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Understanding Lens Power

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January 06, 2001

Objective (outermost) lens size and magnifying power are used to describe binocular and telescope lenses. 10x25 describes binoculars that are 10x power (image appears 10 times larger than with the naked eye) and have a 25mm diameter objective lens. Larger objective lenses allow more light to be gathered, making the image brighter and clearer. 10x50 binoculars have the same magnifying power as 10x25 (and are much larger and heavier), but the image provided is brighter and more detail can be seen. There is also a loss of light as magnification increases. 7x25 binoculars are brighter than 10x25.

There generally is little reason to mathematically relate the relative brightness of different binoculars in a precise manner. It's not readily apparent whether 10x40 binoculars are more, the same or less bright than 7x25, and you don't need to measure that difference to use them.

On the other hand, photographers must precisely measure and regulate the amount of light exposing the film. Where binoculars always pass as much light as possible to the viewer and have a fixed depth of focus, camera lenses have variable apertures that close down to limit the amount of light exposing the film. Varying the aperture also allows the photographer to vary the depth of focus in the picture.

Camera lenses are described by focal length and maximum aperture. Focal length technically is the distance between the film and the optical center of the lens when the lens is focused on infinity. (It is not necessarily the physical length of the lens) Focal length also relates to the angle of view provided. The nominal focal length only has meaning if it's related to the size of the film frame (or image sensor in digital cameras). A 50mm lens gives a "normal" angle of view (about 45 degrees) in 35mm film format, but gives a narrow angle of view (slight telephoto) in APS, and gives a wider than normal angle of view in medium format (120/220 film). In 35mm film cameras, a 28-90mm zoom lens covers wide angle (about 75 degree angle of view) to short telephoto (about 25 degrees). Dividing the focal length by 50mm gives a rough conversion to binocular-style "power". Looking through the viewfinder of a 35mm camera with a 28-90mm zoom lens is similar to binoculars with .5x to 2x power. But because the image on the film can be blown up to any proportion, the x-power of a camera lens is not very relevant.

As with binoculars, longer focal length (higher power) lenses transmit less light than shorter focal lengths. In order to simplify exposure calculations when using different focal length lenses, a uniform measure for aperture was developed. The aperture of a camera lens is measured in f-stops, which are based on the ratio of the focal length to the diameter of the aperture. Because the area of the aperture varies with the square of the diameter, standard f-stops are based on multiples of the square root of 2, or approximately 1.4 (f/1.0, f/1.4, f/2.8, f/4, f/5.6, f/8,...) Setting a camera lens aperture to f/5.6 transmits exactly the same amount of light, regardless of whether that lens is 50mm, 24mm, or 600mm in focal length.

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January 08, 2001