Prime lenses have a single focal length.
Almost every online photography forum contains a lengthy debate about prime lenses vs. zoom lenses. Which type you will use depends on the type of photography you do, so knowing the differences between them is essential to making the decision.
Focal Length
The focal length of a lens is a number expressed in millimeters (mm) that corresponds with the amount of magnification a lens provides. For example, a 100mm lens would make a subject appear two times closer than a 50mm lens. A prime lens has only a single focal length, such as 50mm, 100mm or 500mm. Prime lenses are often referred to as fixed focal length lenses. The alternative to a prime lens is a zoom lens. These lenses cover a range of focal lengths, such as 17 to 70mm, 50 to 200mm or 100 to 400mm. This allows the user to change the amount of magnification by "zooming" in or out.
Optical Quality
In general, prime lenses produce higher quality images than zoom lenses of equivalent construction, focal length and aperture. This is because prime lenses have fewer glass elements for light to pass through, which contributes to sharper, cleaner photos. However, there is great variability among lenses, so this difference in image quality may not be present in every single comparison. Furthermore, advancing technology in zoom lenses is shrinking the gap in quality between primes and zooms.
Size and Weight
Because zoom lenses require more glass elements and moving parts to cover a broad focal range, they are usually bigger and heavier than their prime counterparts. If you are frequently carrying your lenses around in the field, this may be an important factor to consider in your lens selection. It is important to remember, however, that it would take several prime lenses to cover the same range of focal lengths as one zoom lens. If your photography regularly requires multiple focal lengths, the weight savings of a prime lens may be offset by the need to carry more than one.
Lens Speed
The speed of a lens refers to its maximum aperture or f-stop. A wider aperture allows more light to enter a lens, which means the user can shoot at higher shutter speeds. A lens with a wide maximum aperture (for example f/1.4 or f/2.8) is thus considered a "fast" lens. The Digital Photography School website points out that although zoom lenses are gradually improving, prime lenses are still the faster choice. This ability to take in more light is especially advantageous for shooting in low-light situations or when you want to use high shutter speeds to capture fast motion.
Cost
The final difference between prime and zoom lenses is the cost. Because of their generally simpler construction, prime lenses are usually less expensive than zoom lenses of the same quality, aperture and focal length. However, like the differences in size and weight, the difference in cost may be offset by the need to carry multiple prime lenses to equal the focal range of one zoom lens. The type of photography you plan to do will help to determine the lens(es) that will best suit your needs and budget.
Tags: prime lenses, zoom lenses, focal length, focal lengths, prime lens, zoom lens, amount magnification
