Getting to grips with... Aperture

Aperture and exposure

Aperture, which along with shutter speed controls the amount of light that hits your camera's sensor. While shutter speed controls the duration of time that your sensor is exposed to light (we'll go into this in greater depth next month), Aperture controls the strength of the light reaching your chip. This is normally done via a variable diaphragm on the lens that can be opened at different apertures, with the opening either being increased to let more light in or closed down, to limit light hitting the sensor.

Why would you want to do this? Photography is all about light, and because the intensity of light can vary from location to location, time of day or season, you need some control over the level of light hitting your sensor to produce a correctly exposed image. Just like the iris of the human eye would contract on a bright day when light is in abundance, you'll want to reduce the opening of aperture to a small hole to avoid overexposing your shot, while a darkened room will require you to have as much light as possible hitting the sensor.

These aperture openings are measured in what are termed ‘f-stops', or ‘f-numbers'. Each ‘stop' increase in aperture number reduces the size of the lens diaphragm, reducing the amount of light hitting the sensor by half. F-stop numbering doesn't appear to be the most obvious though, so for instance, increasing the aperture of a lens by one stop from f/8 to f/11 reduces the amount of light by half, while opening up the lens from f/4 to f/2.8 doubles the amount of light passing through the diaphragm. The thing to remember is that the higher the aperture f-number you use, the smaller the aperture hole, and less light that can enter the camera.

Wide apertures are more suited to portraits and wildlife, isolating your subject

Aperture and depth-of-field

Why would you want to control aperture yourself, when it would be easier to let the camera do it automatically for you? It's because aperture does not only limit light getting to your camera's sensor, it also allows you to control the depth-of-field of your shot, and is one of the most important creative decisions you can make to your image, and something that's virtually impossible to replicate afterwards in Photoshop.

Controlling depth-of-field allows you to set how much of the shot is in focus - some shots such as portraits may work better when the background is blown out of focus to isolate your subject for instance, while landscapes are normally more successful when everything is pin-sharp through the frame, from the foreground to the background.

With your lens aperture wide open letting in as much light as possible also results in a narrow zone of focus. When you bare in mind that your camera can only focus on one plane of distance, along with a small area in front and behind of that also appearing sharp, you're left with a shallow depth-of-field. Stop your lens aperture down further and you're not only letting less light in, but your zone of focus also improves, extending the area of sharpness in front and behind the plane of distance to increase the depth-of-field. Reduce the size of the aperture even further until your lens is stopped down as much as possible, the zone of focus is much greater, providing you with a sharp focus across the entire frame.

The Aperture Scale

As you stop down the lens, you reduce the size of the aperture, limiting the amount getting to the sensor, but also improving the depth-of-field. Wide apertures such as f/2.8 are more suited to portraits and action to isolate the subject, while smaller apertures such as f/11-16 are more suited to landscapes, ensuring everything is sharp in the frame.

Large aperture f/2.8

Most cameras will also be able to set third or half stop aperture increments - between a stop of f/5.6 and f/8, you'll also be able to set you camera to third stop increments of f/6.3 or f/7.1 for instance, allowing you even more precise exposure control.  

Small aperture f/16

Depth-of-field and your camera

Depth-of-field is not fixed however, and there are a couple of factors to consider. The first is the focal length - the longer the focal length, the shallower the depth-of-field, with telephoto lenses more suited to portraits, while wide-angle lenses, not only fitting more in the frame, are more at home shooting landscapes due to the greater depth-of-field achievable.

As the zone of focus is greater behind the plane of distance than in front, focusing is also key. If you're want to get everything pin-sharp, then focus near the front of the frame and stop your lens right down. Focus in the middle distance at the same aperture and the foreground won't be as sharp.

Finally, sensor size also plays a part when it comes to depth-of-field. As the physical sensor size of the camera increases - from Micro FourThirds (used by cameras such as the Panasonic LUMIX G2 and Olympus PEN series) to APS-C (most Canon and Nikon DSLRs for instance), through to full-frame DSLRs (such as the Canon EOS 5D MkII and Nikon D700), so does the ability to achieve shallow depth-of-field shots become easier.

DSLR at f/2.8

This is because larger sensors require you use a longer focal length if you're going to be shooting from the same distance to your subject to fill the frame as you would with a smaller sensor. That's not to say you can't get lovely shallow depth-of-field shots with a Micro FourThirds camera, but the effect is more pronounced using a camera with a larger sensor. This is also why on a compact camera, even using a wide aperture, shallow depth-of-field shots are hard to achieve as the sensors are physically much smaller (with the odd exception) than a Compact System Camera (CSC) or DSLR, which is compounded by the very short distance from the rear of the lens to the chip.

Compact at f/2.8

Try it yourself

The best way to understand how aperture works and how it effects depth-of-field is to have a go yourself with your own camera. Over the next couple of steps, we'll show you how to set your camera up to achieve a shallow depth-of-field shot, and another one where everything's pin sharp. Let's get started...

Shallow depth-of-field

Step 1: Aperture Priority

Set your camera to Aperture priority mode - this is where you'll set the aperture on the camera yourself, but will let the camera set the shutter speed automatically so you get a balanced exposure.

Step 2: Set aperture

Now set your aperture, so dial in an aperture as wide as possible. This will vary depending on your lens, but even with your lens fully extended, an aperture of f/5.6 should deliver a blurred background if you're far enough away from your subject. To set the aperture, most cameras feature a Command dial to toggle through the settings.

Step 3: Focus

Focus on your subject and with your image framed up, fire the camera's shutter. Review your shot, and you should have a pleasing blurred background to isolate your subject. If it's not as pronounced as you'd like, widen the aperture even further if possible.

Large depth-of-field

Step 1: Increase aperture

Keep your camera in Aperture priority, but change the aperture to f/16. While you may be able to stop down even further, some lenses can suffer from softness, so f/16 is a good compromise for this.

Step 2: Check shutter speed

Reducing the amount of light getting through the aperture will lengthen the shutter speed. If you're handholding your camera and the shutter speed is below 1/60th sec, camera shake may be a risk so you might want to increase the ISO or use a tripod.

Step 3: Focus

Focus on the nearest area possible in the frame to ensure when you take the picture that focus remains in front of the plane of distance - focus further in the distance and your foreground will be out of focus. Fire the shutter and check your shot - you should have an image with focus running through the entire image.