A Concise Look At Camera Aperture
In photography, creating an image requires adequate lighting to get focus on your subject. When you have focus on your subject, you get clearer and sharper details in your photos. In a DSLR camera, the light enters through the lens which is then reflected by mirrors to your view finder and sensor. With mirrorless cameras, the light enters the lens and directly hits the sensor. You can control the amount of light you want to the camera body’s diaphragm by adjusting what is called the aperture.
Inside the lens are blades that open and close the aperture. It functions much like how the iris contracts and expands in response to the amount of light that enters the eyes. The aperture is thus the diameter of the opening. By varying the diameter of the aperture, you are controlling how much light you want to pass thru the diaphragm and into the sensor. Depending on the scene you want to capture, you want to allow more light or a larger aperture when you are in the shade. When you are shooting in a bright area, you don’t require as much light so you have a smaller aperture. Aperture has other uses and I will explain that later on.
The F-Stop
When we adjust the aperture, we are controlling how much light we want to enter the sensor. An analogy to opening a hole explains how this works. When you want a large aperture, you open a bigger hole to let more light in. When you want a smaller aperture, you close the hole to let less light in. Aperture is measured in units called f-stop. Now the f-stop can be confusing at first because a smaller f-stop value indicates a larger aperture, while a bigger f-stop value indicates a smaller aperture. The aperture size is inversely related to the f-stop value. Thus when you want a larger aperture, you have a smaller f-stop value e.g. f/1.4. When you want a smaller aperture, you have a bigger f-stop value e.g. f/22.
The f-stop also called f-number, is a value that determines how much you want to increase or decrease the amount of light you want to the sensor. The value of the f-stop consists of two variables, which is the ratio of the diameter of the aperture to the focal length of the lens. I will now explain the meaning of those two variables. The focal length refers to the distance (measured in millimeters) from the center of the lens to the image plane, where the light converges with the lens to get focus. It is simpler to say how long the lens is, but a better way to explain is that longer lenses also have a smaller angle of view (AOV), meaning it can zoom in much better to isolate the subject from its surrounding. As mentioned earlier, aperture is just the diameter of the opening to the diaphragm measured in millimeters (mm). When you take the ratio of the two variables we get the f-stop.
Let’s say we have a lens with a focal length of 10 mm and an aperture diameter of 2 mm.
Focal Length = 10 mm
Aperture Diameter = 2 mm
f-stop = Focal Length / Aperture Diameter
The f-stop would be f/5 because the ratio is 10/2.
With a longer focal length you can magnify a larger area of your frame to capture a scene or subject. This is why a 400 mm lens will have superior magnification to zoom in on a subject who is >500 feet away. You can use a 55 mm lens, but you won’t be able to zoom in all the way compared to a 400 mm lens.
Here is a rule of thumb to remember regarding the AOV:
The longer the focal length, the narrower the angle of view and the higher the magnification. The shorter the focal length, the wider the angle of view and the lower the magnification.
Another thing to know is that when you step up (increase your f-stop or larger aperture) you make your image brighter. When you step down (decrease the f-stop or smaller aperture) your image is less brighter.
Not every camera has the same f-stop setting, it is not an absolute value. It depends on the type of lens you use based on the focal length. Canon, Sigma, Nikon and other manufacturers have their own specifications for lenses. Consult your lens manufacturers manual for more information.
Depth of Field (DOF)
If you wonder how photographers can take pictures that focus sharply on the subject while blurring out the background, that is due to aperture. This effect is called the depth of field and is the point where light converges with the focal point where the subject is in focus. Aperture plays a big part in this because it affects the result of the exposure.
When shooting portraits, I prefer a shallower depth of field. This means I want to focus directly on the subject I am shooting and not the background or surrounding. I set my focus on the eyes and set my aperture wide, meaning I increase the opening of the aperture. I will set it to a smaller aperture value like f/1.4 which is a large aperture value. This means I am increasing the diameter of the aperture to allow more light to focus on the subject. With a fixed prime lens, I have a smaller or narrower angle of view or likewise I can use a longer focal length and zoom in on the subject. The background is not in focus which is why it will appear blurry. Another term used to describe this is called bokeh.
This is the opposite of when I want to shoot landscape. When shooting landscapes I want a larger depth of field, so I compose more of my scene in focus. For example, if I want to capture the scene of a mountain or valley, I will be using a smaller aperture by increasing the f-stop to let’s say f/13. I am allowing less light into the sensor with a smaller aperture diameter. I will also be using a smaller focal length which has a bigger angle of view so I can capture the surrounding. The result of the photo is more of the scenery is in focus and a much wider angle.
Shutter Speed
Adjusting the shutter speed also affects the aperture. When you change the shutter speed from one f-stop to the next, it doubles or halves the amount of light that gets in. That means if you increase one and decrease the other you let the same amount of light or vice versa. The relationship is inverse to one another. So the following would be equal in the amount of light you are allowing:
f/4.0; 1/800 sec = f/2.8; 1/400 sec
Let me give some examples to further explain this. When you increase your aperture from f/4.0 to f/2.8 and then decrease the shutter speed, you are actually allowing the same amount of light into the sensor. When you lower your shutter speed you are actually allowing more light. The slower shutter speed of 1/400 sec. allows time for more light to hit the sensor. That is akin to opening up your aperture.
When you increase the shutter speed and decrease your aperture, you are allowing less light in. At f/4.0 you are decreasing the diameter of the aperture and a faster shutter speed of 1/800 sec. allows less light thru the lens. You are creating an exposure which does not require as much light but you need to capture the scene quickly enough to freeze motion. This is most common in sports and action photography.
Aperture Is About Focusing
The most important thing to get focus when taking photos is the light. How much light you allow thru the lens via the aperture determines how much focus you want on your subject. Aperture is what allow you to focus on your images based on what part of the scene you want to capture. To get a more sharper focus on a subject, you narrow the angle and open up the aperture for a shallower depth of field. When you want to put into focus more of a scene, you widen the angle and close the aperture for a larger depth of field.
For more about Angle Of View (AOV) I explain that here with examples in surfing photography.
For more about Depth of Field (DOF) I wrote this guide to better understand it.
