Aperture, Zoom and focal length

ApertureThe amount of light that enters to the camera's

imaging sensor (exposure level) plays a large role in the quality of the image. The exposure can be controlled mainly by two parts of a camera: the Aperture and the Shutter. Perfect balances between the settings of these two parts are essential to achieve the desired quality and effect.

The aperture is the opening of the lens where light enters before it goes to the imaging chip. It acts like the pupil of a human eye that contracts to make the opening larger and allowing more light to enter or expands making the opening smaller thus reducing the amount of light that enters to the eye.

In the camera the Aperture Opening is controlled by the Iris which is composed of mechanically interlinked overlapping blades that opens or closes as needed.

Aperture opening is expressed in F-values called F-stops. F-value is equal to the focal length of a lens divided by the aperture's diameter.

A lens with a focal length of 100mm for example with an aperture opening diameter of 36mm has an F-value of 100/36 = 2.8. The aperture value will be expressed as f/2.8 or F2.8. The same way, if the diameter of the opening of the same lens is reduced to 25mm then the F-value is 100/25 = 4, so the F-value is f/4.0 or F4.0. This shows that the larger F-value means a smaller aperture opening.

Common F-values used by camera lenses today are the following: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, etc. The values are rounded off to these numbers to make it easier to remember.

Each step of these values is called a "stop". Each stop higher reduces the opening area by half, and as a result allows only half as much light to pass through. f/4 for example is one stop higher than f/2.8 so it offers only half as much light than f/2.8.

A large (1) and a small (2) aperture

Diagram of decreasing aperture sizes (increasing f-numbers) for "full stop" increments (factor of two aperture area per stop)

The aperture range of a 50mm "Minolta" lens, f/1.4-f/16

At f/16 the background is distracting

At f/1.4 the background is reduced to a blur, but not all of the subject is in focus either.

Focal LengthThe focal length of a lens is defined as the

distance in mm from the optical center of the lens to the focal point, which is located on the sensor or film if the subject (at infinity) is "in focus".

In photography, the focal length of a lens means the magnification or telephoto power of the lens and is expressed in the millimeters of the lens, like 100mm, 300mm, etc. The higher the number the higher the magnification the lens will provide. At the higher magnifications, it will be harder to hand-hold the camera without getting a blurry picture.

The photograph shows a focal length of about 36mm. This lens magnification is great for landscapes or large group pictures. This focal length also works great if you are close to your subject and want to include as much as possible in the frame. Any time you wish to include a wide view of what you are photographing, choose a focal length between 20mm and 40mm.

Here the focal length is 75mm. This focal length is good for cutting out some of the distracting objects in the background. While this lens setting is not quite long enough for a good portrait, this setting would be good for small groups or if you want to include some background.

This photo shows about 125mm of magnification. This length is good for portraits. Move closer to your subject and you can easily fill your frame with a pleasing head shot.

Here is a 300mm focal length shot. Use this setting to get right into the action. This setting is great for nature or sports photography. With this setting, you will cut out most of the background and distractions.

This photo shows 450mm of magnification. This is an example of an extremely long focal length. This length is excellent for bird photography and sports. Use this length if your subject is far away and you cannot get closer.

Diagram shows focal length

The focal length of a lens is usually displayed on the lens barrel. Pictured below is a Nikon lens with a focal length of 50mm. The maximum aperture is f/1.8 (also often written as, F1.8).

A landscape at 18mm, the white box marking 1/5 of the width and height.

The same landscape at 90mm: the focal length is 5 times longer so the area marked by the white box fills the whole scene

ZoomZoom is the ability of a camera to magnify or

de-magnify an image to a certain range. A 3x zoom factor indicates that the camera is capable of magnifying from its lowest zoom value up to a point where the image is three times its size.