• Today, interchangeable lenses are most frequently used with digital single-lens reflex cameras.reflex num??rique image by Sophie Hurel from Fotolia.com
  
  Interchangeable lenses are designed for use with single-lens reflex (SLR) cameras, most of which are digital (DSLR) these days, as film cameras have been replaced. Lenses are distinguished by several factors, including their focal length and speed and whether they are zoom or prime. Each factor will affect the price of the lens.
  
  

Focal Length

• The focal length of a lens can be normal, wide-angle or telephoto.two black photo lenses isolated on white image by Elnur from Fotolia.com
  
  Lenses for DSLR cameras may be wide-angle, normal or telephoto. A focal length of approximately 50mm is generally defined as a normal lens. A normal, or standard, lens renders perspective in a manner closely matching that of the human eye, according to photographer Gary Voth. Lenses also may be wide-angle (less than 50mm) or telephoto (greater than 50mm). Within each category, there are subcategories. A wide-angle lens may be regular, which Scott Kelby defines as between 24mm and 35mm, or super-wide, averaging between 12mm and 24mm. A fish-eye lens, with the widest focal length and greatest distortion, photographs a circular perspective. Whereas a wide-angle lens will allow a broad view, which is great for landscape images, a telephoto lens enables the photographer to get closer to a subject without changing position. Kelby notes that super-telephoto lenses range between 300mm and 600mm. Telephoto lenses are often used to shoot sporting events or wildlife.
  
  

Zoom or Prime

• A prime lens has a fixed focal length, as in 50mm.camera lens image by dinostock from Fotolia.com
  
  Wide-angle and telephoto lenses will be either zoom or prime. A zoom lens offers a variable focal length, as in 28mm to 105mm; a prime lens has a fixed focal length, as in 100mm. The advantage of a zoom lens is convenience. The photographer can move from one focal length to another without having to switch lenses. This is particularly beneficial when shooting in a fast-paced environment. While a zoom lens offers flexibility, some experts contend that a prime lens with its fixed focal length provides a sharper image.
  
  

Lens Speed

• Fast lenses permit shooting in low-light settings without using a flash.Nighttime Studies image by jumedero from Fotolia.com
  
  The speed of a lens is determined by its largest f-stop or lens opening. An f-stop will determine how much light reaches the camera's sensor. Larger f-stops which fall in the range of f1.4, f1.8, f2 or f2.8 permit a photographer to shoot in low-light conditions without having to employ a flash. In many instances, natural light is preferable to the light created by a flash, as the available colors appear more natural. Unfortunately, the faster the lens is, the greater the accompanying price tag.
  
  

Which Lens

• Thoroughly research any lens before making a purchase.computer image by martini from Fotolia.com
  
  There are many factors to consider when purchasing a lens. Probably foremost, you must determine your application for the lens. How do you plan to use it? Secondly, your budget will ultimately affect your final decision. To assist you in making an informed selection, use online resources, including CNET, for research and visit your local camera store. Often, camera stores will allow you to test a lens on-site. From home, you can upload your photos to assess the quality of the lens.