Fundamentals of Photography: Understanding Aperture to Get the Perfect Exposure

 In this article, as part of the Fundamentals of Photography series, we will look in more detail at the role of aperture. Aperture is one of the three parts of the exposure triangle, and having a good understanding is essential to improve your photography.

What Is Aperture?

In photography, the aperture is the opening or hole through which light travels to get into the camera, hitting the sensor or film to be captured as a photograph. Changing the diameter of the aperture changes how much light can enter the camera in a given time period. When we talk about aperture in photography, we mean using different diameter apertures in order to produce the desired outcome. Aperture has a hugely important relationship with shutter speed, as between these two settings we control exactly how much light can enter the camera to hit the sensor by controlling the size of the opening (aperture) and how long it opens for (shutter speed – see the separate article on shutter speed here).

As a simple comparison to make better sense of aperture, consider your own eyes. The iris (the coloured part of your eye) changes diameter depending on how bright your environment is, causing your pupil to be different sizes and controlling how much light hits your retina.

If you are somewhere dark, the iris constricts which leaves you with a large, wide pupil, allowing more light to enter the eye so you can still see in the dark. In bright environments, the iris relaxes leaving you with a small, narrow pupil, meaning less light can enter the eye so you aren’t blinded.

The aperture of a camera works in exactly the same way. By using a large (wide) aperture, we allow more light to enter the camera in a certain period of time. Similarly, a small (narrow) aperture means that less light is able to enter the camera in the same period of time. As a result, aperture helps to control the overall exposure of your image, in other words how bright or dark it is.

Aperture Settings

When it comes to choosing aperture on your camera, aperture sizes are expressed as a value known as an f-stop­ or f-number, for example f/8 or f/11. It can feel backwards, but the lower the f-stop, the wider the diameter of the aperture. For example, f/2.8 is a large, wide aperture, and f/11 is a small, narrow aperture. This can take a while to get used to, but eventually becomes second nature.

To remember this, it helps to know that your f-stop is a fraction of the focal length of the lens. When you see an aperture of F/4, the actual diameter of the opening is the focal length (F) divided by 4. For example, if using an 80mm focal length at F/4, the physical size of the opening is 20mm (80 divided by 4). To remember that larger f-stops mean narrower aperture, just remember this relationship. At 100mm focal length at F/4, the physical opening will be 25mm (100 divided by 4). The same 100mm focal length at F/8 will be 12.5mm (100 divided by 8).

Aperture and Stops

Much the same as shutter speed, aperture affects how many stops of light enter the camera. The maths is slightly more complex than for shutter speed due to the aperture being the area of a circle. For example, a 50mm focal length at F/11 means that the total area of the aperture (not just the diameter) is 16mm2. The same 50mm focal length at F/8 has a total area of 32mm2, so the amount of light that can go through has doubled, or increased by one stop. Similarly to shutter speed, most cameras will allow you to change the aperture in increments of one third of a stop for better control.

Aperture and Exposure

By controlling how much light enters the camera, aperture has a direct effect on exposure. A wider (larger) aperture allows more light in and gives you a brighter image. In contrast, a narrower (smaller) aperture allows less light to enter the camera giving you a darker image. By controlling the aperture size, along with shutter speed, you change how bright or dark your photo will be.

In addition to controlling the exposure of your photos, the aperture also controls the depth of field.

Using Aperture to Control Depth of Field

Image taken at f/13, for a deep depth of field.

In photography, the depth of the field is the area within your image that appears in focus and sharp. Take the image on the left as an example. The trees at the bottom of the image are nearest the camera, with the tower on the lake in the middle, and the far mountains in the background. All three parts of this image are in focus and sharp, therefore the depth of field is very deep. This is achieved by using a narrow aperture, such as F/11 or F/13. This is ideal for landscape photography where you generally want everything in your image, from close foreground to distant background, to appear sharp.

Image taken at f/2.8, for a shallow depth of field.

In contrast, looking at the image on the right, the only part of the photo that is in focus and sharp is the stack of rocks, whilst everything else in the photo appears blurred. This is due to having a very narrow depth of field. This is achieved by using a wide aperture, such as F/2.8 in this case. Wider apertures are commonly used for portraits or macro photography, where you want your subject to be in focus but everything else is soft and blurred. This gives you beautiful bokeh, which is the visual effect of blurring in the parts of your image that are not in focus.

How Aperture Affects Sharpness

As well as exposure and depth of field, aperture has an impact on the overall sharpness of your image. Sharpness is the level of detail achieved in a photo in terms of both focus and contrast. In a really sharp image, you will be able to clearly see smaller details, such as individual blades of grass.

As we’ve mentioned above, a smaller aperture gives you a deeper depth of field, so should we just use the smallest aperture we can? Unfortunately, it’s not that simple. Lenses perform best in a ‘sweet spot’ of aperture where images will appear sharpest. You can check the best aperture range for your individual lenses online, but as a rule of thumb the ‘sweet spot’ is 2 or 3 stops below the maximum aperture. For example, if you have a lens with a maximum aperture of f/2.8, the sweet spot will be between f/5.6 (two stops below) and f/8 (three stops below).

As you use smaller and smaller apertures past the sweet spot, you introduce more diffraction into your image. The physics of this get complex, but essentially the waves of light that go through the lens when taking a photo will bend as they go through the aperture. With a wider aperture, light doesn’t bend very far, so there is minimal diffraction and the image is sharp. With very narrow apertures (such as f/22), light bends significantly enough that things get blurry in your photos.

How Do I Choose the Best Aperture?

Image taken at f/4 to give a blurred (bokeh) background.

Your choice of aperture will depend entirely on the situation, there is no one size fits all aperture in photography. Here’s a few examples of general apertures to use in different styles of photography.

Landscapes

On the whole, most landscape photographers want the entire photo to be sharp, so use a smaller aperture range such as f/8 to f/13, but be cautious not to go too small or you run the risk of introducing diffraction and losing focus. You can also get creative with landscapes and add an element of bokeh to your foreground/background using a wider aperture.

Portraits and Macro

This will depend on your circumstances, but often these styles of photography can look stunning with a nice wide aperture, such as f/2.8, to give you a pin-sharp subject and gorgeous blurred bokeh through the rest of your image. You need to be cautious with how wide you go with aperture as you can end up with a depth of field that does not actually include your whole subject. Depending on your surroundings you may want to make sure that the foreground and/or background are also sharp, so will need to go a bit smaller with your aperture.

Night or Astro Photography

Due to the dark environment, you’ll need to use wider apertures for night and astro photography in order to get a decent exposure. Again, you need to be cautious that you do not end up with too small of a depth of field, but this will depend on the camera and lenses you are using.

I hope this article has helped clear up a few mysteries to do with aperture, as well as given you some inspiration and motivation to get out there and test out some settings to see what you like and works best for you. Happy photographing!