Monday, January 29, 2007
Lens Buying Guide
Everything you need to know about focal lengths, maximum apertures, new technologies and more!
A casual glance through any manufacturer's lens catalog shows a wide range of available maximum apertures. Many lenses open up to ƒ/2.8 or wider, others to no more than ƒ/5.6 (or even less), and you'll often have a choice between fast and slow versions of the lens you're thinking of buying. Just choose one according to your needs.
Fast lenses are bigger, heavier and more expensive, but they let you shoot in dimmer light with a faster shutter speed or with a lower ISO. The main difference between an expensive ƒ/2.8 zoom and its less expensive ƒ/3.5 counterpart is that often the ƒ/3.5 lens is only that ƒ-stop at its widest focal length, while the ƒ/2.8 lens is constant throughout the range.
That smaller, less expensive lens may be ƒ/4.5 or even ƒ/5.6 at the longer focal lengths, which is a considerably smaller ƒ-stop. That can mean a difference of two whole stops of shutter speed, which in low light also may mean the difference between sharp and not sharp. If you're not working at the limits of available light, though, the slower lens could do as well for you, and it will be more compact, too.
If you check compact digital camera lenses, you'll find this applies to them as well. The extra speed of a faster lens may not be important to you, but it's worth checking.
When you're deciding, keep in mind that with D-SLRs, you'll find it easier to compose your pictures with the brighter viewfinder image produced by faster maximum apertures, and your D-SLR's autofocus system will perform better. Although AF systems commonly operate with lenses as slow as ƒ/5.6, they're neither as discriminating nor as quick-working with lenses of that speed as they are with lenses that open to ƒ/2.8 or faster. If you like to focus manually, you'll find that easier with faster optics, too.
D-SLRs accept bayonet-mount interchangeable lenses, each of which has its own complete optical system. Along with all the glass, these optics have built-in focusing mechanisms, as well as an internal lens diaphragm to control your ƒ-stop settings.
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