In optics, an aperture is a hole or an opening through which light is admitted.
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 degree of exposure to light. Typically, a fast shutter speed will require a larger aperture to ensure sufficient light exposure, and a slow shutter speed will require a smaller aperture to avoid excessive exposure.
Diagram of decreasing aperture sizes (increasing f-numbers) for "full stop" increments (factor of two aperture area per stop)
A device called a diaphragm usually serves as the aperture stop, and controls the aperture. The diaphragm functions much like the iris of the eye—it controls the effective diameter of the lens opening. Reducing the aperture size increases the depth of field, which describes the extent to which subject matter lying closer than or farther from the actual plane of focus appears to be in focus. In general, the smaller the aperture (the larger the number), the greater the distance from the plane of focus the subject matter may be while still appearing in focus.
The lens aperture is usually specified as an f-number, the ratio of focal length to effective aperture diameter. A lens typically has a set of marked "f-stops" that the f-number can be set to. A lower f-number denotes a greater aperture opening which allows more light to reach the film or image sensor.
The largest aperture opening is f/2.8 an the smallest is f /22
The Aperture Controls Light and Depth of Field
Aperture is referred to the lens diaphragm opening inside a photographic lens. The size of the diaphragm opening in a camera lens regulates amount of light passes through onto the film inside the camera the moment when the shutter curtain in camera opens during an exposure process.
Aperture is the ratio of the focal length of the lens to the diameter of the actual opening of the lens.
Aperture size is usually calibrated in f-numbers. 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
Each of this value represents one time the amount of light either more or less in quantity. 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.
Area of aperture opening:
If f: 50mm and you set the aperture at f4 then
Ratio: 50/d= 4 = d: 50/4: 12.5mm Area of the opening: ( d^2)/4
: (3.414x12.5^2)/4
: 122.7 sq.mm
For aperture at f5.6:
Ratio: 50/d= 5.6 =d: 50.5.6: 8.93mm
Area of opening: ( d^2)/4
: (3.414x8.93^2)/4
: 62.63 sq.mm
122.7 is nearly double the area of 62.63 sq.mm.
Hence at f4 the amount of light let in (brightness) is double that of f5.6.
In the same way at f16 : 7.7 sq.mm
f11 : 16.2 sq.mm
f8 : 30.6 sq.mm
f5.6 : 62.63 sq.mm
f4 : 122.7 sq.mm
f2.8 : 250.4 sq.mm
Hence, the squares of the f-numbers are inversely proportional to the amount of light admitted
Aperture and exposure
The aperture can be opened up to let in more light or closed (stopped down) to let in less. Like the shutter speed, the aperture is used to control exposure. The larger the aperture opening, the more light reaches the image sensor in a given period of time. The more light, the lighter the image.
Aperture and depth-of-field
Changing the aperture changes the depth of field, the depth in a scene from foreground to background that will be sharp in a photograph. The smaller the aperture you use, the greater the area of a scene that will be sharp.
For some pictures-for example, a landscape-you may want a smaller aperture for maximum depth of field so that everything from near foreground to distant background is sharp. But perhaps in a portrait you will want a larger aperture to decrease the depth of field so that your subject's face is sharp but the background is soft and out of focus.
* Images from website:uni. of Victoria & www.tpub.com
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