A camera is only as good as lens you have attached to it, and only if that lens is right for the job at hand. Imagine setting your camera in front of an amazing landscape that stretches far off in every direction. You would need a wide angle lens with a short focal length capable of capturing all that grandeur and spectacle. If all you have is a telephoto, then that lens is going to limit the kind of image you could take there. You could also find yourself in a field of sunflowers, wanting to get an extreme close-up of the morning dew on those beautiful yellow flowers. But all you have in your bag is a standard lens with a minimum focusing distance that puts the dew-covered bluebell so small in the frame that it just doesn't work as a good image. After choosing the right camera for you, your next port of call is a choice of lens.
Notably lenses can be divided into two main categories: zoom lenses which have a variable focal length; and prime lenses, which have a fixed focal length. Zoom lenses have the advantage of being more versatile, so having just a couple of lenses you can easily cover a wide range of focal lengths, but they also have several disadvantages. They are heavier than prime lenses, the optical quality is usually not quite as good as prime lenses, and they usually have a slower (smaller) maximum aperture. Prime lenses, on the other hand, are lighter and generally of better optical quality, however you have to carry more of them in your bag if you want to cover a range of focal lengths. That being said both types of lenses are available in a huge range of sizes and prices, from cheap standard 50mm lenses to ultra-fast telephoto zooms costing over $25,000.
There is actually relatively little difference in cost-per-focal length between zooms and primes; both vary widely depending on quality, focal length and maximum aperture, without both popular and premium varieties of both types.
Some photographers prefer the convenience of zoom lenses, others prefer the superior performance of prime lenses, while others prefer to use a mixture of prime and zoom lenses depending on the circumstances.
TYPE OF LENSES
Pancake lens
Typical focal length
Wide, Normal, Telephoto
As suggested by it's name, a pancake lens is a very flat prime lens. It is shorter than it is wide and it is very small and light. Used with DSLR and Micro Four Thirds cameras they are primarily used by photographers who prefer a small and light camera/lens system. Relative to their miniature size, they can produce some good images
Standard Zoom
Typical focal length:
28-80mm (full-frame equivalent)
This is undoubtedly one of the most common focal length, suitable for everything from landscapes to portraits. Most camera systems will include a couple of lenses within this focal length range, usually a cheaper, and included as a kit lens with a new camera, and a premium quality lens often costing a lot more.
Ultra-wide zoom
Typical focal length:
16-35mm (full-frame equivalent)
Ultra-wide zoom lenses are mostly used for landscape photography. They are more specialized than standard zooms, and for this reason are generally more expensive.
Macro lens
Typical focal length:
50-100mm (full-frame equivalent)
A true macro lens will be able to record an image at a 1:1 scale on the image sensor or medium it was shot on at it's closest focusing distance allowing the lens to fill the frame and reveal amazing detail on very small objects.
Medium telephoto zoom
Typical focal length:
80-300mm (full-frame equivalent)
The medium telephoto zoom is generally useful for amateur wildlife, sports photography, and portraits at the shorter end of its focal length range. Telephoto zooms have a smaller effective aperture than standard zooms.
Specialist lenses
Typical focal length:
400-1200mm (full-frame equivalent)
Specialist lenses are largely used by professionals and advanced enthusiasts. These include both zoom and prime ultra-fast telephoto lenses used by sports and wildlife photographers.
CONVERSION FACTORS
Since most digital SLR and CSC cameras use sensors that are noticeably smaller than a frame of 35mm film, when using lenses designed for older film cameras the field of view is reduced by a certain amount. This has the effect of increasing the apparent focal length, so what had previously been a wide-angle lens now becomes closer to a standard view lens.
This "crop factor" or "conversion factor" is an key consideration when buying a lens. To convert the focal length of a digital SLR lens to the equivalent focal length for a 35mm camera, it must be multiplied by the conversion factor.
For APS-C cameras this is approximately 1.6 on Canon bodies, so a standard 18-55mm zoom lens, as supplied with many DSLRs, is roughly equivalent to 28-88mm, close to a standard zoom on 35mm cameras. If one were to user that 28-88mm lens on a DSLR it would be the equivalent to a 44-140mm zoom.
Four-Thirds systems have a conversion factor of approximately 2:1, so a standard zoom for these systems is usually 14-42mm, which again is roughly equivalent to the 28-80mm full-frame standard.