Beginner Tutorial – Aperture and Stops of Light

In the last post, I provided an introduction to aperture.

Here’s a quick summary of that information:

  • Aperture indicates whether the opening in your lens is wide open or narrow
  • Your digital SLR communicates with the lens via electronic contacts
  • You can control the width of the opening by changing the aperture setting from the camera
  • A small aperture number (2.8) represents a WIDE opening in the lens
  • A large aperture number (22) represents a NARROW opening in the lens
  • Aperture numbers follow a standard scale called f-stops.

The aperture f-stop scale looks like this:

f/2.8, f/4.0, f/5.6, f/8, f/11, f/16, f/22

Now, this particular scale shows each aperture number changing by something called a full stop of light. When the aperture changes by a full stop, the amount of light landing on the camera’s sensor is either doubled or halved.


  • f/2.8 lets in twice as much light as f/4.0
  • f/4.0 lets in half as much light as f/2.8

Once you realize that the lens aperture can be changed by full stops, a question arises: what about half stops? Yes, aperture numbers can also be broken down into a half-stop scale:

f/2.8, f/3.5, f/4.0, f/4.5, f/5.6, f/6.7, f/8.0, f/9.5, f/11, f/13, f/16

While a simple half-stop scale might make the most sense, most digital SLR cameras are actually set up by default to change aperture in third-stop increments:

f/2.8, f/3.2, f/3.5, f/4.0, f/4.5, f/5.0, f/5.6, f/6.3, f/7.1, f/8.0, f/9.0, f/10, f/11, f/13, f/14, f/16

Let’s take a quick break from the numbers and talk about HOW you actually change the lens aperture using a digital SLR camera.

  • First, you have to set the camera in a mode that allows you to change aperture – a good one to start with is “Aperture Priority” mode (often marked as “A” or “Av”)
  • In order to change the aperture value, you have to spin the main command dial which is located near the shutter release under your index finger

All main command dials don’t just spin freely: they “click”. Let’s say that your digital SLR is one of the many where aperture numbers are broken down into third-stops. Let’s also say that currently your aperture is set to f/8.0. Now, if you want to OPEN the lens by a full stop, you click the main command dial three times in one direction to go from f/8.0 to f/5.6. If you want to NARROW the aperture back down to f/8.0, you click the main command dial three times in the opposite direction.

The reason that you have to click three times is because of the third stops.

  • f/8.0 to f/7.1 = +1/3 stop of light
  • f/8.0 to f/6.3 = +2/3 stop of light
  • f/8.0 to f/5.6 = +1 full stop of light
  • f/5.6 to f/6.3 = -1/3 stop of light
  • f/5.6 to f/7.1 = -2/3 stop of light
  • f/5.6 to f/8.0 = -1 full stop of light

Before we wrap things up on aperture control, a word about customization: the factory default setting for most digital SLR cameras is to use a third-stop aperture scale. However, some cameras allow you to customize the scale and change from third-stops to half-stops if you so desire. Both work equally well, the third stop scale just provides you with a slightly finer level of aperture control.

By kind permission of Chris Roberts of