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December 22, 2006
[ PRIME SUSPECTS ]
DPEXPERT WAS ASKED THE OTHER DAY FOR ADVICE on buying a lens for a digital single lens reflex camera. The inquirer has a choice when buying a Pentax K100D -- a Sigma 18-125mm zoom or, for $50 more, an 18-200 lens. It looks like a bargain.
Zoom lenses are a wonderful invention but, as with just about everything else to do with photography, you get what you pay for.
These multi focal length lenses are mechanically and optically complex when compared with a fixed focal length -- what is sometimes called “prime” -- lens. The glass elements must be moved in relation to each other in order the change the focal length. It is difficult to remove distortions and colour aberrations from zooms and the longer the range the harder it gets to correct the problems. This in turn means that it is only the most expensive zooms that begin to approach the optical quality of primes.
With the current crop of digital SLRs coming into a competitive market the kit lenses are being made down to a price. One way of keeping the price down is to have the lenses manufactured, or at least assembled, in China, the Philippines, Indonesia, Thailand or Vietnam. You may think you are buying a proud Japanese brand but Sony get their kit lenses from China and Nikon have theirs made in Thailand. Canon’s come from the Philippines and Pentax from Vietnam.
When it comes to their top quality and expensive lenses it is a different matter. They are made in Japan.
More important than the location of assembly is the quality of the optics and mechanics of the lens. Corners are cut in the cheap kit lenses. The cost cutting is apparent in barrel and pincushion distortion, low contrast, poor colour accuracy and chromatic aberrations which can appear as purple fringing around sharp outlines in high contrast lighting.
Barrel distortion occurs at the wide angle end of the range and appears as straight lines near the edge of the frame -- walls, for instance -- bulging outwards. Pincushion distortion is the opposite. It occurs at the telephoto end of the range and appears as lines curving inwards.
Canon give customers a choice of kit lenses. You can buy the camera body plus a cheap lens of ordinary quality or you can opt for a more expensive and much better lens in what they call the “Enthusiast Kit”.
There is another approach to lens purchase. Consider the value of prime lenses. They are simpler to make with fewer moving parts and generally fewer optical elements. They usually offer better resolution edge to edge than zooms and produce sharper images.
Imaging’s Nikon D70 is used more frequently with a Nikkor 60mm or 180mm lens than with the kit zoom. A 60mm lens is ideal for portraiture and most prime lenses at around this focal length are also macro lenses. The 180mm fitted to a DSLR is the equivalent of 270mm on a 35mm camera and that is enough even for the Zoo. Both of these lenses were bought second hand at reasonable prices.
At the moment we are testing the Pentax K10D with a Sigma 70mm f2.8 macro lens. It is optically impressive even though it doesn’t have the smoothest mechanical actions. The on-line photo review site, Photozone, has comprehensive objective tests of most lenses and is worth checking before making a buying decision. A quick look at the reviews will show that the optical performance of cheap zoom lenses is generally mediocre while prime performance, at lower prices than the zooms, is usually good to excellent.
Of course the zoom is more convenient and versatile but there is another trade-off -- maximum aperture or the “speed” of the lens. The Sigma 70mm macro is f2.8. The Nikkor 18-70mm kit zoom is f3.5 at the wide end, reducing to f4.5 at the 70mm end.
Camera makers these days offer enticing packages of body plus two zoom lenses in what looks like an unbelievable bargain. It is -- unbelievable, that is. The rule of thumb is to buy the body and then add the best zoom in the 18-70mm range that you can afford. After that consider adding prime lenses to the gadget bag. You will get better photographic value for your money.
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Posted by terry at December 22, 2006 12:42 AM
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