Depth of Field

Aperture Triangle

The Exposure Triangle (from Digital Camera World)

There are 3 fundamental controls on our cameras and they form the sides of this triangle:

1. Shutter Speed - the length of time the camera is open to allow light to reach the sensor.
2. Aperture - the size of the hole in the lens that allows light to reach the sensor.
3. ISO - The sensitivity of the sensor to light.

The first 2 controls limit the amount of light getting into the camera, the third dicates what happens when the light gets in there.

All 3 sides of the triangle have to be in equilibrium, change any one and you have to change one or both of the others to compensate.

As we move away from always using automatic modes on our cameras, we need to understand this triangle.  We spoke about 2 settings on the main camera control dial: 

EOS90D Control Dial

on my camera, Canon EOS90D, they are (all cameras have these but the terminology varies):

1. Tv (Time Value) controls the shutter speed.
2. Av (Aperture Value) controls the aperture.

These controls allow the user to set either shutter speed or aperture while the camera sets the other according to the amount f light in the scene.  The ISO is set elsewhere and often has an auto setting, we will consider this in a later meeting.

This post looks at Aperture Value and its impact upon depth of field.

Depth of Field

With the camera set to Aperture Value the camera will have a selection control to set the size of the aperture using a number scale such as that shown in the following image, f/2.8 to f/22:

Do not worry about the exact numbers and the strange sequence for now.  Just remember the smaller the f/No the larger the aperture.

Why change the aperture size?

The main reason we adjust the aperture size is to control the amount of a scene that appears to be in focus, the depth of field.  The larger the aperture, the smaller the f/No, the less depth of field there will be.  We control the Depth of Field to get different visual effects.

For example:

• In a portrait we often want our subject to be sharp with a blurred background.
• In a landscape we often want a front to back sharpness from foreground interest to our subject and onto the distant background.

Small f/No Short DoF

In the example above a large aperture (small f/No) has resulted in a short DoF (the blue area).  The foreground logs, the distant tree and the background are all out of focus.

Note that 1/3rd approx of the DoF extends to the front of your subject, 2/3rd is behind it.

If we reduce the size of the aperture (increase f/No) the DoF will increase and, in this exampe, includes the foreground and background interest.

Large f/No Long DoF

If we focus on our subject we risk "wasting DoF because much of it is behind the subject and may not be in view.   I briefly mentioned how we can often optimise the Depth of Field by focusing in front of our subject.  That ideal focus point may be surprisingly close to our camera.