Exposure Triangle Explained: The relation between aperture, shutter speed and ISO

The exposure determines  how bright or dark an image will look like when you take a picture. If the picture is too bright, we call it overexposed. If it is too dark, then it’s underexposed. The light that reaches the camera’s sensor depends on three basic settings:

These factors form what is known in photography as the exposure triangle.

In order to produce professional looking pictures, it is essential to understand how changing one of these settings affects the others. To help you understand the relationship between these three elements, I have the following graphic ready. It will give you some suggestions for where to start with your settings. After that, it’s entirely up to your experimentation.

What is exposure?

Achieving the correct exposure is a tremendously hard thing to do, even for experienced photographers. Let’s compare this with the action of collecting rain in a bucket. You can control three factors no matter how heavy the rain is: the bucket’s width, the duration you leave it in the rain, and the amount of rain you want to collect. Your job is to ensure you don’t collect too little and at the same time, not too much.

watter-bucket-exposure-comparison

 

The key is that there are many different combinations of width, time and quantity that will achieve this. For example, for the same quantity of water, you can get away with less time in the rain if you pick a bucket that’s really wide. Alternatively, for the same amount of time you leave in the rain, a really smaller bucket can be used as long as you plan on getting by with less water.

In photography, the exposure settings of aperture, shutter speed and ISO speed are analogous to the width, time and quantity discussed above. Furthermore, just as the rate of rainfall was beyond your control above, so too is natural light for a photographer.

Controlling The Exposure Triangle

Each setting controls exposure differently:

  • Aperture: controls the area over which light can enter your camera
  • Shutter speed: controls the duration of the exposure
  • ISO speed: controls the sensitivity of your camera’s sensor to a given amount of light

You can therefore use many combinations of the above three settings to obtain a similar exposure. While you adjust your camera’s settings, you must take into consideration the fact that each one of the above triangle elements also influences other image factors as follows:

  • Aperture affects depth of field
  • Shutter speed affects motion blur
  • ISO speed affects image noise

shutter-speed-aperture-iso

Here are a few tips on the priority of changing the exposure factors:

  • Evening conditions, no tripod available: increase ISO to reduce the shutter speed for the same exposure value, or else your picture will suffer from camera shake.
  • Portrait photography, outdoors good light: control the aperture to obtain a shallow DOF, adjust ISO only if exposure time takes too long.
  • Waves crashing lightly into the water: shutter speed should take longer to obtain the silk effect, and ISO should be small enough not to add too much grain to that silky area of the water.

Exposure Triangle Factors:

(click on the image to enlarge)

EXPOSURE_TRIANGLE

SHUTTER SPEED OR EXPOSURE TIME

A camera’s shutter determines when the camera sensor will be open or closed to incoming light from the camera lens. The shutter speed specifically refers to how much time passes before the diaphragm closes and the picture is ready. “Shutter speed” and “exposure time” are expressions of the same measurement, where a faster shutter speed means a shorter exposure time. Shutter speed’s influence on exposure is simple: it correlates exactly 1:1 with the amount of light entering the camera. For example, when the exposure time doubles the amount of light entering the camera also doubles.

APERTURE

A camera’s aperture setting controls the area over which light can pass through your camera lens. It is specified in terms an f-stop value. The area of the opening (diaphragm diameter) increases as the f-stop decreases. In photographer slang, when someone says they are “stopping down” or “opening up” their lens, they are referring to increasing and decreasing the f-stop value. Every time the f-stop value halves, the lightcollecting area quadruples. To learn more about aperture, click here.

ISO SPEED

The ISO speed determines how sensitive the camera is to incoming light. Similar to shutter speed, it also correlates 1:1 with how much the exposure increases or decreases. However, unlike aperture and shutter speed, a lower ISO speed is usually preferred, because higher ISO speeds dramatically increase image noise. As a result, ISO speed is only increased from its minimum value if the desired aperture and shutter speed aren’t otherwise easy to setup.

EXPOSURE COMPENSATION

Even if the exposure triangle is known for the three before mentioned factors, there’s a forth parameter that can change the luminosity of your image. Exposure compensation (shortened with EV) is an easy way to correct for improper exposure.  It is a “sliding scale” found on most digital cameras, usually indicated with a “plus/minus” sign and a sliding scale, usually ranging from -2.0 on the left to +2.0 on the right.  Plus means the image will be lighter, while a minus value will darken the picture.


This article is an excerpt from Complete Technical Photography Manual
(for any camera and any photographer!)
It is an exceptional ebook available only on BetterPhotosAcademy.com.

To get the ebook, click here!

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Laura

Laura

I started photography as a hobby in 2005, during college. My passion slowly became a more important part of my life since 2008. Because of using a combination of my photographic knowledge, with those of internet marketing, I like to call myself a "photomarketer".
Laura

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