Last updated on October 1st, 2018

Understanding aperture, shutter speed, and ISO is crucial for beginner photographers. They are the foundations of digital photography. So let’s get to the basics.

What Are Aperture, Shutter Speed, and ISO?

Aperture Definition: The adjustable opening in a camera lens that light passes through and reaches the camera’s shutter and image sensor.

Shutter Speed Definition: The amount of time the shutter (a flap or curtain covering the image sensor) is open to control the amount of light passing through the lens to the image sensor and creating a photo.

ISO Definition: The camera image sensor’s sensitivity to light. A lower ISO means your camera is less sensitive to light. ISO stands for International Standard Organization.

Now you have an idea of the basics of photography. But what does all that exactly mean? Let me elaborate a bit more without going into the science. Besides, I’m not an optical physicist.

Understanding Aperture

The size of the aperture or lens opening can change and is measured by “f-numbers.” Lenses have a particular set of f-number sizes called f-stops.

F-stops are inversed. A large f-stop leads to small aperture; a small f-stop leads to a large aperture.

The part of the lens that controls the aperture size is the diaphragm, similar to the pupils of your eyes.  It’s made up of small blades which rotate synchronously.  A large aperture admits more light and a small aperture admits less light to the shutter and image sensor.  Aperture is one component in controlling depth of field.

aperture definition

Aperture Size Example: An aperture of 16 (f/16) is smaller than 4 (f/4).

 

Understanding Shutter Speed

The camera’s shutter remains closed until you press the shutter release button (to take picture) at which time the shutter opens and closes to allow the camera’s image sensor to capture a photo.  The shutter can open and close in less than 1/3000 seconds but can also be programmed to remain open for 30 seconds or more.

Typically when we are speaking about shutters, we are referring to mechanical shutters in a DSLR camera. However, there are also electronic shutters that are utilized by mirrorless cameras.

Most images will be taken with a shutter speed less than a second. Specialized photography such astrophotography will require a shutter speed of more than a second.

Understanding ISO

A lower ISO is used in daylight settings because the camera’s image sensor doesn’t need a lot of light to process an image.

A higher ISO is usually used in darker environments. A common issue is higher the ISO, the more noise (grain) your photo will have. Think of noise in this sense; imagine you took the best photo in the world but then someone sprinkled a little bit of salt and pepper on the photo. The salt and pepper is noise!

Ideally you want to have your ISO as low as possible but can be challenging at night. Typically ISOs ranges from 100 to 12800.

Now you know the meaning of aperture, shutter speed, and ISO. Next, we’ll go into two common camera settings: aperture and shutter priority.

What is Aperture Priority?

Aperture priority mode allows you to choose (set) the aperture size; whereas in automatic mode, the camera sets the aperture size. When you photograph the subject, the aperture will be fixed while the camera calculates the shutter speed and ISO for standard exposure. You can choose the ISO as well.

How to Use Aperture Priority Mode

Aperture priority is usually abbreviated with an A on the camera mode dial. Canon cameras abbreviate it with Av.

Canon 6D Aperture Priority Setting

Canon mode dial set to aperture priority (Av).

 

To use aperture priority:

  1. Set the mode dial to aperture priority.
  2. Use the main dial to select the desired aperture size.
  3. Set the ISO if required.
  4. Take the photo.

It is best to refer to the camera’s manual.

When to Use It

Use aperture priority when you want a shallow or narrow depth of field but are unsure of the shutter speed or ISO. Or you don’t have time to balance the shutter speed or ISO because you only have an instance to photograph but you know what depth of field you want.

The below photos are an example of aperture priority. The left photo was set with an aperture of f/22 and right with f/2.8. Both photos focused on the middle bottle of coke with a tripod. The ISO was auto and the camera determined the shutter speed for standard exposure. Focal length was fixed at 24 mm. Ideally, it’s better to keep the ISO as low as possible to reduce noise. This is why choosing the ISO is important instead of allowing the camera. Look at the difference between the depths of fields.

Aperture priority example

Left – Aperture set at f/22, 24 mm; Right – Aperture set at f/2.8, 24 mm

 

High-dynamic-range photography requires aperture priority (or manual) mode. When you take bracketed photos, you need to have the same depth of field when merging the photos. Otherwise, the merged photo will be blurry.

Downfall
If shooting handheld in low-light and the camera chose a slow shutter speed, then the image will be blurry. In this case use a flash, manual mode, or a tripod.

What is Shutter Priority?

Shutter priority mode allows you to choose (set) the shutter speed; contrast to automatic mode, the camera chooses the shutter speed. When you photograph the subject, the shutter speed will be fixed while the camera calculates the aperture size and ISO for standard exposure. You can set the ISO as well.

How To User Shutter Priority Mode

Shutter priority is usually abbreviated with an S on the camera mode dial. Canon cameras abbreviate it with Tv.

canon 6d camera dial set to shutter priority

Canon mode dial set to shutter priority (Tv).

To use shutter priority:

  1. Set the mode dial to shutter priority.
  2. Use the main dial to select the desired aperture size.
  3. Set the ISO if required.
  4. Take the photo.

It is best to refer to the camera’s manual.

When to Use It

There aren’t many needs to use shutter priority. For example, when you want a smooth and blurry waterfall you can use shutter priority to slow the shutter speed. But what depth of field do you want for the rest of the image? It’s better to choose the aperture size and shutter speed with manual mode.

The same goes for moving objects such as cars. Do you want the car to be slightly blurry or sharp? Choose a slow or fast shutter speed, respectively. But what depth of field do you want? Again, use manual mode.

The below photo was taken using shutter priority mode. It was sundown and I took a photo of a SUV. The SUV appears to be idle but was traveling approximately 40 km/h (25 mph). I set the shutter speed to 1/1000 secs and ISO to 2000. The camera calibrated the aperture to f/4.0 for stand exposure. If I took this photo with automatic mode, then the shutter speed would have been slower and the SUV blurred.

 

Shutter priority at 1/1000: 24 mm, f/4.0, ISO 2000

Shutter priority at 1/1000: 24 mm, f/4.0, ISO 2000