by UrMessage

Created on: January 02, 2010

What is the Focal Length and how is it calculated?

The definition of the focal length in simple terms: when light strikes the lens at a specific area the rays meet at the focal point. The length then is measured by the distance from the center of the lens itself to the focal point.

Have you ever wondered what makes a camera a telephoto or a wide angle lens? Well, it is based on the focal length of the lens. For example, a 50mm or larger focal length usually means it is a telephoto lens. If it happens to be less than that, it is a wide angle lens. A zoom lens offers a wide array of focal lengths and therefore can be classified as several ranges. For example, a zoom lens can cover from 26mm - 108mm, which means it is a normal, wide angle, and a telephoto lens. There are a wide array of zoom lenses from Nikon to Cannon, each with their own ranges and capabilities.

Please be aware that the focal length of 35mm film cameras lenses (digital length) are not equivalent (to SLR equivalent) when those lenses are placed onto a digital SLR camera base. You will need to convert the 35mm lens using a specified factor. This is based on the image sensor and since most professionals use the high end image sensors due to their price and you the common consumer use digital low end sensors, you will need to use a different multiplier depending on your equipment.

Image sensor sizes vary in size in all digital cameras. Large sensors require larger sized lenses, while smaller sensors require smaller sized lenses. The optical zoom may also affect the physical surface space of the lens and therefore affect the sensor and lens size.

For example, lets assume you have an 80mm lens and your DSLR camera has a focal multiplier of 1.6 - the actual focal length is 80mm x 1.5 = 120mm. So when you go shopping you will need to keep an eye out to ensure you are purchasing the required focal lengths due to the differences between 35mm Digital Lenses and SLR lenses. Since the common consumer is use to 35mm specs, even new SLR lenses have the 35mm specs on them which can confuse the unprepared shopper. The downside to all of this is that you might not be getting the ranges you desire. If your zoom lens is 26mm - 108mm, but that is reflected in digital 35mm terms, than you will need to increase the low end of the range by the factor based on your lens. Depending on your factor, the low end might be 48mm and not 26mm as you thought, thereby limiting your photographic experiences. This can be rectified by purchasing certain types of lens accessories.

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