Photography Basics – Aperture

What is aperture?

The aperture is the opening in the lens that allows light through to the camera's image sensor, and determines the light rays that come to focus in the image plane. The smaller the aperture opening, the less light gets through to the image sensor, but the more of the image will be in focus.

Lens with aperture partially closed

Aperture is also known as f-stops or f-numbers, and is usually expressed as the focal length divided by the aperture. For example, a lens with a focal length (f) of 50mm and an aperture of f/2 would give an aperture opening size of 50mm/2 = 25mm. The same lens with an aperture of f/8 would give an aperture opening size of 50mm/8 = 6.25mm. So, as you can see, f/2 is a larger aperture than f/8. When looking at apertures, the smaller the f-number, the larger the aperture.

The reason that aperture is expressed as an f-stop rather than an actual diameter is because it allows us to easily compare depth of field and light gathering ability across lenses of different focal lengths. A 25mm lens with an f/2 aperture (12mm physical aperture size) will let through the same amount of light and give the same depth of field as a 50mm lens with an f/2 aperture (25mm physical aperture size). The longer the focal length, the larger the diameter of the aperture will have to be for a given focal length. This is why you can't purchase a 800mm f/2 lens - the aperture diameter would need to be 400mm! That would make for a very large, heavy, and expensive lens.

Lenses are usually marketed using their maximum (largest) aperture. For zoom lenses, the maximum f-stop will usually change throughout the zoom length, and so these lenses will be marketed with their maximum aperture range, e.g. a 18-55mm f/3.5-5.6 lens means that the lens' largest aperture at 18mm is f/3.5, while at 55mm the largest aperture is f/5.6. You can purchase zoom lenses with a constant aperture, but these tend to be more expensive.

There are a few different formats that you may see used for aperture, for example, a 50mm lens with a maximum aperture of 1.4 may be written as:

• 50mm f/1.4
• 50mm/1.4
• 50mm 1:1.4

f/aperture is by far the most common way of writing an aperture value though.

The aperture setting on your camera can be changed by putting your camera into aperture priority mode (usually indicated as A or Av on the mode dial), or Manual mode (usually indicated as M on the mode dial).

Mode dial on Canon 450D indicating camera is set to Aperture Priority mode

Top LCD panel on Nikon D200 indicating camera is set to Aperture Priority mode

Some lenses also feature an aperture ring that lets you set the aperture directly on the lens by turning the ring.

Aperture ring on Nikon 50mm/1.4 D lens

F-stops are numbered starting from 1, and then increasing by a factor of √2 (about 1.4). Each increase represents a halving of the light transmitted. Full f-stops, starting from f/1 would be: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16... Aperture is not always measured in full stops, most cameras allow you to change the aperture value in 1/3 stop increments, and many lenses fall somewhere in between two full f-stop values. For example, f/1.8 is 2/3 a stop slower than f/1.4 and 1/3 of a stop faster than f/2.

50mm/1.4 lens at f/1.4

50mm/1.4 lens at f/2

50mm/1.4 lens at f/2.8

50mm/1.4 lens at f/16

A larger aperture allows more light through the lens, and is known as being 'faster' than a smaller aperture - it allows a faster shutter speed. E.g. f/2.8 is faster than f/4. Lenses with large maximum apertures are also known as fast lenses. Typically these are lenses with an aperture of f/2.8 or less.

Using aperture to affect exposure

Because the aperture controls how much light your camera's sensor receives, it can be used to modify the exposure / brightness of an image, or to allow you to use a faster shutter speed or slower ISO. One stop change in the aperture value equates to one stop change of shutter speed, or one stop change in ISO value. So for an image with the same brightness, you can open your aperture by one stop to allow you to increase your shutter speed by one stop. For example, the following settings would all give an image with the same exposure:

Or you can open your aperture to keep the same shutter speed, but decrease your ISO speed:

When photographing fast action, such as sports, you may want to use a fast shutter speed. Likewise, when shooting handheld in dimly lit conditions, you may want a fast shutter speed to prevent blur from camera shake. In both these situations it would make sense to use a fast aperture (e.g. f/2.8) to allow you to use a fast shutter speed while keeping the exposure correct. You would probably want to use a fast ISO as well in these situations.

Malta International Swim Meet 2011 by Michele | Mic. Agius Photography on flickr - shot at f/2.8, ISO1250, 1/2000s (licensed CC-BY)

IMG_3429 by Lachy Leung on flickr - shot at f/1.4, ISO1600, 1/20s (licensed CC-BY)

In some situations you want to use as slow a shutter speed as possible, for example when trying to blur moving water. Here you would use a small aperture, e.g. f/16, along with a slow ISO e.g. ISO100.

carezze by why 137 on flickr - shot at f/29, 1/4s (licensed CC-BY)

Although ISO can also be increased to allow for a brighter exposure, or a faster shutter speed, increasing the ISO can also result in image noise. And in some cases, you may already be using a high ISO, but still need a brighter exposure without dropping the shutter speed too low. So in some cases it is preferable to use a faster aperture. Unlike ISO, changing the aperture doesn't affect image noise, but it does instead affect depth of field.

Depth of Field (DoF)

Depth of Field, or DoF for short, describes how much of an image is in focus. For a given perspective, a larger aperture, e.g. f/2 results in a small depth of field (i.e. the plane of sharp focus is quite narrow), while a smaller aperture, e.g. f/8 results in a larger depth of field (i.e. the plane of sharp focus is much deeper).

DoF at f/2.8 is quite shallow - only the central portion of the image is in sharp focus

DoF at f/5.6 is larger - most of the image is in focus except for the immediate foreground and distant background

At f/11 all the image is in focus, from the nearest fence post to the trees on the hill in the background.

For images where you want to focus on a single subject and remove background distractions, such as a portrait, a large aperture that will give a shallow DoF e.g. f/2 works well.

Anna by Max Khokhlov on flickr (licensed CC-BY-SA)

For images where you want nearly everything to be in focus, such as a landscape, a small aperture that will give a large depth of field e.g. f/8 works well.

Yankee Hat by nosha on flickr

When you view an image through the optical viewfinder of a camera, the lens will be wide open. The lens is only stopped down to the aperture you have chosen on the camera when you press the shutter to take the photo. To preview the affect on depth of field the aperture will have, you will need to press the DoF preview button on your camera. This only applies when previewing the effect through an optical viewfinder - electronic viewfinders and Live View will usually automatically stop the lens down, giving you an accurate preview without you having to press a DoF preview button.

Bokeh

Bokeh is a japanese word that refers to the way out of focus areas are rendered in am image.

Busy bokeh:

natures tinsel by Grant MacDonald on flickr (licensed CC-BY-NC)

Smooth bokeh:

Bokeh Canon!!! by Darren Kirby on flickr (licensed CC-BY-SA)

Lookout by Andrew Butitta on flickr (licensed CC-BY-SA)

The aperture of a lens determines how out of focus areas (bokeh), are rendered. Generally the more aperture blades a lens has, the more rounded the out of focus areas will be. Many lenses also feature rounded aperture blades, which help create a more pleasing bokeh.

A lens with a 9 bladed aperture and the blades rounded produces an almost circular aperture, and pleasing bokeh in photos taken with the lens

Focusing in dim light

As well as allowing more light through to the camera's sensor, a large aperture makes it easier to see through your camera's optical viewfinder in dim light. The way most optical viewfinders are constructed means that for apertures larger than f/2.8, the amount of light that reaches your eye does not change. But for apertures smaller than f/2.8, it can make quite a difference.

Viewfinder with f/1.4 lens

Viewfinder with f/2 lens

Viewfinder with f/2.8 lens

Viewfinder with f/4 lens

Viewfinder with f/5.6 lens

Viewfinder with f/8 lens

Many cameras also have special autofocus sensors that work only with lenses that have a maximum aperture of f/2.8 or larger. So a lens with a large maximum aperture (e.g. f/2.8) will be easier to see through in dim light, and will also help your camera's autofocus system to focus quickly and accurately.

Vignetting

Most lenses will exhibit some vignetting. This is where the corners and edges of the image are darker than the center. Vignetting will usually be most pronounced with the lens set at its largest aperture, and will disappear as you stop the lens down. Usually closing down the aperture a couple of stops will be enough to eliminate vignetting.

Vignetting with aperture wide open at f/1.4

With the lens stopped down to f/2.8 the vignetting disappears

Although using a lens wide open can cause vignetting, most photo editing software can easily remove vignetting, so don't think you must stop a lens down to prevent vignetting. Likewise, if you like the vignetting effect, you don't have to use a lens wide open, and can instead add a vignette to the photo on the computer.

Cost, weight, and size

While having a lens with a fast aperture gives you more creative possibilities, allowing you to shoot in lower light / use a faster shutter speed / use a shallower depth of field, the faster a lens, the more it will cost.

As you can see, you pay more and more for each small gain in maximum aperture. The larger aperture lenses will also be larger and heavier than smaller aperture lenses. If you would like to experiment with shallow DoF, a 50mm f/1.8 lens makes a reasonably cheap introduction to the world of fast primes. You can also purchase many older lenses with large maximum apertures quite cheaply on ebay.