Aperture Priority Tutorial

Aperture Priority Tutorial

In the previous article we motivated why its hard to take your photography to the next level until you understand concepts like exposureaperture and ISO.

We had a look at the three side of the exposure triangle and observed that to expose an image properly, if the aperture (hole that lets light in) is opened wider, then either ISO (sensor sensitivity) needs to be reduced or the shutter (blocking light) needs to be opened for a shorter period, otherwise the image will end up overexposed.

Three Manual Modes

We investigated the three shooting modes where you are in control of the settings:

  • aperture priority,
  • shutter priority and
  • and manual mode

We made a case for using aperture priority 95% of the time. Sure there are hardcore photographers out there that claim that one should always use manual mode. Studio photographers love manual mode, and for good reason. It gives them great control, especially as they normally employ flash photography. On the other hand, a street photographer would end up with many under or overexposed photos if clouds pass overhead and light conditions change continually. For them it would make more sense to have the camera help with exposure. The easiest way to achieve this is to preset ISO and aperture (as they want control over depth of field), and have the camera only worry about shutter speed.


Aperture Priority Tutorial

Let us go ahead and take our first photo. I am using a Canon T3i/600D DSLR camera, but the principles should apply to any camera that can shoot in manual mode.


Taking a Photo In Aperture Priority Mode

Let’s take the chess board I used in a previous article as an example.

Aperture Priority Tutorial

Our aim is to take a handheld photo with the bishop in focus and the knight out of focus using aperture priority. We do not want to end up with a soft or blurry photo, as the camera will keep the shutter open long enough to expose the image properly. The longer the shutter keeps open, the higher the risk that the camera will record your tiny handshakes. To ensure a fast shutter speed, two options for enough light exposure on the sensor is to

  • increase the sensor sensitivity (ISO),
  • open the aperture even wider for more light to flood in

Step 1

Set the camera to aperture priority mode. On some cameras it is indicated by A.

Aperture Priority Tutorial
Aperture priority is set


Step 2

Have a look through the viewfinder.

If your camera does not have a viewfinder, use the display screen at the back. I do recommend that you consider getting a camera that has a viewfinder, as there are distinct advantages in shooting this way. This will be vital to the development of your photography skills. Please refer to the Essential Gear page for more information.

Step 3

When looking though the viewfinder you will see several marks on the screen:

Aperture Priority Tutorial
The central focus spot is on your subject


These are called the focus spots. If you depress the shutter button halfway, two things will happen:

  • one or more of the spots will light up in red, and
  • the camera will focus on whatever part of the image is located under the red spot/spots.

By depressing the shutter button completely, a photo will be taken with the focus spot(s) selected when the shutter was depressed halfway.

For this exercise, set your camera to have only one spot illuminated in red. That means that the camera will only focus on that spot. To select what focus spots are active, press the “Auto Focus point selector” button.

Aperture Priority Tutorial
Auto focus selector button


Now, while looking though the viewfinder, scroll with the left/right scroll wheel until the middle point is selected.

Aperture Priority Tutorial
Left/Right Scroll Wheel


This means your camera will now focus on whatever is situated in the middle of the image.

Step 5

Set the aperture (f-stop) and ISO. As we want to have a shallow depth of field (bishop in focus, knight in the background out of focus), the aperture needs to be set wide open. On a normal kit lens, like the 18-55mm lens for example, the aperture can open to f/3.5 (A larger number represents a smaller aperture).

Aperture Priority Tutorial
An aperture of f/3.5 and ISO 100 yields a shutter speed of 1/15.

The ISO is set by pressing the Quick menu button,

Aperture Priority Tutorial
Q Menu button

and moving the left right arrow buttons

Aperture Priority Tutorial
Left Right button

until the ISO item is highlighted. Move the left/right scroll wheel to adjust the ISO.

Let’s set the ISO to a low number. ISO 100 is the lowest we can go on a Canon T3i. A low ISO means a clean image with little noise (no grain or white spots). With an aperture we set to f/3.5 and the IS set to 100, the camera indicates that a shutter needs to open for 1/15th of a second to expose the image properly.

Step 6

Everything is now setup. Aim through the viewfinder, depress the shutter halfway and place the red spot on the bishop. The camera will focus on the bishop. Press the shutter all the way. Below is my result.

Aperture Priority Tutorial
The knight is out of focus, but the bishop suffers from camera shake blur

OK, so that photo did not come out sharp. The knight is out of focus, but the bishop in the foreground is soft and suffers from motion blur.

What went wrong? The shutter had to remain open for too long to expose the image properly. It recorded the shake of my hand which caused the blur. The camera shows that it would open the shutter for 1/15 of a second, which is a relatively long open time for a camera. What can we change to have a faster shutter speed? Aperture is already (on a 18-55mm lens at least) on its widest open setting, so no luck on that front. Our only remaining option is to make the sensor more sensitive so it can record the image faster. That means increasing ISO.

Let’s take ISO up from 100 to about 400. At 400 the image is still relatively noise free, but let’s see what the shutter speed will be at this juncture.

Aperture Priority Tutorial
An aperture of f/3.5 and ISO 400 yields a shutter speed of 1/100.

A shutter speed of 1/100 is a lot faster than 1/15. What should the shutter speed be to ensure no camera shake blurring? A good rule of thumb is to take the focal length of the lens – its reciprocal value should not be more than your current shutter speed for a handheld shot. So if the lens we are using is a 50mm, then the shutter speed should not be slower than 1/50 seconds. Let’s take a photo and see what happens.

Aperture Priority Tutorial
A sharper photo thanks to the faster shutter speed


You have just shot your first photo in aperture priority mode. Hope you enjoyed the short tutorial.

In the next article we will look at the different shooting scenarios, for example kids running around indoors, landscape shots, and the settings needed to capture great looking photos under those conditions.



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