Choosing a Suitable Focal Length
Focal length is probably the most important factor that should be considered when choosing a lens, and for good reason: focal lengths determine the field-of-view of the photos you will be able to take successfully with your camera.
The two main types of focal length are telephoto and wide-angle, and while telephoto lenses have a narrow field-of-view and are best suited for close-up shots and portraits, wide-angle lenses have a wider field-of-view which is perfect for indoor photography and landscapes.
Keep in mind that the performance of lenses can differ from camera to camera, with the magnification power behind a lens generally being greater on a digital camera than on a 35mm film-based camera.
The Need For Speed
When you hear about fast and slow lenses, reference is being made to a lens’s maximum aperture, which is the maximum amount of light that a lens can let in. A simple rule of thumb is that a fast lens lets in a lot of light, while a slow lens lets in less light, which defines how your photos will look.
Maximum apertures are measured in f/stop numbers, which are actually a ratio of the size of the lens aperture and focal length. The smaller the f/number, the more light is let in. An increment in the f/stop number doubles the amount of light let in, so f/2.0 lets in twice as much light as f/1.4.
This may seem quite confusing at first, so the easiest way to make sense of it is to remember the following: fast lenses are best suited towards successful photography in darker lighting conditions, and slow lenses are targeted towards photography in lighter conditions.
The Ins and Outs of a Zoom Lens
Unlike a fixed-focal-length lens, a zoom lens often gives you the diversity of a range of focal lengths all rolled into a single adjustable lens. This can be great if you often have to switch between various lenses for different shots, but it is important to remember that not all zoom lenses have a constant maximum aperture, and those that do are often larger and more expensive.
Although the maximum aperture may be reduced as you zoom in using a lens with a variable maximum aperture, this may not be as important to some photographers as the reduced cost and size of such lenses. Keep this in mind when purchasing a zoom lens.
Add-on or accessory lenses are targeted towards compact digital cameras, and allow owners of such models to significantly lengthen or reduce the camera’s built-in focal length while at the same time being able to automate camera functions including f/stop settings and focusing.
These lenses can be an excellent low-cost add-on to your digital camera, with telephoto add-on lenses being able to increase focal lengths by up to 300%, and wide-angle versions allowing for reduction in focal lengths of up to 30%.
There are several other terms to take into consideration when buying a digital camera lens to make sure you are making the best purchase. If your lens utilizes aspheric lens elements, then you can rest happily with the knowledge that your lens will help produce sharper photographs and help keep lens weight to a minimum.
Lenses using internal and automatic focusing also keep lens weight down thanks to less moving parts, and of course allow for faster focusing. Low-dispersion glass leaves photos looking less hazy or fuzzy, while stabilization systems help to keep images sharp when taken using slow shutter speeds.