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The Exposure Triangle – A Beginner’s Guide

For many starting out in photography, the relationship between aperture, shutter speed, and ISO can be confusing. To further muddy the water, the terms ‘stop’ and ‘f-stop’ are often used interchangeably but refer to different things. In this article,

I would like to take some of the mystery out of these concepts by talking about the exposure triangle and why it is important to understand for those who are starting out. Please keep in mind that the concepts in this article are oversimplified to make it easy for beginners to understand, especially when it comes to ISO.


1) The Exposure Triangle


Aperture, shutter speed, and ISO make up the three sides of the exposure triangle. They work together to produce a photo that is properly exposed. If one variable changes, at least one of the others must also change to maintain the correct exposure. For more information on this topic, you may which to check out our detailed summary of exposure.


triangle

2) Stop!


Before we go too far, let’s start our discussion by talking about a stop of light. Understanding what a stop is is key to understanding the exposure triangle. In photography, a stop refers to the doubling or halving of the amount of light that makes up an exposure. Each photo that we take requires a certain quantity of light to expose it correctly. Adding a stop of light by doubling the exposure will brighten an underexposed image. Conversely, decreasing an exposure by one stop (halving the amount of light) will darken an overexposed image.

So how do you add or take away a stop of light? To do this, we need to change the aperture, shutter speed, and/or ISO. Let us look at each of these individually.


3) Shutter Speed


Shutter speed is the length of time light is allowed to hit the sensor. It is measured in seconds. Shutter speed is probably the easiest of the exposure triangle sides to understand. To double the amount of light, we need to double the length of the exposure. For example, moving from a shutter speed of 160 s to 130 s will add a stop of light because the shutter will remain open twice as long. Changing from a shutter speed of 1s to 1/8 s will decrease the exposure by three stops. Why? From 1s to 12 s is one stop. Then 12 s to 14 s is another stop. Finally, 14 s to 18 s is a further halving of the time the shutter remains open or the third stop.


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4) Aperture


Aperture refers to the size of the circular hole in the lens that lets in light. The bigger the hole, the more light that reaches the sensor. In fact, each time you double the area of that opening, you double the amount of light or increase the exposure by one stop. On the other hand, if you half the area of the opening, you half the amount of light hitting the sensor. And you guessed it; that will decrease the exposure by one stop.


Now without getting too technical, an f-stop is a ratio that relates to the size of that opening. Mathematically it is equal to the focal length of the lens divided by the diameter of the lens. At first glance the values on the f-stop scale are confusing. The numbers don’t seem to make any sense. Why do small values correspond to larger openings and vice versa? For a simple explanation, keep reading.


To understand why large f-stop numbers refer to small openings and small f-stop numbers refer to large openings requires a bit of math. Don’t worry; I will try and keep it simple. If you take the ratio I mentioned above:

f-stop = focal length/diameter

and rearrange it for diameter, you get:

diameter = focal length/f-stop

What this means is that for any given focal length, we can calculate the diameter of the aperture by dividing the focal length by the f-stop value. But when you divide a given focal length by a large f-stop number, the result is a small diameter. Therefore, the area of the opening is small. Conversely, if you divide the same focal length by a small f-stop number, you get a large diameter. And a large diameter means a bigger area and more light passing through the opening.


Also, it turns out that to double the area of the opening, the f-stop needs to be divided by the square root of two (1.414). That is why the f-stops are not nice round numbers. To half the area, the f-stop needs to be multiplied by the square root of two.