Knowledge & Skills: Aperture, Shutter Speed, and ISO

In this article I am going to talk about the most basic tools available to you for controlling the exposure of your images, using aperture and shutter speed.

Those tools are shutter speed, aperture, and ISO, which make up the exposure triangle. After I explain what each one does, I’ll explain why we need three separate tools to control the brightness or darkness of the photo.

What is the Exposure Triangle?

In photography, the exposure triangle explains the relationship between Shutter speed, ISO and Aperture. Whether you’re shooting old school film or with a mirror-less, these three factors are at the center of every exposure. Hopefully this image might help you to understand the relationship.

what is aperture

Aperture

What does aperture mean?

The aperture of your camera works exactly like your eye. When there is a lot of available light your iris or aperture closes down, the pupil contracts to limit the amount of light reaching the retina or sensor.

In low light the iris opens up and the pupil expands, letting more light hit the retina or sensor.

Your camera does exactly the same thing when set to auto, but when you are using aperture value (AV) you control how much light hits the camera sensor.

aperture and shutter speed - human iris reacting to light

The human eye is represented in your camera by the aperture, which is a small set of blades in the lens that controls how much light will enter the camera. The blades create an octagonal shaped hole that can be widened, or closed down to a small hole.

As explained above, if you shoot with the aperture wide open, (f/2.8) more light is allowed into the camera than if the aperture is closed down to only allow a tiny hole of light to enter the camera, (f/22.)

aperture and shutter speed - image showing aperture blades inside lens

When you take a picture that is over-exposed,  how do you fix it? The easiest way is to choose a smaller aperture. For example if you have taken the shot using f/5.6 try closing it down by a couple of stops to f/8.

NOTE: Aperture is measured by f-stops. A mid-range lens would give you f/5.6 (wide open) to f/22 (shut down.) It is worth noting here that the aperture has a direct relationship with the DOF or depth of field or plane of focus.

aperture

 

Depth-of-field (depth of focus or focal plane)

DOF is how much of the picture is sharp from front to back. Landscape photographers need the whole image to be sharp from the front to the back whereas portrait and wildlife photographers such as myself, prefer the subject to be sharp with a blurred background.

This is demonstrated quite well in this image of a Woodpecker.

aperture and shutter speed - Great Spotted Woodpecker

To achieve the blurred background I shot wide open, which on my 100-400mm telephoto lens is f/5.6. Using a wide aperture created a shallow depth of field.

If you want to take a landscape image, you need to use a small aperture size (high f-stop number such as f/11) so that the entire scene is in sharp focus. This image of the Wrekin was shot at 18mm at f/11 because I wanted the whole image to be in focus, from the stump to the horizon.

aperture and shutter speed - Develop your Photography Skills

Shutter Speed

What is shutter speed in photography?

The shutter is a small ‘curtain’ in the camera that exposes the image sensor to light. The longer the shutter allows light to hit the image sensor, the brighter the picture since more light is gathered.

The opposite applies when using fast shutter speeds.

The length of time the shutter allows light to hit the image sensor is called the ‘shutter speed’. Shutter speed is measured in fractions of a second.

A shutter speed of 1/30 sec will allow more light to hit the image sensor and will produce a brighter picture than a shutter speed of 1/400 of a second. However if the subject is moving, the 1/30 sec will create motion blur whereas the 1/400 sec will freeze the action.

The image of the Red Kite was taken using a shutter speed of 1/2000 sec which froze everything including the fast beating wings.

aperture and shutter speed - red kite hovering

shutter speed

However with this image of a Vulture the the shutter speed was only 1/100 sec, the other settings were ISO 250, f/25 @100mm. Obviously 1/100 sec is far too slow to freeze the motion and prevent motion blur, despite the fact that I was panning, (hence the streaky background.)

aperture and shutter speed - blurred image of a vulture

If we look at the histogram below you can see that the image is also under exposed. I could have cured the exposure problem by either upping the ISO or opening the aperture.

As you would expect, the shutter speed affects more than just the exposure. As mentioned above, the shutter speed is also responsible for controlling the amount of motion blur in your images.

Aperture and Shutter Speed Count but Let’s Not Forget ISO!

This article would not be complete without mentioning ISO. The ISO controls the exposure by using software in the camera to make it extra sensitive to light.

So what is the definition of ISO in photography?

ISO is an acronym for ‘International Standards Organisation‘. ISO can range from 50 all the way through to 6400 and higher.

What is a high ISO?

High ISO capability refers to a camera’s ability to deliver clean, as opposed to noisy, images at high ISO’s. ISO is a number that represents a sensor’s sensitivity to light, and while using a higher ISO brightens an image, it also diminishes images quality by introducing noise. Noise in an image can look like this image.

aperture

A high ISO such as f/1600 will produce a brighter picture than a lower ISO such as f/100. As I explain in detail in my article, ‘Understanding  ISO‘ the drawback to increasing the ISO is that it makes the picture noisier or grainier. Using a high ISO in low light situations makes the sensor hunt for light and this results in noise or grain in the image.

ISO setting

What constitutes a “high” ISO is constantly changing. I use the Canon 7D mkii because of its ability to use high ISO without creating much noise. However, as camera technology improves so does the ability to operate at higher ISO settings.

Since each camera is different, it might be an idea to take some test images with your camera to see how high of an ISO you can shoot at without making the image overly grainy.

Aperture, Shutter Speed, and ISO – Putting It All Together

If you are finding this confusing, join a huge club! This weekend I was trying to teach my son. He was getting frustrated, asking questions like, “Why do I need three tools to control the exposure. Why can’t they just use the one?” I will try to explain why with an example.

In November this year I took a short trip to the Stiperstones. I was having a play with my new lens, the Canon 10-20mm. In these three images I deliberately used different settings for the purpose of this article.

Image #1 – the Stiperstones, totally over-exposed!

ISO 100, f/11, 1/1.6 sec @10mm. Yes there really is an image here. If you look very closely you can just make out the dark ridge line to the right!

aperture and shutter speed - totally over exposed

The settings for the above image were ISO 100, f/11, 1/1.6 sec @10mm. As you can see from the histogram the image was totally blown out or over-exposed.

histogram

Image #2 – the Stiperstones, blown sky

ISO 100, f/18, 1/0.6 sec @10mm. This image is slightly better but the sky is still blown out and over exposed. This is caused by too much light hitting the sensor.

aperture and shutter speed - stiperstones

Looking at the histogram the mid-tones are fairly well exposed but the sky is totally blown or over exposed and the shadow areas need more depth.

histogram

Image #3 – the Stiperstones, under exposed

ISO 100, f/11, 1/125 sec @10mm. Almost there. Increasing the speed has once again reduced the amount of light hitting the sensor. This has resulted in a far better looking histogram although it is apparent that the shadows are still under exposed.

aperture and shutter speed

Looking at the histogram you can see that the sky is correctly exposed but the shadow areas are under exposed.

In these images the camera was on a tripod and I played with the shutter speed only. I increased the speed from 1.6 sec to 1/125 sec.

Can you take a guess as to what speed might have lead to a perfectly exposed image?

If you guessed 1/250 sec then you were correct – well done.

Hopefully you can see why you need to know how aperture and shutter speed, AND ISO, work together and how to use them independently.

If you are a hands-on learner and want to really explore your camera, then be sure to check out my ‘Get Off Auto‘ or ‘Develop your Photography Skills‘ workshops.

Happy shooting

Roger