One of the benefits of using a DSLR is that you have access to a variety of interchangeable lenses. All the lenses for the DSLR are specially designed to maximize your creative potential.

For identification purposes, camera manufacturers classify lenses by focal length while those made for specific purposes are then classified for their unique optical or imaging characteristics.

Lens Focal Length

Lenses are generally categorized according to their focal length as this determines the area of the image that can be captured. The most common categories are macro, wide-angle, standard, telephoto and zoom lenses. In optical terms, focal length is defined as the distance in millimetres from the optical center of a lens to the focal plane (image sensor plane) when a subject at infinity is in sharp focus.

Angle of View

Lens focal length also affects the angle of view. To simplify the definition, the angle of view of a particular lens determines the area of a scene that can be captured by the lens. So, a longer focal length means the image area captured will be narrower or smaller while a shorter focal length lets you capture a wider or bigger image area.

Lenses can also be further classified by their fixed or variable angle of view. A lens with a fixed focal length is often called a prime lens, while lens with a variable focal length range is called a zoom lens. Zoom lenses are categorized in the same way according to the focal length range they cover.

Equivalent Focal Length

'Angle of view crop' and 'field of view crop' are terms commonly used in DSLR photography to help 35mm format SLR users understand how their existing range of lenses would perform on DSLR cameras.

APS-C size sensors used in DSLRs are smaller than traditional 35mm film, the resulting image circle is also smaller. Another way to look at this is to consider the image area of the two respective formats shown above. When a 35mm format lens is mounted on an APS-C format camera, the camera only "sees" the central portion of the image, effectively cropping the image to a smaller field of view.

All lenses carrying the DT prefix are specifically made for use with APS-C size sensors and cannot be used on 35mm format SLR cameras. However the 1.5x 'angle of view crop' is still applicable when mounting these lenses on APS-C format DSLRs. This is because focal lengths listed on lenses are always in their nominal values. The advantages of such DT lenses are they are able to offer outstanding image quality, and yet compact in size and light in weight.

To illustrate an example, the SAL20F28 20mm F2.8 lens is made to conform to the 35mm format image circle; and when used on DSLR cameras, it would give you an equivalent focal length of 30mm (20mm x 1.5).

Circular Aperture

Obtaining beautiful defocusing is difficult with conventional aperture designs because defocusing produces polygonal shapes with the same number of sides as the number of aperture blades. Take for example a lens with a 6-blade aperture: the defocused point light sources will have a clearly defined hexagonal shape. While one can open the aperture all the way to achieve a more circular defocusing pattern, vignetting may result and cause the defocused point light sources to have an irregular shape near the image periphery.

The aperture blades in most lenses on the other hand are formed in a composite curve, with the curvature and tapering near the tip are designed to form a near-perfect circle when the aperture is 1-2 stops down from the maximum - ensuring a smoother and more natural defocusing not just for images with point light sources in the background, but virtually any type of background.

ED Glass and Aspherical Lenses

The color of light is determined by wavelength and because different wavelengths travel at different speeds when they pass through a lens (also known as dispersion), the focal points of various colors do not all fall in the exact same focal plane - some may fall in front, while some may fall behind. This causes distortions in captured images, leading to color and image aberrations. One way to correct this is to use ED (Extra-low Dispersion) glass.

High quality ED glass elements are used to enhance the performance of many lenses, notably in telephoto and zoom lenses. Because of its low refractive index and the extra-low dispersion index, it helps to ensure that light rays of all colors are brought closer to the correct focal point. Another means of controlling such color distortion is to use aspherical lens elements where the optical glass is designed to ensure that light waves passing through different areas of the lens arrive at a common focal point. Aspherical lens elements are also effective in controlling the curvilinear distortion that can occur with wide and super-wide angle lenses, and chromatic aberration that can cause defocused point light sources at the periphery of an image to have comet-like tails.

Due to their effectiveness in compensating for various types of aberration and distortion, aspherical elements also help to reduce the overall number of elements in a lens—making it possible to create extremely compact high quality zoom and fixed focus lenses.

SSM (Super Sonic Wave Motor)

Super Sonic Wave Motor (SSM) is a technology normally found in large telephoto and zoom lenses. Conventional electronic auto focus drive technology use motors, coils and magnets to drive the lenses into focus while SSM technology utilizes piezoelectric elements. One of the main advantages of SSM drive technology is its extremely smooth and accurate auto focus response. The on/off response is instant, and the ultra-sonic vibration emitted by the drive unit is inaudible to the human ear - thus ensuring extremely quiet operation.

Another benefit of SSM technology to the photographer is it offers greater focusing freedom by enabling both fulltime direct manual focus (DMF) and standard direct manual focus-allowing a photographer to focus manually too while using continuous AF mode.