Understand: aperture

Exposure is the quantity of light detected by the photosensitive element (photographic film for analog cameras, or a digital sensor for digital cameras). To influence the quantity of light detected by the photosensitive element, you can either change aperture, exposure time or sensitivity. I will now talk about aperture, how to quantify it, how to change it, how it will affect the other two parameters, and what is the main side effect of aperture.

How to quantify aperture?

If you look through a lens, you will notice an opaque structure with an opening at its center. This structure is called the iris diaphragm, it’s made of 3 to about 20 blades depending on the lens, and the opening at its center is called the entrance pupil or aperture.

By turning an aperture ring located on your lens or by pushing the Av button while turning a wheel located on your camera body (on most DSLR cameras), you can increase or decrease aperture. In other words, you can increase or decrease the diameter of the entrance pupil in the center of the diaphragm, letting more or less light into your lens. Aperture size is usually calibrated in f-numbers and a lens typically has a set of marked f-stops that the f-number can be set to. Common f-stops are 1.4, 2, 2.8, 4, 5.6, 8, 11, 16 and 22. The f-number is the ratio of focal length (noted f) to effective aperture diameter. Lets clarify this a little bit:

F-number = Focal length / effective diameter of the entrance pupil. Imagine that you have a 50mm lens and the entrance pupil diameter is 25mm. The f-number is 2 (as 50 / 25 = 2), and the aperture diameter would be expressed as f/2. Also, you should know that the smaller the f-number, the greater the aperture. It seems counter-intuitive but it’s logical, as aperture diameter is a ratio of focal length to this f-number. If you set the aperture of your 50mm lens to 5.6 (also noted f/5.6 as the ratio of focal length), it means that the effective diameter of the entrance pupil is equal to the focal length of your lens (here, 50mm) divided by 5.6. If you set aperture to 16, it means that the diameter of the entrance pupil is equal to 50mm divided by 16. The higher the f-number, the smaller the aperture, because you divide focal length by a greater number.

In a nutshell:

Aperture is one of three parameters that you can use to change exposure.
It corresponds to the diameter of the entrance pupil at the center of the diaphragm at the moment the picture is taken.
Changing aperture is changing the quantity of light you let into your lens at the moment the picture is taken.
Aperture is usually calibrated in f-numbers (or f-stops).
Common stops are f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 and f/22. But some lenses can go up to f/0.95 or down to f/32.
The smaller the f-number, the greater the aperture, f/1,4 is a greater aperture than f/8.
Every lens has a specified maximum aperture. Maximum aperture is reached when the diaphragm is fully retracted.
A lens maximum aperture is clearly specified in its name, along with its focal length. For instance, a 35mm f/1.4 (also noted 35mm 1:1.4) lens has a 35mm focal length and a maximum aperture of f/1.4. A 18-55mm 1:3.5-5.6 zoom lens has a maximum aperture of f/3.5 at 18mm and a maximum aperture of f/5.6 at 55mm.
For each f-stop the diameter of the aperture is multiplied or divided by 2, letting twice as much or twice as less light into the camera to the previous or next stop. For instance, at f/2, you let twice as much light into your camera than at f/2.8. Exception: it’s not always the case for maximum aperture. For instance, f/0,95 lenses exist, but it doesn’t let twice as much light into your camera at f/0.95 than at f/1. The maximum common stop of the lens is f/1, but it can go a little further, up to f/0.95. Also, note that some lenses feature half-stops or third-stops (i.e. one or two stops between each common stop).

How to change aperture?

You can change the value of the f-number (and therefore aperture size) in two modes: manual mode (M) or aperture priority mode (A or Av). Change aperture by turning the aperture ring on the lens. It will click at each stop (even at half-stops or third-stops on some lenses). If there is no aperture ring on the lens, change aperture by holding the Av button and turning the wheel on the back of your camera body.

What it does

Changing aperture will change the amount of light that enters the lens. Therefore, if you increase aperture (by selecting a smaller f-stop), you will increase the amount of light that enters the lens. So, in manual mode (M), if you take a picture then increase aperture one f-stop and take the same picture again, the second picture will appear brighter than the first one.

Manual mode (M)

In manual mode, if you increase aperture one f-stop but want to keep the same exposure, you need to either decrease exposure time one stop or to decrease sensitivity (ISO) one stop. Yes, exposure time and sensitivity are calibrated as stops that match aperture stops, so if you increase one of them by a given amount of stops, you have to dicrease the other two by the same total amount of stops to keep the same exposure and vice-versa. For instance, in manual mode, if you increase aperture four stops but you want to keep the same exposure, you can either :

Decrease exposure time four stops.
Decrease sensitivity (ISO) four stops.
Decrease exposure time two stops and sensitivity (ISO) two stops.
Decrease exposure time one stop and sensitivity (ISO) three stops.
Decrease exposure time three stops and sensitivity (ISO) one stop.

In manual mode, if you decrease aperture four stops but you want to keep the same exposure, you can either:

Increase exposure time four stops.
Increase sensitivity (ISO) four stops.
Increase exposure time two stops and sensitivity (ISO) two stops.
Increase exposure time one stop and sensitivity (ISO) three stops.
Increase exposure time three stops and sensitivity (ISO) one stop.

Aperture priority mode (A or Av)

In aperture priority mode, all you have to do is to change aperture. Your camera will calculate proper exposure time for you (and also proper sensitivity if ISO is set to auto). Hence the name aperture priority.

Depth of field

Depth of field is the proportion of the image that will be sharp (or in focus).

If depth of field is deep, most of the image (or all of it) will be in focus.
If depth of field is shallow, only a little part of the image will be in focus.

The link between aperture and depth of field

In a few words, the greater the f-number (the smaller the aperture), the deeper depth of field.

The link between focus and depth of field

Depth of field (the proportion of the image that will be sharp) can be seen as a range around the focus point. If you subject is five meters away from you, you have to set the focus point at five meters and depending on depth of field, more or less of the image will be sharp around this distance.

If depth of field is deep, the sharp range around focus point will be deep (and can cover the total range of the image, from the closest point to the farest point of the image). That’s why most of the image will be in focus.
If depth of field is shallow, the sharp range around focus point will be shallow. That’s why only a little part of the image will be in focus.

Also, the closer your focus point, the shallower depth of field.

For portrait photography, you can use a great aperture lens to isolate your subject from the background. For landscape photography, you may want to set a smaller aperture to maximize sharpness at every point of your image. Use aperture to give more sense to your images and to guide the eyes of your spectators to the main subject of your photograph.

In a nutshell: