Focal Length

September 12th, 2011

 

Focal Length

When parallel rays of light hit a lens, the distance from the centre of the lens to where the rays meet (focal point) is called the focal length.

 

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On older lenses the focal length shown was quite literally the length of the lens, however modern lenses use multiple lens setups which means the actual size of the lens can be considerably shorter and more compact.

In photography lenses come under 3 main categories- you have the wide angle, the normal and the telephoto. The normal lens refers to the lens which gives the closest reproduction of a scene that would be witnessed by the human eye. On a 35mm camera this can be achieved with a 50mm lens or what is sometimes called a normal lens. Anything over 55mm would be considered telephoto where as anything less than 50mm would be considered wide-angle. On a digital cropped sensor camera you have to take into account the multiplication factor of the camera. This can vary but for most digital SLR’s a 28-35mm lens can be considered normal.

Wide Angle Lens

So a wide angle lens is great for taking pictures where you want to show a lot of a scene or need to photograph in close quarters. They work great for landscape, architecture and indoor photography. They do however suffer from distortion and this can be quite obvious especially when photographing close up. They tend not to work well for portrait shots because of this and you can end up with distorted features like the nose and ears. Wide angle lenses also give a good depth of field and even at low apertures you can get relatively sharp images from front to back.

Normal Lens

This is a photographers most basic lens and usually has a very low f-number making it ideal for low light photography. Unlike the telephoto which can compress the scene and the wide angle which can distort an image the normal lens creates the most natural view.

Telephoto

This type of lens has a narrow field of view and long focal length, they work well for sports, nature and some types of landscape photography. It has the effect of compressing a scene which can be used very creatively. A lot of portrait photographer find that a 80-105mm telephoto creates the best portrait photographs because of its flattering perspective. Telephotos also have a narrow depth of field and are great for isolating their subject. They are however harder to focus in low light because of their narrow field of vision which takes in less available light.

Zoom vs Prime

Nowadays many of the lenses are zoom lenses offering a range of focal lengths all in one lens. Some of these offer focal lengths right the way from 18mm-200mm in one lens. Although this can be extremely versatile and is great for general purpose, the zoom lenses tend to suffer more from distortion and have less optical quality such as sharpness and contrast than the fast primes available. They also tend not to be as fast so they are not great for low light or very shallow depth of field.

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