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Lens Properties - Focal Length
Category: Lenses
Updated 4 months ago by renan


Lens Properties

A lens system in a camera is quite sophisticated.  Inspite of all those mechanical and electronic components though, it is primarily an assembly of various lenses that is aimed to focus and to control the amount of light that goes to the sensor of the camera at a particular focal length. There are three main properties that must be considered in every camera lens.

Focal Length - The most basic property of a lens is it's focal length. It is the distance from the optical center of a lens to a point where parallel rays of light converges after passing through the lens (focal point).  This property dictates the angle of view or how much of the view in front of the camera will fit in the sensor. This is typically measured in millimeters (mm). It is classified to three basic groups:

     a. Normal - A focal length of approximately 50mm on a full frame camera, represents what a typical eye can see.

focal length 50mm - normalView with a 50mm lens

     b. Wide Angle - Focal lengths that are less than 50mm will let in a much more of the view into the camera's sensor. Focal lengths that are shorter than the shortest side of the sensor is considered Ultra-Wide Angle. For full-frame sensors, that would be anything less than 24mm.

wide angle - 18mm focal lengthView with an 18mm lens

     c. Telephoto - Any focal length that is longer than 50mm will let you capture a much closer view of the image.  For full-frame cameras any lens that has a focal length longer than 300mm is considered Super-Telephoto.

telephoto - focal length 150mmView with a 150mm lens

Zoom - This is the ability of a lens to be able to operate at different focal lengths. Zoom is measured by the range of focal lengths that the lens can handle or the amount of magnification between the widest end to the longest focal length.

A typical kit lens for example, is a zoom lens with a focal length ranging from 18mm to 55mm. This could be identified as an 18-55mm lens or a 3x zoom lens. The latter is not very specific and is often a source of confusion and frustration to buyers as the actual focal lengths might not be the one you need.

Some manufacturers also provide a "Digital Zoom" feature within the camera. This feature digitally extends the magnification of the camera by interpolating the pixels from the image of the actual maximum optical range of the lens. As a result, the fake pixels are created which does not give any additional details and often lacks sharpness.

optical digital zoom

In a photographic point of view the digital zoom is more of a marketing gimmick than a useful feature. It is probably best just to ignore it when choosing a lens or camera system.



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