The Exposure Triangle

By Ash Davies
January 14, 2010 from Beginner's Guides

The Triangle by Thackman

The Triangle by Thackman

Imagine you’ve just put a pie in the oven. Now, the back of the packet said it needed 20 minutes at 120 degrees, but you haven’t defrosted your pie yet and the guide on the pack is for a fully thawed out pie. How hot do you want it? How long should you leave it in for and at what temperature? Welcome everyone to the world of photography.

Confused? This is where The Exposure Triangle can help. Imagine that the time you leave your pie in the oven for is your cameras Shutter Speed, the temperature of the oven is the ISO, and the state of your pie (frozen or thawed) is your cameras aperture. These three aspects of photography help to make up what is called ‘The Exposure Triangle’. Each one is directly related to the other, and a change in one will require an adjustment in the other two to ensure a properly exposed photo.

This analogy may seem strange, but I find this is one of the clearest ways to understand the relationship between Shutter Speed, Aperture and ISO. In order to understand the relationship though, you first need to know what each one is, and how it affects your photo.

Shutter Speed: The Shutter Speed is a fairly simple concept. It basically defines how long the shutter is open for. A longer shutter speed, such as two seconds (2″), means that your camera’s sensor (the CCD) is exposed to the light for longer, and this subsequently results in a brighter photo. Likewise, if you use a shorter shutter speed, such as one two hundredth of a second (1/200), then the camera’s sensor is exposed to the light for less time resulting in a darker photo. In the pie analogy, the shutter speed relates to the time your pie is in the oven. If you leave it in the oven for 30 minutes, it will be much hotter compared to leaving it in the oven for 10 minutes.

If you want to leave it in the oven for at least 30 minutes however, but don’t wish for it to get burnt, then the easiest way to avoid this is to turn down the temperature of the oven…

ISO: The ISO is the temperature in our pie analogy. ISO refers to your camera’s sensitivity to light and usually ranges from ISO 80 to ISO 1600. When light passes through the lens it is detected by the camera’s CCD (the camera’s sensor). If you have a higher ISO, such as ISO 800, then your photo will appear very bright. If however you choose a lower sensitivity such as ISO 100, then the image will appear darker. In photography it is best to leave the ISO at a lower level, around the 200 mark. Higher ISO’s will result in a noisier image, a bit like how having an oven at 400 degrees will burn the edges of your pie, even if you leave it in the oven for a very short time.

So you’ve got a low ISO, and you’re compensating for this low sensitivity with a slightly longer shutter speed. The additional light let in by exposing the sensor for longer should help you to properly expose your photo. But what about the frozen pie? After all, if the pie is frozen then you might need to leave it in the oven for longer, or perhaps turn the temperature up…

Aperture: The aperture refers to the size of the opening within your lens. Aperture is measured in F-Stops, where a smaller F stop such as F3.2 means a large opening, and a larger F-Stop such as F22 means a much smaller opening. If you increase the size of your aperture to around F3.2 then there is more space for light to enter, which subsequently causes your photo to be brighter. If however you choose a smaller aperture of F22, then there is very little room for light to enter, and less light enters resulting in a darker photo. In the pie analogy, a smaller aperture means a frozen pie, whereas a larger aperture means a well thawed pie. If your pie is frozen, you will need to leave it in the oven for longer or turn the temperature up. Thaw it out however, and it will be ready much faster.

A well exposed photo is a result of the correct shutter speed, the correct ISO and the correct Aperture. A nice warm pie is similarly the result of the correct time, the correct temperature, and a pie in the correct state.

The Exposure Triangle defines the relationship between all three of these photographic aspects. Hopefully this pie analogy will help to explain just how they all affect each other. There is no right or wrong setting for taking a photo. Sometimes you may want a longer shutter speed to capture the flow of a waterfall, in which case you will need a smaller aperture and a less sensitive ISO to compensate for this extra light. Sometimes you may be photographing at night in which case you’ll want to increase the sensitivity, increase the shutter speed and use a wide aperture to capture all the light you need.

The best way to fully understand The Exposure Triangle though is to get out there and take photos. Move the dial to M and shoot on full manual, and proceed to adjust each setting to see how they change your photo. Even the pro’s get it wrong the first time, but having the understanding of how they work together will allow you to end up with a nice tasty photo.

  • http://www.gpuss.co.uk Charlie

    Haha; I’ve always used toast to describe exposures to people. You can have the grill on for longer or hotter and you’ll get the same result. And it works like a negative – going blacker the more you expose it. You could even “expose” a negative picture on toast by masking some of it off. There’s also the law of reciprocity failure, which can be analogous with overheating toast making it burnt on the outside and soggy on the inside, but by then people have usually stopped listening and are fiddling with a lens cap. Of course, my explanation never included the third element – the all-important ISO, which I took as a given. Funny to see cooking used thus … everything comes back to food in the end.

  • http://digitalslrphotographyguide.net Prince Vasquez

    Hehehe great analogy my friend. Pie on an oven :)

    Great article. Thanks for sharing.

  • Caroline

    Finally! Now I know I will never confuse one for the other! To me as a newbie this was very helpful. I got a fancy DSLR for Christmas but haven’t the faintes clue on how to make it take the pictures I can see in my mind… Now I FINALLY understand the relationship and can’t wait to start practicing :) Thank you!

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  • http://www.photoguides.net/?fbconnect_action=myhome&userid=1&height=400&width=370 Ash Davies

    Glad you like it. PhotoGuides is based on ‘Headlines’ by WooThemes.
    http://www.woothemes.com/2009/11/headlines/

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  • http://photo.stackexchange.com/questions/6598/what-is-the-exposure-triangle Matthew Miller

    Hi Ash! I think its funny that you start with the “triangle” as popularized by Bryan Peterson, and quickly discard it for the better metaphor of baking (even while still calling it a triangle). You might be interested in this article, where I discuss problems with the triangle model: http://photo.stackexchange.com/questions/6598/what-is-the-exposure-triangle