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Exposure triangle

Cover Image for Exposure triangle

This is a short guide explaining the exposure triangle and its uses in photography.

Exposure what?

It's called the exposure triangle because of the three components namely: aperture, shutter speed, and ISO. Let's look at each of them and then how they each play a role in making a well-lit and balanced image.

Aperture

Aperture is the measure of how much light you let into the camera. A wider aperture (or a lower F-stop number) will let in more light, and a narrower aperture (or a higher F-stop number) will give you less light. In the diagram below you see the scale, from the left the iris is wide open letting in as much light as possible and at the other end the iris is as narrow as it gets letting in only a small amount of light.

Image with low F-stop

Some characteristics change depending on how wide or narrow you set the aperture. The depth of field for example varies when you change the aperture and this can create some interesting effects. When you want to separate the subject from the background this can be a handy technique. By lowering the F-stop you create a shallow depth of field, blurring the background. This can be handy when you want only your subject in focus. When you open the aperture this wide you also let in more light so to compensate for this you have to play with the other two settings in the triangle which will be covered later in this guide.

Shutter speed

The shutter speed tells you how long the shutter remains open and how long the sensor is exposed to light. The shorter the shutter speed you have the less light have time to enter the camera and the longer the shutter speed will let in light for a longer time. With this setting, you have another way to control how much light you expose the camera sensor to. You also control the motion in the image, either how much the camera is shaking or moving or how much motion you have in your subject. This is often the apparent difference between a blurry image and an image with a subject in sharp focus.

ISO

Although technically not a part of the exposure triangle these days the ISO, or film speed, is a way to capture more light in the image when you have exhausted the other two options above. Back in the day when the image was captured on a physical medium, the ISO was embedded in the film and not something you could change after it was loaded in the camera. Today cameras both for photography and video are digital and the sensor is taken over the job. You can tell the sensor to brighten up the image after it is taken (increasing the ISO) but this has some tradeoffs. You can increase the ISO number, e.i. from 100 to 200 and 800 to 1600, effectively doubling the amount of light in the image. By doing this you introduce more and more noise or grain into the image so use it only when you can't go lower with the aperture and won't go lower with the shutter speed if this makes the subject blurred.

Creating an image

By balancing the aperture, shutter speed, and ISO you can capture subjects in fast motion like birds, a bright moon at night, or a smile in a sharp and well-lit portrait.

Image with high F-stop

F/18 1/100 sec ISO-800 70mm

Image with low F-stop

F/3.5 1/320 sec ISO-100 70mm

The two images above are taken at the same distance with the same lens at 70mm. The left image is taken with an aperture of f/18 with an ISO of 800 to compensate for the low light the iris is letting in. The image on the right is taken with f/3.5 with the ISO at the lowest of 100.

Hope this small guide has been helpful be sure to check in later for more!

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