APERTURE

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

TYPES OF LENSES

What is focal length?

The focal length of any lens is the distance between the optical center of the lens and the point at which it focuses an image.

However, a given focal length lens may be a wide angle lens on one camera and a telephoto lens on another. This is because descriptions such as "wide-angle" or "normal" depend on the size of the film or image sensor being used. As these get smaller, a given focal length lens magnifies more.

1. Standard Lens:



A "normal lens" for a 35mm camera usually refers to a lens with a 50mm focal length.

Negative size: 36 x24mm
Diagonal distance BD: 43.26mm

AC (axis of the lens) is perpendicular to the plane of the film and it bisects the diagonal BD at A.
BD: 21.63mm

CAB is 90º
CAB is a right angled triangle.
Tan θ : AB/AC
: 21.63/50
: .433
: 23.30º


BCD = 47 which is nearer to 50º

If the focal length of a lens is equal to the diagonal distance of the negative then it is called the normal (standard) lens for a camera.

If a camera has standard lens, it means that the angle of field of coverage is the same as what a human eye would see, with the other eye closed ≈ 50º




2. Zoom Lens:

A zoom lens lets you choose any focal length within the range the lens is designed for. When you change focal lengths by zooming the lens, two important effects are immediately obvious in the lens’ angle of view and its magnifying power.
Zoomed out, you have a wide-angle of view that captures a wide expanse of a scene. As you zoom in, the field of view narrows and you can isolate small portions of the scene without moving closer to the subject.
Magnification is related to the lens’ angle of view. Since zooming out includes a wide sweep of the scene, all of the objects in the scene are reduced to fit into the image. Zooming in gives a much narrower angle of view, so objects in a scene appear larger.
Note: A zoom lens is an excellent portrait lens, especially for head-and-shoulders portraits. When zoomed in you can keep your distance and still fill the viewfinder frame with the subject. Keeping at a distance eliminates the exaggerated perspective caused by working very close to a subject with a shorter focal length lens. It also helps relax your subjects if they get uneasy, as many people do, when a camera comes close.

3. Wide-Angle Zoom:

A lens zoomed to a wide-angle also has great depth of field. This great depth of field makes short lenses good for street or action photographs. When out to capture quickly unfolding scenes, keep the lens zoomed out to a wide angle so you’ll have maximum depth of field when you respond quickly to a photo opportunity.
Short lenses also let you focus very close to your subject, and the effect this can have on the perspective in your images can be dramatic. Objects very close to the camera loom much larger than those farther in the background. This distortion in the apparent size of objects can deliberately give emphasis and when carried to an extreme, give an unrealistic appearance to a scene.


4. Telephoto Zoom:

A lens zoomed in on a subject acts somewhat like a telescope: It magnifies the image of your subject. This is especially useful when you can’t get close to your subject—or don’t want to. Zooming in like this is ideal for wildlife, portrait, and candid photography, whenever getting close to a subject might disturb it.
When you zoom in on a subject, depth of field gets shallower so you must focus carefully. Also, zooming in visually compresses space, making objects in the scene appear closer together than they actually are.
The primary drawback of zooming in is that it gives you a smaller maximum aperture. This smaller maximum aperture may require a longer shutter speed and since a long lens magnifies movement, just as it magnifies the subject, you may have to use a tripod instead of hand-holding the camera.
For a telephoto view, you can zoom the lens all the way in. For even more magnification, some cameras have optional lens converters that give you even longer focal lengths.

5. Fish-eye lens:

For image angles greater than 110°, it becomes difficult to bring the lens close enough to the film to allow the rays between the lens and film to diverge sufficiently. The fish-eye lens overcomes this difficulty by making the rays diverge less behind the lens than they do in front. The resulting image shows appreciable distortion, with image details near the edges and corners progressively compressed. Fish-eye lenses usually cover angles between 140° and 210° and are used for unusual wide-angle effects where the distortion becomes a deliberate pictorial element. They also have certain scientific applications, for instance, to cover a horizon-to-horizon view of the sky in recording cloud formations.