Camera Lens

A camera lens is made up of multiple lens elements assembled and fitted together in a tube called a lens barrel. One of the major advantages of using an SLR camera is its ability to change lenses. You could go for wide angle to squeeze more in, a telephoto to magnify a distant subject, or a macro lens for taking great quality close-ups. There’s almost no limit to what you can do, with lenses to suit all occasions and budgets.

Focal Length

Every lens is defined in millimeters, and this measure is called its focal length. This measurement is the distance between the sensor and the optical centre of the lens when the lens is focused on infinity. A short focal length allows you to stand close to your subject so that they fill the frame and things in the background seem farther away than normal. Examples of short focal lengths are 15mm, 20mm, 35mm. A long focal length allows you to stand far away from the subject so that it still fills the frame, but that things in the background seem closer than normal. Examples are 85mm, 105mm, 300mm. The focal length range is printed on the lens barrel along with the most commonly used focal lengths.

Types of DSLR Lenses

Standard Lenses - Also known as a "normal lens", is one which produces an image that roughly matches what the human eye sees, and which looks natural to the viewer. Standard lenses have an angle of view of around 50 to 55 degrees diagonally. This is roughly the same as the angle that the human eye can comfortably view, which is why it gives a natural-looking perspective. Normal lenses make great general-purpose lenses, and can be used to photograph everything from close-up portraits to landscapes. They tend to be very "fast" lenses (i.e. they have a wide aperture), making them great for indoor and low-light photography.

Kit Lenses – Lens that is offered as a package with the DSLR is generally called the ‘kit lens’. It is generally an entry level quality zoom lens. They are usually a fairly general purpose lens designed for everyday shooting. Personally I prefer to buy the DSLR as body only and buy the lenses separate giving me more range as the kit lenses are generally at a lower to medium end of the spectrum of lenses. 18-55mm and 18-105mm are common Kit lenses.

Prime Lenses – Majority of the lenses that you see are zoom lenses (18-55mm, 55-200mm, 18-200mm), meaning they can be adjusted from one focal length to another. Prime lenses are lenses that have only one focal length. (35mm, 50mm, 85mm). The advantage of a prime lens is the fact that they are comparatively cheaper than zoom lenses and also faster, meaning the apertures on them are large (f2.8, f1.2). 

Telephoto Lenses – Or Zoom lenses are lenses that let you photograph distant objects without getting closer to the subject. They are the most common type of lenses you will find in the market. A common question asked around these days is, what zoom do you have for this camera? The question is a result of point and shoot (P&S) and bridge camera manufacturers advertising their cameras focusing on the zoom (X) and megapixel (MP) aspect. Zoom is a feature of the lens and not the camera. Therefore in DSLR field, it’s the wrong question to ask, because zoom factor changes with the changing of lenses. Zoom is calculated by dividing the largest focal length of the lens by the smallest one. So an 18-55mm lens has a 3x zoom and so does 100-300mm lens, irrespective of the focal length covered. 

Macro Lenses - A macro lens is defined as one that is capable of producing an image on the sensor that is the same size or larger than the actual object being photographed.   This is expressed as a reproduction ratio of 1x or 1:1. True macro lenses enable you to get in incredibly close to the subject you are shooting.

Wide Angle Lenses - As their name suggests, these lenses enable their users to take shots with a very wide perspective. They are useful for landscapes and for getting in nice and close to subjects still fitting a lot of them in. Wide angle lenses come both as prime lens and lower end zoom lens.

Source: Nikon Website, Digital SLR Photography