Exposure Triangle

Aperture, shutter speed, and ISO all work together to create a properly exposed image.

The exposure triangle is a relatively new term first bursting onto the scene somewhere between 2005 and 2010. Before that, it was “time + intensity = exposure.” With digital cameras and their ability to change ISO values, that old mentality went out the window.

The exposure triangle, in my opinion, is meant to teach the concept of equivalent exposures. Assuming that you already have a properly exposed image, when you adjust one setting, you need to adjust something else in an equal, but opposite way.


What makes up the exposure triangle?



Aperture is just a hole that lets light hit some sort of recording medium (film, plate, sensor, etc.). They can be mechanical, like those found in modern lenses or leaf shutters. Or, they can be a simple hole, like those found in pinhole cameras. 

Apertures limit the amount of light which can reach a sensor by increasing or decreasing the size of the hole. Changing the size of the hole will also change the depth of field. (visit the Aperture page for a more in-depth understanding of aperture)

Shutter Speed

A shutter controls the amount of time that light is entering the camera. Like an aperture, a shutter can be extremely simple, like using your hand to cover the lens or a pinhole. Or, they can be very complex like leaf shutters.

All shutters limit the amount of light entering the camera by blocking and unblocking the light entering the camera. Shutter speed also dictates the amount of motion blur present in the final image. (visit the Shutter Speed page for a more in-depth understanding of shutter speed)


In broad terms, ISO is the sensitivity of the recording medium. The higher the number, the more sensitive the medium.

Unlike film, digital ISO can be changed with each photo. However, like film, the high you set your ISO, the worse the image quality will be. As you go up the ISO scale you will notice grain or noise, especially in the shadows.

Generally speaking, you should use the lowest ISO possible which produces the desired aperture and shutter speed settings. (visit the ISO page for a more in-depth understanding)

Why is it important?

The exposure triangle tells you nothing about what the proper exposure for a photo should be. So what makes it important?

The exposure triangle is all about equivalency. Equivalent exposures are when your photos from the same scene have the same brightness (or density if you’re using film). It is when you change one leg of the triangle, like decrease the aperture, and make up for the loss of light by increasing the ISO or decreasing your shutter speed.

Equivalent exposures become important as you look to control your depth of field through changes in the aperture, motion blur through changes in the shutter speed, or control noise with changes in ISO. Knowing that you have to make an adjustment in the other direction (more or less light/sensitivity) becomes very important to your creative process.



ISO is the final part of the exposure triangle. It controls the sensitivity (or gain) of your recording medium.

The important thing to remember with ISO is that the lower the number, the less grain or noise your image will have. Your film or sensor will also be less sensitive so you will either need to increase your aperture size or decrease your shutter speed to maintain the same equivalent exposures.

ISO is rated in whole numbers. Like shutter speed, if you double the ISO value (say, from 200 to 400), you have increased the sensitivity by one whole stop because you doubled the sensitivity. If you halve the ISO number, you have decreased the sensitivity by one whole stop.

I grew up shooting film so I personally believe that you should set your ISO according to where you are shooting first and adjust your shutter speed and apertures later. I know of other photographers who adjust it as they need (or use auto-ISO modes which fix your aperture and shutter speed to specific values and change the ISO as needed).

Generally speaking, the more light that a scene has available, the lower your ISO should be. 200 ISO is a great starting place for photos taken in daylight while 800 ISO is a good place to start for indoor photos. Adjust as needed to get the results that you want.


In order to increase the sensitivity of the film stock, larger grains of silver are used. This lessens the amount of detail present and leads to noticeable grain in the finished print.

ISO in digital cameras is handled a bit different. All camera sensors have a base ISO (also called native ISO). From there more voltage is applied to the sensor to reach higher ISO ratings. As this happens, digital noise is introduced to the final image which degrades it the more you increase the ISO.

Just like with film, as you increase the sensitivity of your sensor, you lose dynamic range (the range of values and colors present in an image). This is why photos taken at night often look washed out or have a yellow/orange/brown tinge to them, especially in the shadows.

Marian McBride


Sally Bishop

Creative Director

Lelia Meyer