Online Tutorials: Which Lens?
Short focal length lenses are known as wide-angle lenses, with typical focal lengths of between 18mm and 35mm. These are often used for landscape, architectural and social photography. Cropped sensor lenses need to be wider than their film / full sensor cousins. The two lenses in the picture are specifically for a cropped sensor (10-20mm ultra wide and 18-50mm standard zoom lens).
Lenses of between 35mm and 135mm are quite popular with portrait and macro work, as the photographer can give the subject a little space. These lenses are commonly used on cropped sensors and on full frame cameras and come in a variety of different qualities and costs. Faster lenses (those that have a wider aperture) often cost more than a "standard" lens.
80mm-300mm lenses are considered to be telephoto lenses and anything above 300mm (such as 400mm or 600mm) are regarded as super-telephoto. Lenses in the higher regions of this group are popular with wildlife, action and sports photography, although lenses which are fast (small F-Stop number) in this group are very expensive compared to shorter focal length lenses.
Prime lenses lack the optical functions that are found in a zoom lens. This means that the space can be taken up with the optics specialised to the lens' focal length. In the case of this 50mm f1.7 lens, the amount of light available to the sensor is twice that of a standard zoom lens. The optics of this type of lens give it optimal performance in low light and much more control over depth of field.
Teleconverters can add a multiplier to the focal length of your lens. With a x1.5 teleconverter, a 300mm lens will become 450mm. Alternatively a x2 converter will increase the focal length to 600mm. There is a trade-off for this as there is a loss of light passing through to the sensor. This results in the optics being "slower". A typical x1.5 teleconverter will reduce the light by 1-stop and a x2 teleconverter will reduce the light by 2-stops. A 300mm f5.6 lens will become a 450mm f8 or a 600mm f11 lens. This pushes the limits of most cameras' focus and light sensors.
A lot of camera sensors typically have a 1.5 times magnification on the focal length compared to 35mm film, thus a typical focal length of 100mm on a 35mm film SLR would be 150mm on a digital SLR (this is why D-SLRs come with 18mm wide-angle lenses as an equivalent to 28mm.) This multiplication factor does not suit all as landscape photographers prefer to shoot wider and perhaps have to invest in a 6 or 8mm lens. It is, however, a benefit for sports or wildlife photographers, who appreciate the extra focal lengths.