An Aperture is an adjustable opening inside your lens which controls how much light is allowed to reach the image sensor (or film). A series of blades controlled by the aperture mechanism increase and decrease the size of the hole which in turn increases or decreases the amount of light permitted to pass through. Here is a terribly simple sketch I made to illustrate the aperture inside a lens:
The photographer controls the aperture by adjusting the f number. In modern digital cameras this is done by turning a dial or pressing a button, while older lenses require the photographer to manually adjust an aperture ring on the lens barrel. Here are the key points to remember with regard to choosing an aperture setting:
Smaller f numbers correspond to a wider opening; larger f numbers correspond to a narrower opening. Therefore, smaller f numbers let in more light; larger f numbers let in less light.
Beginning photographers often find this confusing, so spend some time committing the idea of the last paragraph to memory — lower number, larger opening, more light. Higher number, smaller opening, less light. f/1.8 lets in more light than f/32.
If you put your camera into full manual mode, chose a shutter speed and ISO setting, then took a series of photos using a lower f number each time, each successive frame would be brighter than the last. This is because as you chose lower f numbers, you increased the size of the aperture opening and permitted more light to hit the sensor. Conversely, if you chose a higher f number each time then each successive frame would be darker than the last.
Lenses with wide maximum apertures are considered fast lenses because the aperture can be opened up to a very large diameter in order to let in a great deal of light, thereby enabling the use of a faster shutter speed. Think about taking a photo in a dark bar. If your lens is only capable of f/3.5 then you will need to keep the shutter open for a long time in order to take a photo. But if you switch to an f/1.2 lens you can choose a wider aperture to let in more light at once, which in turn gives you the option to use a faster shutter speed.
Fast lenses are helpful for hand-holding your camera in dark environments (as opposed to using a tripod). The fast shutter speed minimizes the blur that would otherwise result at a slower shutter speed from your shaking hands.
Depth of Field
Aperture has a direct effect on depth of field:
- Wider apertures (low f numbers) result in shallower depth of field.
- Narrow apertures (high f numbers) result in deeper depth of field.
You can create a blurry background behind a single subject by choosing a wide aperture such as f/1.8, or get everything sharp and in-focus for a landscape photo by choosing a narrow aperture such as f/22.
How to Get the Most Background Blur Possible in Your Photos
Lots of photographers love selective focus — blurring the background of an image with a single subject, such as a portrait, can really make that subject pop. The most widely understood method of getting a blurry background is to use a wide aperture, however there many more things you can do to increase this effect. Here’s the rundown:
- Choose your widest aperture.
- Choose your longest focal length.
- Bring your subject close, and put a lot of distance between it/them and the background.
Choose your widest aperture
Depth of field decreases as you enlarge the hole through which light passes into your camera and onto the sensor/film. This setting is your aperture. If you are not familiar this, here’s a crash course: lower f/ numbers = wider apertures. f/3.5 is wider than f/22. Choose the lowest number supported by your lens. In order to change this setting, you need to be in either Aperture Priority mode (Av on Canon, A on Nikon) or Manual (M) mode.
Choose your longest focal length
You can take advantage of telephoto compression to decrease depth of field and increase background blur. Wider lenses emphasize the distance between your subject and background while telephoto lenses compress them together. So, given the choice between a 24mm lens at f/1.4 and a 135mm lens at f/2, the 135mm lens is capable of producing images with a blurrier background in many situations. A kit lens that does f/5.6 at 55mm will produce a little bit of blur while a 300mm lens at f/5.6 can get you a great deal of blur.
Increase the distance between subject and background
The farther away your background is from the subject, the blurrier it will be. Get the subject as close to the lens as possible while still being able to focus and get your desired framing, and put as much distance between it and the background as you can.
By combining all of these things together — widest aperture, longest focal length, and greatest distance between subject and background — you’ll be able to maximize the amount of background blur in your photos.