-stop numbers or apertures, they do not make use of such comprehensive language each time. A common way of talking about the apertures, f-stop numbers, amount of light, etc., is to tell whether you are going up stop or down stop. From everything that we have discussed till now, it is clear that going up stop will mean widening the aperture and doubling the amount of light entering the camera, simultaneously. You may also hear some photographers say that their camera has a fast lens or a slow lens. Fast lens means that the lens has a large focal length and the camera has a wide aperture, which allows a lot of light to enter.

What is F stop in photography? – Conclusion

So, in this article, you learned what is f stop in photography. A lot of calculations and technical aspects are involved in measuring and estimating the appropriate exposure for a scene. While these calculations can sometimes be confusing, understanding the f-stop scale before using it, is the textbook method of learning photography. To become a good photographer, you should know your camera very well. For many photographers, it is difficult to remember the f-stop scale. However, there is no alternative to remembering the f-stop scale because of its impact on the aperture width. While some people have their tricks to learn it, the bottom line is that you need to remember the scale by all means, for adjusting the exposure in your frames.

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Marco Downs

Marco Downs

Marco Downs is our second founder and he is the creative head of this website. Marco stumbled upon photography only in college when he joined the photography club. His parents could never afford a camera for him as a child and it was in college that he saved up and bought his first camera. He now writes in-depth buyer guides and informational articles to assist the buyers. 
Marco Downs

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