Beginner's guides to digital SLR Photography

Understanding aperture

Like shutter speed, aperture is one of the fundamental things involved in getting a correct exposure. Here’s what you need to know.


To make the new growth in this Banksia plagiocarpa stand out I only had to blur the background away. And I did that by using a big aperture.

What is it?

The aperture setting is the one that determines the size of the hole in your lens. The bigger the aperture the bigger the hole, which means more light gets into your camera and so your images end up brighter.

But first …

big and small apertures

The aperture setting controls the size of the hole letting light into the lens.

This article assumes you’re starting to venture beyond the Auto settings of your camera. Because if you work in Auto mode, your camera makes all of the aperture decisions for you.


Aperture settings are described as f-stops or f-numbers. That might seem just plain weird, but there’s a really good reason for it, and I explain it in my article on getting started in digital SLR photography. Also, a small f-number (such as f/1.4) represents a big aperture (a big hole), and a big f-number (such as f/22) represents a small aperture (a small hole). Why does the big represent the small? Rather than repeat myself here, you can find that explained too on that page about getting started with Digital SLR photography.


Sometimes photographers talk about ‘stops’. For example, when they’re talking about exposure compensation they might want to go up or down a stop.

A stop can be thought of as a unit of aperture. Huh? Okay, then think of it this way: if you increase your aperture by one stop then you’re doubling how much light gets onto your sensor. Go down a stop, and you’re halving the amount of light.

Big apertures

Big apertures do two things:

freshwater turtle

A big aperture blurred the background in this picture of a freshwater turtle. That helped draw attention to the turtle’s face.

Small apertures

Just like you’d expect, small apertures do the opposite to big apertures:

Northern Green Jumping Spider

In macro photography it can be a struggle to get a whole spider in clear focus. A small aperture increased the depth of field, which allowed most of this Northern Green Jumping spider to look nice and sharp.


Your maximum and minimum aperture (the biggest possible hole or smallest possible hole for letting in the light) will be constrained by the mechanical limits of your lens.

While you might want to often use your lens at its maximum (biggest) aperture, it’s unusual to want to use your lens at its smallest aperture. That’s because of a sneaky little problem called diffraction.



100% crops showing the same part of the same photo taken at f/5 and f/32. Diffraction caused the fuzziness at f/32.

Very small apertures (letting the light in through a really tiny hole) cause diffraction, which makes everything look a little bit blurry.

For that reason, I think it’s a good idea to never use an aperture smaller than about f/16, or to put it in other words, try not to use an f-number bigger than 16. In fact, even that’s pushing it, because in an SLR camera you’re going to get some diffraction at f/16 too but for things like macro photography, where you often need every bit of depth of field you can get then a little bit of diffraction is the price you sometimes have to pay. If you really want to get cleaner shots then try not to go for a bigger f-number than about f/10.

Weak light


A big aperture helped capture the weak light from stars in the night sky. This pic shows the Southern Cross.

A big aperture is handy if the light isn’t very strong. Because letting light in through a bigger hole in the lens means more photons are captured by the sensor in less time, which translated into photography language means shorter shutter speeds. Those shorter shutter speeds can come in handy if your subject is moving because they mean you’ll capture less motion blur.

How to take control of your digital SLR’s aperture

When your camera is in one of its automatic modes, it looks after the aperture settings for you. However it’s easy to take back control of aperture if you want. For example, you might want to manage how much depth of field you’re getting — which is something controlled by aperture.

All you have to do is to switch your camera to aperture priority mode. Most likely that will be done with a big dial on the camera and you select the setting that says ‘A’ (for Aperture) or ‘AV’ (for Aperture Value, which means the same thing).

Once you’ve done that, then your camera sticks with whatever aperture you choose. When getting its automatic exposure right the camera will make its own adjustments to shutter speed and maybe also to ISO. But the aperture will be whatever you decided.

If you want, you can switch to manual mode (‘M’ on the dial) and then your camera sticks with your choices of aperture and shutter speed.

Shameless plug

So that’s aperture for you, but if you don’t properly understand how it relates to depth of field and shutter and speed and how it fits in with why SLRs (digital or otherwise) are such fantastic cameras, then I really urge you to read my web page called the beginner’s guide to digital SLR photography. Yeah I know I keep shamefully plugging that article, but the feedback I’m getting tells me it’s actually helped an awful lot of people make sense of their fancy new cameras. Which means you can spend less time thinking about technical stuff and more time taking pictures. SLR cameras and the fancy new mirrorless offerings are fantastic tools and you’ll be amazed at how capable and versatile they are once you understand the fundamentals.


Beginners’ guides to digital SLR photography


Before you start


The essential basics


Making sense of technical stuff

Photography words

Photography words explained


Sneaky stuff


Common problems and their solutions

Preying mantis

Taking things further


Photography at night

Other photography stuff

Copyright © Mark David. All rights reserved