Digital Photography: The Exposure Triangle

The Exposure triangle is the most important aspect of photography.  Its three primary components are, Aperture, ISO, and Shutter Speed, but before we move further let me discuss a few things in detail.

Shutter speed is easy to understand, but it's complicated in practice. In its simplest form shutter speed is how long the shutter will remain open when pressing the shutter release button. It is represented as fractions of a second to as long as minutes or hours.  The shutter controls three things.  

1. Shutter speed controls the length of time light is let into your camera to correctly expose the frame. While looking at your light meter you can see how under or overexposed the image will be based on a set amount of time, and is directly related to how open the lens iris is (your aperture setting) and the sensor's light sensitivity (your ISO setting).  Increase the speed and the exposure will darken. Decrease and it will lighten.

When exposing for low light with a slow shutter speed it's recommended you use of a tripod and a remote trigger to avoid camera shake.

2. Shutter speed either freezes or blurs action.  If you are photographing live action sports and you want to freeze movement you want a fast shutter, or you could slow the shutter to create motion blur.

3. A slow shutter speed can show how slow moving objects change over time. For example, in the film days when photographing star trails you would keep the shutter open for an hour or more.  

One thing to note is that long exposure photography is a relic from the films days. Today, long exposure photography is done by taking many photographs of the same scene then compiling them with software. So instead of leaving the camera's shutter open for two hours to capture star trails you would instead take 1,440 five second exposures stitching them together with software to achieve the same affect all while making a cleaner image. 

The Reciprocal Rule With A Twist

Zooming with a telephoto lens can cause unwanted camera shake leading to blurry images. To avoid this you need to employ the Reciprocal Rule. That is, your shutter speed should not be slower than the focal length of your lens as measured in fractions of a second.  This is to avoid motion blur when hand holding a telephoto lens.

For example, if you are shooting at a 100mm focal length then your shutter speed should not be less than 1/100 of a second. If you are at 200mm you should shoot at a minimum of 1/200 of a second.  

But here's the twist!  If you are not shooting with a full frame camera then you have a camera with a cropped sensor. This adds a multiplier that is based on the size of your camera's sensor (i.e., Nikon = x1.5, Canon = x1.6, Lumix & Olympus micro 4:3 = x2).  

For example, you have a 200mm focal length with a Nikon DX then your are effectively shooting at a 300mm equivalent and should not lower your shutter below 1/300 of a second. A Lumix or Olympus would be at 1/400 of a second. This takes a little thought, but with practice you can quickly do the calculations and be on your way to making tack sharp photos.

Stops of Light

Light is designated with an f/number where f represents doubling or halving the amount of light entering the camera.  You most often see this on your lens with an f/number designation (e.g., f/2.8, f/4, f/5.6, etc.). One stop from f/2.8 to f/4 will cut the amount of light entering your camera in half. Opening your aperture up from f/11 to f/8 will double the amount of light.

This is the primary reason why large aperture lenses are so expensive. A fast f/2.8 telephoto lens requires a lot of glass, and is big and heavy. Whereas, slow kit lenses are much less expensive because they have variable apertures. To learn more, I wrote extensively comparing single aperture with variable aperture lenses. 

Exposure Triangle

Armed with these three pieces of information (Shutter Speed, Aperture, and ISO) you create the Exposure Triangle.  Keep in mind, the exposure triangle is always in flux.

Think of it this way.  Proper exposure is in the center of the triangle.  If you move up or down on one side you need to move in the equal and opposite direction for one of the other sides. Or a half stop on both opposite sides to maintain the correct exposure.

For example, your camera's sensor has a specific native sensitivity for a properly exposed image.  If you have a correctly exposed image with the settings of 1/125 shutter speed, aperture set to f/5.6, and ISO 800, and you speed up the shutter to 1/250 to freeze action then you either need to open the aperture or increase the ISO by one stop, f/4 or ISO 1600 to maintain the correct exposure.  If your camera is capable of half stops then you can do one half stop of each instead of a full stop of one or the other.  

If you changed any one of the three without changing either of the other two then your image may be over or under exposed by one stop of light.  To see this in action put your camera in manual mode and start turning the shutter and aperture dials while leaving the ISO constant.  You will see your light meter move from a proper exposer to under and over exposed.  

Exposure is always in flux as light is never constant. With a strong understanding of the exposure triangle you will capture great images.  To get out of Auto and into Manual I recommend you shoot in Auto first to see what your camera is doing, then emulate those settings and adjust them by one or two stops up or down to see how each setting affects your image. 

Now get out there and make great pictures.