The Exposure Triangle (Myfotoguy Series on Exposure Part 1 of 5)

 Myfotoguy series on exposure part 1 of 5

In photography, exposure is the total amount of light allowed to fall on the film or image sensor during the process of taking a photograph. (Wikipedia)

Controlling exposure is one of the basic elements of photography. The word "Photography" itself comes from two latin words:

  • foto- which means "light",
  • and grafis - which means "control"
So, you could say that photography is about "the control of light". That's what photographers learn to do, control the light that enters a camera so the image is produced properly (or as desired if going after a certain look or effect).

The “Exposure Triangle” is a common tool to help explain what factors contribute to exposure.
This article is just a primer. In parts 2-5 we’ll go into greater detail on each element of the triangle.
 
It’s represented as a triangle, because if you change one of the elements, the others are impacted. The three elements are:
ISO – This is the sensitivity of the medium recording light (in a digital camera this is how sensitive you have set the image sensor to be). A lower number means the sensor is less sensitive to light, a larger number means it’s more sensitive.
Shutter Speed – This controls the length of time the shutter is open. (The length of time you allow light to be recorded by the image sensor).
Aperture – This is the volume of light, or how much light you are letting through to the image sensor. How big the hole is that the light can pass through on the way to the image sensor.

 

Working Together
Since these three things work together to contribute to the exposure, you can’t change one without changing the other. For example:
Let’s say your exposure is all set, and you decide you want to use a faster shutter speed to stop the action, so you increase the shutter speed.
This means the image is recorded for less amount of time to stop the action…
That means light comes to the sensor for less time, so you need to either:

  1. Allow more light to get to the sensor in this shorter amount of time. By that I mean more volume of light, so make your Aperture a bigger hole to get the light through faster. Or….
  2. Make your image sensor more sensitive so that it gathers the light info more quickly. To increase the sensitivity you would make the ISO a larger number.
Okay, are you confused? A couple metaphors may help. At some point metaphors usually tend to break down, but I think these will help.

 

Filling a Glass of Water

Think of filling a glass of water at your kitchen sink.
 
Water Valve (Aperture) - You turn the water on to the desired flow. If you open the water valve so that more water is getting through, you have increased how much water is flowing. If you close the valve down a bit, then less water is coming through (a smaller hole).
Time (Shutter Speed) – How long you leave the water on determines how much water goes into the glass.

Glass (ISO) – For this part of the analogy a small glass is more sensitive (because it takes less time to fill) and a large glass less sensitive (it takes longer to fill).

If you open the valve (aperture) wider, it takes less time (shutter speed) to gather the desired amount of water you want.

If you close the valve (aperture) so it’s smaller, the flow slows and it takes more time (shutter speed) to gather the amount of water you want.

If you are after a full glass of water, the size of the glass (ISO) will determine the requirements to “fill” the glass. You can open the valve wider if you want to fill it quicker. Or you can leave the glass under the tap longer and wait for the water to fill it at the rate the valve is allowing it to flow. If you switch to a larger glass (lower ISO number, less sensitive) it will take more time to fill unless you open the valve more. If the valve were opened more (aperture) then you get water coming faster (shutter speed) and you could fill it the same amount of time it took to fill the smaller glass.


Window Blinds on your Bedroom Window

Another metaphor is window blinds.
 
Blinds (Aperture) – How far the blinds are open determines how much light fills the room.
Time (Shutter Speed) – How long the blinds are open determines how quickly the room fills with light.
Eyes (ISO) – Your eyes represent ISO. If you just woke up, your eyes are more sensitive to light (high ISO number). If you walked into this room after being in another bright room, your eyes would not be as sensitive (lower ISO number).


Understanding Exposure

Understanding Exposure: How to Shoot Great Photographs with a Film or Digital Camera (Updated Edition)
Bryan Peterson has written an excellent book titled "Understanding Exposure" which has helped thousands of photography students across the globe gain a better understanding of exposure. In it he explains the Exposure Triangle in greater detail.
This is the first photography book I purchased at the recommendation of many others on photography forums. I highly recommend this book.


 
Now What?
Practice! Change your settings and see what happens. Keep in mind:
  • Changing one of the three elements impacts the exposure.
  • Changing one of the elements also requires changing one or both of the other elements.

 

Semi-Auto Modes

Most cameras allow you to automate the process a bit. If you set the camera to modes such as Aperture Priority or Shutter Priority, you decide on one or two of the elements and the camera will decide the other.

Note: This is exposure in its simplest form, you could add other variables to the equation, such as flash, but that will be for a future lesson.


What do you think?
What metaphors have you heard for the exposure triangle?

 
 

Articles in this series:
Part 1 - The Exposure Triangle
Part 2 - Stops, Values, and Sensitivity
Part 3 - Aperture
Part 4 - Shutter Speed
Part 5 - ISO
 

 
 

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