When you're taking quality photos, it's important to know how your camera's settings will affect one another. I’ve focussed upon three elements of the ‘exposure triangle‘ – ISO, Shutter Speed and Aperture.

Definitions of Aperture in the 1707 - Wikipedia

This is a quick and simple explanation of aperture, one of the most important features on your camera. Aperture is referred to the lens diaphragm opening inside a photographic lens. Most simply – Aperture is "the opening in the lens".

The size of an aperture in a lens can either be a fixed or the most popular form in an adjustable type (like an SLR camera). Aperture size is usually calibrated in f-numbers or f-stops. i.e. those little numbers engraved on the lens barrel like f22 (f/22),16 (f/16), f/11, f/8.0, f/5.6, f/4.0, f/2.8, f/2.0, f/1.8 etc.

One thing that causes a lot of new photographers confusion is that large apertures (where lots of light gets through) are given f/stop smaller numbers and smaller apertures (where less light gets through) have larger f-stop numbers. Meaning to say, f/16 will let in 1X the amount of light than a diaphragm opening of f/22 and so forth; while on the other hand, an aperture of f/4.0 will let in 1X lesser than that of f/2.8 etc.

The aperture stop of a photographic lens can be adjusted to control the amount of light reaching the film or image sensor. In combination with variation of shutter speed, the aperture size will regulate the film's or image sensor's degree of exposure to light. Typically, a fast shutter will require a larger aperture to ensure sufficient light exposure, and a slow shutter will require a smaller aperture to avoid excessive exposure.

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How Aperture Affects Depth of Field
Depth of field is defined as "the zone of acceptable sharpness in front of and behind the subject on which the lens is focused." Simply put: how sharp or blurry is the area behind your subject. Here's the equation:

• The lower the f/stop—the larger the opening in the lens—the less depth of field—the blurrier the background.
• The higher the f/stop—the smaller the opening in the lens—the greater the depth of field—the sharper the background.

How Aperture Affects Shutter Speed

The f/stop also affects shutter speed. Using a low f/stop means more light is entering the lens and therefore the shutter doesn't need to stay open as long to make a correct exposure which translates into a faster shutter speed. Again, the reverse is true: using a high f/stop means that less light is entering the lens and therefore the shutter will need to stay open a little longer which translates into a slower shutter speed.

Where to Find the Aperture

Modern Autofocus SLR cameras may have a different ways in manipulating the aperture. One of the trend is - the aperture value is now control via a thumb wheel on the camera (usually near the shutter release button) and the AF lens has no aperture ring to alter the value. Each camera manufacturer usually has their own series of lenses under a trade name to verify its usage, various compatibility issues with their previous camera model's function etc. For an instance, Canon manual focus lenses are called "FD" or "FL"; while their newer series of autofocus lenses (AF) designed for their Canon EOS Series cameras are referred as "EF" (Electro Focus). Each of these MF/AF lenses has their own respective way to illustrate the control of aperture in the camera. When you turn the aperture ring on a lens to vary the aperture, you will be able to check visually the set opening of the lens diaphragm (Opens bigger or stopping smaller).

There is 3 popular camera brands to illustrate the whereabouts of these aperture on the lens. (A) is a typical FA lens by Pentax; (B) is a typical manual focus Nikkor zoom lens from Nikon; while the (C) is a Zuiko lens by Olympus.

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Newer range of autofocus SLR cameras use a new method of controlling aperture. You will find there is no need to set aperture via the lens aperture ring; instead - aperture is controlled by the thumb wheel for both shutter Speed and Aperture.

Choosing the Aperture

Now that we know how to control depth of field, what determines the choices we make in selecting the aperture? We use focus and depth of field to direct attention to what is important in the photograph, and we use lack of focus to minimize distractions that cannot be eliminated from the composition. While there are no rules, there are some guidelines.

For classic portraiture we separate our subject from the surroundings by using "selective focus." Choosing a large aperture (lower f/stop, like f2.8) creates very shallow depth of field with only the subject, or just a portion of the subject, in focus. This helps direct the viewer's attention to the subject.

In a landscape or scenic photograph we usually want to see as much detail as possible from foreground to background; we want to achieve the maximum depth of field by choosing a small aperture (higher f/stop, like f/8 or f/11).

While we can get the maximum or minimum depth of field by working at each end of the aperture range, sometimes we want a more intermediate level of depth of field, limiting focus to a specific range of distances within the overall photograph. One way to do this is to choose a mid-range f/stop, like f/5.6, and shoot a test frame. In image playback, use the magnifying function of the LCD to zoom in and check the depth of field; make adjustments if necessary and reshoot.

Reference: MIR | Nikon