Photography Basics #3: Understand Exposure Triangle – Aperture, Shutter Speed, ISO

Exposure TriangleLight is the essence of photography. Without it, or with a lack of it, taking a good picture can be almost impossible. To understand how a camera works, one of the key principles is the relationship between aperture, shutter speed, and ISO. This is known as the ‘exposure triangle’ and is one of the first things you’ll want to master on your journey to becoming a good photographer. Each aspect has its own nuance, and will impact both the exposure of a photo and the other two aspects in the triangle. We’ll examine what exposure is, as well as the basics of how aperture, shutter speed, and ISO interact.

What is Exposure?

To correctly understand the exposure triangle, we must first know what exactly exposure is. Essentially exposure refers to the amount of light that enters through the lens to the camera’s sensor. It’s a measure of the brightness or darkness of a photograph. Correctly exposing an image can be tricky; if the sensor receives too much light, the picture becomes overexposed. On the other hand, if too little enters, it becomes underexposed. Getting this balance right depends on a variety of factors, and the trinity of aperture, shutter speed, and ISO play a big role in controlling it.

You may hear people refer to the measure of exposure in relation to ‘stops’. To brighten an underexposed photo you need to increase the exposure by a stop or two, and likewise decrease an overexposed photo by the same amount.

An Exposure Histogram

A useful feature that digital cameras have is a histogram. You may need to consult your guide book to find out how to activate it, but it can save you a lot of headaches in the long run. To get a fairly accurate measure of how exposed a photo is or will be, you can use a histogram. Reading from left to right shows the spread of image tones from the shadows, to the mid-tones, to the highlights.

Data on the graph that is too far to the left means that the darker areas of your picture will be ‘clipped.’ This means it’s too dark, there’s no detail in the shadows, and image quality is lost. The opposite effect (if the data is too far to the right) means the same is true for brighter areas which are ‘blown out’ or too bright. Some small amount of either isn’t too much of a problem and can create some interesting effects, but be aware of your histogram when you’re taking a photo.

Overexposed and Underexposed

The use of the terms underexposed and overexposed may give you the idea that there is one ‘perfect’ level of exposure. Unfortunately this isn’t the case, and it can be quite a subjective and situational thing. Despite this, if there’s not enough light entering the sensor, then there isn’t enough data in darker areas and thus the image becomes underexposed. Similarly, if there’s too much light then data in the highlighted areas is unreadable and the photo becomes overexposed. Unfortunately, there’s no way to combat this loss or lack of data once the photo has been taken, even in post-processing. It’s therefore important that you strike the right balance between the two and get a ‘good’ exposure.

Understanding Aperture, Shutter Speed, and ISO

Now that we’ve learned a bit about exposure, it’s time to focus in on the exposure triangle of aperture, shutter speed, and ISO.


The aperture refers to how wide the iris of the lens opens. The wider it opens, the more light enters through the lens to the sensor. A lens’s aperture is measured in ‘f/ stops’. The lower this number is, the wider the aperture, and the more light enters. For example, f/5.6 is wider than f/16, but not as wide as f/3.5.

A wider aperture (wider opening and lower number) means that it’s possible to capture more of a scene in low-lighting. However it also creates a shallow depth of field, creating a background blur. For landscape photography, a narrow aperture (narrower opening and higher number) is beneficial. This is because it creates a greater depth of field, meaning more of the scene is in focus.

Generally speaking, the wider the maximum aperture of a lens, the more expensive it is. Lenses are also usually sharpest at around f/5.6 or f/8. However using a wider aperture of f/1.8 or f/2 means you can isolate your subject more creating a stunning portrait.

Shutter Speed

Shutter SpeedAs the name suggests, shutter speed refers to how fast the shutter opens and closes. The faster the shutter speed, the less time there is for light to enter the sensor. This results in a less-exposed final result. A slower shutter speed means the shutter stays open for longer, allowing the sensor to gather more light.  This makes for a higher exposure.

As the name suggests, shutter speed refers to how fast the shutter opens and closes. The faster the shutter speed, the less time there is for light to enter the sensor. This results in a less-exposed final result. A slower shutter speed means the shutter stays open for longer, allowing the sensor to gather more light.  This makes for a higher exposure.


ISO measures how sensitive your camera’s sensor is to light. The higher the number, the more sensitive the sensor is. Typically this can range from as low as 50-100 all the way up to 16,000 or higher. By increasing your ISO, and thus your sensor’s light sensitivity, you’re able to use a faster shutter speed when the light is poor. However, this increased sensitivity comes at a cost. The higher the ISO, the more digital noise will be present in the image. This noise will make your photos seem more grainy, and will become more noticeable the higher the ISO you use. Although modern digital cameras are much better at controlling this noise than they once were, it will still affect your images to a degree.

To take the best possible image in a given light situation, you must balance ISO against shutter speed and aperture.

EVs and Stops

The combination of the three elements we’ve explored so far, aperture, shutter speed, and ISO, are collectively referred to as the Exposure Value, or EV. When measuring the EV, a doubling or halving of the amount of light entering the sensor (or the sensor’s sensitivity to light) is referred to as a stop. They are measured in the following ways:

  • ISO: If the ISO doubles or halves, it goes up or down by one stop. This means going from ISO 200 to 400 is an increase of one stop. Going from ISO 200 to 100 is a decrease of one stop.
  • Shutter Speed: This works in a similar way to ISO in the sense that a numerical doubling or halving is a change of one stop. So going from 1/30 second to 1/120 second is a decrease of two stops.
  • Aperture: The measurement of aperture is a little trickier, as the numbers involved aren’t a numerical half or double. Instead, for reasons we don’t need to know right now, they follow a specific pattern. For example, in the following sequence each f/stop represents a decrease of one stop: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. You’ll soon get to grips with the sequence after you get used to using your camera and changing f/stops,

Grasping how exposure values affect your final images is one of the fundamentals of becoming a good photographer. Although it may seem daunting at first, the best way of getting to grips with the different variables it to take a lot of pictures. Keep the subject the same and change various settings to get a feel of how each one changes the appearance. Remember that most digital cameras will store meta data for each image file. This means you can see the specific settings that were used when you get a chance to view them on a bigger screen.

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