Exposure Explained

HOME

Exposure Explained

One of the questions that I am asked about more often than I can remember is  "How do I get my exposure right?"

Your camera has a built in light meter which is used to calculate the correct exposure for a given subject. But what if you need to adjust things for different circumstances. You might be shooting a landscape and wish to get everything in focus. You'll need to use a small aperture to allow for increased depth of field f/11, f/16 or f/22 may be required. How does this then affect the other settings.

What if you need a fast shutter speed to freeze a bird's wings in flight or minimize the effect of camera shake whilst hand holding a heavy lens? You may want to use 1/1000s or faster to achieve this. How does this then affect the other settings on your camera?

Apart from the available light, there are three things that combine to give you the correct exposure.

• Aperture

• Shutter Speed

• ISO

Adjusting one of these three settings has an impact on each of the others. In other words if the available light produces a perfect exposure with an aperture of f/8, a shutter speed of 1/200s and an ISO set at 800, a change in one of those three settings will necessitate a change in at least one of the other settings.

The diagram below demonstrates how this works. Notice that in the second group of settings the aperture has been reduced to f/11. Leaving the ISO at 800 means that the shutter speed has to be slowed down by 1 stop to 1/100s to maintain the correct exposure.

The final group of settings demonstrates a combined movement of aperture, shutter speed and ISO whilst maintaining the correct exposure. In this group we have reduced the aperture even more to f/16 to increase depth of field. We have decided to reduce the ISO to 400 to improve image quality (the lower,  the better) which has required a reduction of the shutter speed by 2 stops down to 1/25s. It may seem complicated but it really isn't. If one thing goes up something else must go down by a corresponding amount.

To try and clear the situation up even further I have drawn up an Exposure Triangle which shows the effect one setting has on another.

You will notice that any two pieces of the triangle need to be adjusted in order to maintain the correct exposure.

Have a look at Aperture and Shutter for instance. Assuming the ISO is fixed, as the Aperture gets smaller (reducing the light transmitted to the sensor) the Shutter Speed needs to be slower (to allow more time for the light to reach the sensor). As you can see one balances the other.

Now take a look at moving ISO and Shutter Speed and keeping the Aperture fixed. If you need to use a faster Shutter Speed, to freeze action for example, you will need to increase the ISO to maintain the correct exposure.

Please feel free to save the Exposure Triangle image and print it off to keep with you as an easy reference tool.

You can set everything manually where you set both Aperture and Shutter Speed but to make things easier for you, your camera has two main semi-automatic modes for you to choose from:

• Aperture Priority or Aperture Value - You set the Aperture and the camera automatically chooses the correct Shutter Speed based on the built in light meter reading.
• Shutter Priority or Time Value - You set the Shutter Speed and the camera automatically chooses the correct Aperture based on the built in light meter reading.

The most commonly used setting is Aperture Priority which gives you the creative control over the DOF (Depth of Field) or how much of the scene is in focus. You'll find this setting very easy to manage and your photography will improve as you learn how to take control.