Monday, June 28, 2010
Focal Length Facts—06/28/10
A beginner’s guide to focal lengths and how they affect photographs
6. Photographers shopping for point-and-shoot or compact cameras often encounter zoom lens descriptors such as 2X, 3X or 10X. This isn’t actually a representation of the precise focal length of a lens, but rather the zoom range that lens covers. A 2X lens, for example, doubles its focal length from its widest to its longest setting—as in a 35-70mm lens. A 3X zoom triples the focal length (like 35-105), and a 10X zoom multiplies it by a whopping factor of ten (as in a 35-350mm lens). The bigger the X factor, the larger the range of focal lengths covered by a lens. Remember though, just because two lenses offer 2X zooms doesn’t mean the lenses have the same focal length. For that, you’ll have to compare actual millimeter measurements in 35mm equivalent terms.
7. The longer the focal length of a lens, the shallower the inherent depth of field that lens will produce. The shorter a lens, the greater the depth of field will be even at wide apertures. In practice that means you have to be more precise when focusing a telephoto lens, whereas wide- angle lenses have such depth of field they can be very forgiving of improper focus. Many photojournalists for years have utilized this “benefit” of wide-angle lenses in difficult shooting environments, not only because they take in more of the scene and provide context, but because they have so much depth of field to provide focus from near to far.
8. The longer the focal length of a lens, the more difficult that lens will be to handhold. This is true not only because longer lenses tend to be physically longer and heavier than wide-angle lenses, but also because subtle vibrations and camera shakes are amplified dramatically when using a telephoto lens. A good rule of thumb is to use a minimum shutter speed equivalent to the focal length—for example, when handholding a 500mm telephoto lens, be sure to set the shutter speed no slower than 1/500th of a second.
9. Some lens designations mean that even though the focal length may be the same, the lens won’t perform the same. A macro lens, for instance, can focus extremely close, allowing for great magnification of small objects and fine details. One 100mm lens may be designated macro, while another is not. You’re bound to pay a premium for the added capabilities, but if making big photos of little objects is important to you, it’s well worth the investment.
10. Many photographers utilize special devices to change the effective focal length, or at least the performance, of a lens. Teleconverters are popular among wildlife photographers and those who want to double or triple their lens’ focal length (with a 2X or 3X teleconverter) without carrying an additional, and often quite expensive, supertelephoto lens. Extension tubes are a similar device, but rather than changing the lens’ effective focal length they simply change the focusing range—making a lens focus much closer and behave more like a macro lens would. Like macro lenses, extension tubes are used to allow close focusing are ideal for flower photography and other close-up uses. The downside with both extension tubes and teleconverters is that each requires a sacrifice in available maximum aperture—often as much as two full stops that turn an ƒ/2 lens into an ƒ/5.6. Worth it, though, if you’re working at smaller apertures, with flash or if you simply need the close focusing or telephoto extension effect.
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