Exposure triangle

The "exposure triangle" is a term you have likely heard mentioned if you've been looking up photography tutorials but you might not really understand what it's all about. That's okay. It is reasonably simple... once you understand it.... but it's also a little complicated because each part of the triangle has an affect on the other parts.

The exposure triangle is made up of three parts.

• Shutter
• Aperture
• ISO

The exposure triangle.

Shutter speed.

The shutter speed is how quickly the shutter opens and closes and allows light to hit the image sensor in the camera. The faster the shutter speed, the less time the sensor is exposed to light. The shutter speed is most often measured in fractions of a second.

So, you will see on the camera, or in the exif data for an image, a shutter speed such as 1/200. This means, the shutter was opened and the sensor was exposed to the light for one two-hundredth of a second.That's pretty quick. But, cameras can go much faster. My DSLR camera allows the shutter speed up to 1/8000, or one eight-thousandth of a second.

Fast shutter speed freezes the dirt being flung up behind the bike so it looks as though it is still.

Conversely, you could see an image with motion blur or light trails showing and read the shutter speed as 1 or even 15. If you see a shutter speed in whole numbers like this, it is indicating seconds. So a shutter speed of 15 means the shutter was open and allowed light to hit the sensor for a full fifteen seconds.

In this image, a slower shutter speed was used which captures the movement of the people in front of the building.

So, bearing those in mind you can easily see that a fast shutter speed will only let a very small amount of light to hit the sensor, whereas a slow shutter speed lets lots of light in. So clearly, the shutter speed controls the amount of light hitting the sensor.

NO! Well, no and yes. Yes, it does. BUT.... the main purpose of the shutter speed is controlling motion. Wait.. what?

Yes, controlling motion. For example, say your taking a photo of a car driving past on a racetrack. If you were to set a shutter speed of 1/4 (one-quarter of a second) you're going to end up with the car looking quite blurry because it moves quite a lot in that 1/4 second. Whereas if it was set to say 1/1000 (one thousandth of a second) it will look still and sharp because the sensor is only the recording light for such a short time that there isn't much time for the car to move in that time. (One thousandth of a second is pretty quick.)

So, the real purpose of the shutter speed is to control motion blur and freeze movement or show movement depending on the photo you want to take.

Click here if you want a slightly more detailed look at shutter speed, when and how to use it.

This leads us to the aperture.

Aperture.
The aperture is what we use to control how much light is let through the lens onto the sensor and in doing so, controlling the depth of field (sometimes known as depth of focus).

The aperture setting opens or closes an iris within the lens on the camera and that iris can open wide or be stopped down to a pinpoint hole. Obviously this controls how much light can get through and therefore the aperture goes hand in hand with the shutter speed. If you stop down the aperture (less light comes in) you also probably need to adjust the shutter speed to let that light hit the sensor for longer in order to get the correct exposure.

So, a small pinpoint aperture lets very light light through and therefore requires a much slower shutter speed. Meaning it's useful if you aren't trying to capture fast moving subjects. Whereas a wide open aperture lets lots of light in and therefore can use a faster shutter speed and will be able to capture fast moving subjects.

The other effect that the aperture has as I mentioned above is controlling the depth of field. A wide open aperture (such as f/1.8) will have a very shallow depth of field (meaning only a very small part of the image in front and behind the focus point is actually in focus).

You can see in this portrait that the background behind is blurry and so is the models hand in front of her. This is because the wide open aperture creates a very shallow depth of field, meaning only a narrow band in front and behind the focus point will be in focus and the rest will blur.

A closed (stopped down) aperture (such as f/16) will have a very large depth of field, meaning that a lot of the image will be in focus.

A stopped down aperture gives a much larger depth of field so much more of the image will be in focus at the same time. A very narrow aperture can ensure that as much as possible on the image from front to back is in focus at the same time. (But it will come with a significant reduction in light on the sensor which requires slower shutter speeds or higher ISO)

A shallow depth of field can be very good to isolate subjects such as people, animals, cars, etc from the background. Whereas a large depth of field can be great for capturing vast landscapes or big groups of people.

Last but not least... ISO.

ISO.

ISO controls how sensitive the image sensor in the camera is to the light that is hitting it.

So, the higher the ISO value, the better the sensor can record small amounts of light in dark environments.

Great... you say... so the camera can see in the dark by turning that up a lot.

Well, yes... sort of. It's true it will capture more light in a dark environment, but it still does need some light or it won't capture anything.... AND... there is a big trade off.

The more sensitive you make the image sensor, the more noise you will get in the image. Noise being grainy coloured dots randomly splattered everywhere as a result of the extra sensitivity of the sensor. So, you'll get a picture, but it won't look much good if the ISO is too high.

That said, if you're indoors and the light isn't great but you need a faster shutter speed to avoid blur, you can raise the ISO a bit to help without too much of an issue.

So to summarize.

Shutter speed, Aperture and ISO all affect the overall exposure of an image and all tie together so a change to one usually means changes to one or both of the other depending on the situation and the style of photo and what you need to achieve.

I will write a post in more detail regarding shutter speed, depth of field and ISO soon to cover in more detail the differences and give examples of when to change which setting.

Hopefully this has helped you get a basic understanding of how the exposure triangle fits together and is used. Keep an eye out for the future posts about each part of this in more detail for more examples and help to understand it better.

Also, if you have questions or topics you'd like more information on, leave a comment and let me know what it is and I will try to address those topics as soon as I can.

Also be aware that any camera that allows it (such as DSLR's) you can set the camera in a mode that allows you to set one aspect of the triangle that is the most important for the shot you want and the camera can (to some extent) set the other values accordingly to get a good exposure. So, it's good to understand how it works and know when to use what mode.

See you next time.