A Viewfinder Darkly
Phil Northeast is Feature Writer for Photography at 
Prime and Zoom Lenses on DSLRs
© Philip Northeast
10 November 2009
Prime lenses have advantages that sometimes make them the first out of the camera bag as zoom lenses are not the best choice for all occasions.
Zoom lenses have come a long way from the cheap alternative to a range of prime or fixed focal length lenses for the amateur photographer. They now have a permanent spot in the professional photographer’s camera bag. With the DSLR’s ability to change lenses, photographers choose the best lens type for their own style of photography.
35mm prime lens
Prime lenses, because of their optimization for one focal length, are the best quality lenses. They usually are a faster lens, due to the larger maximum aperture setting or smaller f-stop. This lets more light pass through the lens, making them preferable for low light situations. The wider aperture is also useful when the photographer is seeking to minimize depth of field. They tend to be lighter and more compact than a zoom lens, making the camera easier to handle. The old 50 mm standard lenses with maximum apertures of f1.2 are proving to be popular alternatives to zooms when the lights go down.
Modern Zoom lenses benefit from computer-aided design and automated manufacturing techniques. They are now capable of achieving excellent optical performance on digital cameras. The claim is that they replace all the prime lenses in their zoom range, so the photographer only has to buy and carry around one or two zoom lenses, rather than a bag full of prime lenses. The consumer grade standard zooms that cover the focal length range from 17 up to 125 mm are very versatile and are good walking around lenses. They can cover most of the common situations from wide-angle lens to telephoto lens.
zoom lens
Professionals find the flexibility of the zoom enables them to get the perfect in-between focal length for a wider range of settings. This flexibility allows them to fill the frame with the image so that they get everything in the picture. Cropping, or using only part of the image, wastes those all important megapixels, lowering the quality of the image.
Photographers in sports and photojournalism often have limited options of shooting positions. This partly explains the proliferation of large lenses in the areas designated for accredited photographers at sporting events. The long focal length zoom is essential to bring the action to the photographer and the zoom facility aids composition of moving subjects. The zoom allows rapid focal length changes needed as the action develops and presents different photographic opportunities. There may not be time to change prime lenses in a fast paced event. The zoom's ability to alter the focal length reduces the need for lens changes in the field, so minimizing problems of dust getting onto the sensor of their digital cameras..
While there may be some saving in buying a couple of zooms instead of many primes the search for quality can erode this cost advantage. Zooms contain more lens elements to make them faster and to give them variable focal length. This makes them heavier and bulkier than individual prime lenses. This also influences the cost; a good fast zoom that goes to f2.8 at 200 mm may cost thousands of dollars.
So when considering lenses for your DSLR, a compact, quality prime lens which will deliver good performance in low light may have a place alongside your zooms.
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