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What Is Focal Length?

Learn About How Focal Length Works in Lenses

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When shooting with any type of camera, but especially a DSLR camera, it's important to answer the question: What is focal length?

This article will help you understand focal length, along with showing the importance of focal length in digital photography.

What is Focal Length?

Here is the scientific explanation of focal length: When parallel rays of light hit a lens focused at infinity, they converge to form a focal point. The focal length of the lens is the distance from the middle of the lens to this focal point. The focal length of a lens will be displayed on the barrel of the lens. 

Types of Lenses

Lenses usually are categorized as wide-angle, standard, or telephoto. The focal length of a lens determines the angle of view, so wide-angle lenses have a small focal length, while telephoto lenses have a large focal length.

Here is a list of the accepted focal length definitions in each category of lens:

  • Less than 21mm: Super wide-angle lens
  • 21-35mm: Wide-angle lens
  • 35-70mm: Standard / Normal lens
  • 70-135mm: Standard Telephoto
  • 135-300mm (or more): Telephoto

There are two types of lenses -- prime (fixed) or zoom. A prime lens will have only one focal length (e.g. 50mm), while a zoom lens will cover a range of focal lengths (such as 17-40mm). 

Focal Length Magnifier

The focal length of lenses was set back in the days of film photography and relates to the focal length of a lens on a 35mm camera. (Keep in mind, though, that 35mm refers to the type of film used and not a focal length!) If you are lucky enough to own one of the professional full frame DSLRs, then your focal length will be unaffected.

If, however, you use a crop frame (APS-C) camera, then your focal lengths will be affected. Because crop frame sensors are smaller than a 35mm strip of film, magnification needs to be applied. The magnification varies slightly between manufacturers, but the standard is x1.6. Canon uses this magnification, but Nikon uses x1.5 and Olympus uses x2.

So, for example, on a Canon crop frame camera, a standard 50mm lens becomes a standard telephoto 80mm lens. (50mm multiplied by a factor of 1.6 to result in 80mm.)

Most manufacturers now make lenses to allow for this magnification, which only work on crop frame cameras. This is particularly useful at the wide-angle end of things, where magnification can turn these lenses into standard ones!

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