lens focal length

introduction

the focal length of a lens is a measure of how strongly the system converges (focuses) or diverges (defocuses) light.
for a thin lens (simple lens), the focal length is the distance from the centre of the lens to the point of focus when focussed at infinity.
more details at wikipedia
for distant subjects, longer focal length (lower optical power) is associated with larger magnification, and a narrower angle of view (field of view).
a zoom lens is a lens with a variable focal length.
a rough rule of thumb to prevent camera shake when hand holding a camera, is to use as steady a camera as possible and a shutter speed at least as fast as 1/(focal length in mm) in secs
- cameras with image stabilisers may allow longer shutter speeds hand held.

many photographers have come from the days of 35mm film cameras (35mm referring to the size of the film which gives an image size measuring 36x24mm) which is also the same size image as current “full frame” dSLR camera sensors.
thus there is a historic bias of photographers relating to lens focal lengths and their photographic effects as to how they are on these 35mm systems.
thus a “normal” kit lens for 35mm cameras which was close to the angle of view of the human eye is a 50mm focal length
a “portrait” lens for 35mm cameras is generally regarded as having a focal length of about 90mm to reduce facial distortions from being too close to the subject.
a 28mm lens for 35mm cameras is regarded as a wide angle lens and lenses with focal length shorter than this are regarded as super wide angle lenses.
a 100-135mm lens is regarded as being a short telephoto lens
a 200-400mm lens is regarded as being a telephoto lens
lenses greater than 400mm are regarded as being super-telephoto lenses.

the conundrum arises in the digital world as most digital cameras have sensors smaller in size than “full frame 35mm” and thus they are said to have a crop factor which is a measure of how the diagonal of the sensor relates to the diagonal of a full frame sensor.
for instance, the Olympus Four Thirds dSLR system and Micro Four Thirds system have a diagonal half the length and thus are said to have a 2x crop factor. This means that the equivalent field of view of a lens on such a camera will be similar to a lens with 2x that focal length on a full frame camera. Hence a 50mm normal lens when used on a 2x crop camera becomes in effect a 100mm lens on a 35mm camera in terms of field of view as one only images the central area. This is great if you want telephoto reach, but not so handy when you want wide angle effects.
when a lens is used on a cropped camera, we often refer to the “effective” focal length of that lens which is the true focal length x crop factor, and allows people to predict its field of view based on their experience with 35mm full frame cameras.
see also digital camera sensors

keeping the subject size constant by moving closer or further away for different focal length lenses results in profound differences to your photographic image:
- the longer the focal length:
  - the narrower the area of the background (“perpective”)
  - assuming the f/ratio (aperture) is kept constant, the narrower the depth of field (DOF), and thus the less will be in focus and the more the background will be blurred.