Tuesday, February 25, 2014
Super Telephoto Zooms
We decipher the technobabble behind these nature photography mainstays
The aperture is the opening in the lens that lets light in. The larger the aperture, the more light the lens can transmit to the image sensor, so you get a brighter viewfinder image for composing and focusing, and you can shoot at a faster shutter speed in a given light level. But the larger the maximum aperture, the bulkier the lens. Apertures are expressed as ƒ-numbers. An ƒ-number is simply the focal length of the lens divided by the diameter of the aperture (actually, the diameter of the effective aperture, which is what you see when you look into the front of the lens, rather than the diameter of the physical aperture itself). For example, ƒ/4 means the diameter of the effective aperture is 1⁄4 the focal length of the lens, or 25mm for a 100mm lens.
You can see from this why there aren't a lot of fast long lenses, and why the ones there are cost a bunch. A 500mm ƒ/2.8 lens (the fastest 500mm you can buy today) has an effective aperture diameter of 500/2.8 = 178.6mm (seven inches!) wide open. That means a big, heavy and costly front element (that Sigma 200-500mm ƒ/2.8 zoom has a street price of over $25,000 and weighs more than 30 pounds!). And, yes, the fastest 500mm lens available today is the long end of a zoom—not a prime lens—quite an engineering feat.
Zoom lenses generally aren't as sharp as prime lenses of equal focal length and price. That's because a prime lens must be corrected for only the one focal length, while a zoom must be corrected for a whole range of focal lengths. And corrections that help at one focal length can make things worse at another. Various aberrations and distortions tend to be more visible in zoom lenses.
That said, however, today's better zoom lenses are excellent, and for many—including working pros—the benefits outweigh the drawbacks.
Tamron's new SP 150-600mm features the second-longest focal length in a current telezoom, yet it costs just $1,069—about $7,000 less than the only current zoom with a longer focal length (Sigma's 300-800mm). Think about that: A quality zoom that goes all the way to 600mm, for just over $1,000. And with built-in Vibration Compensation. And it covers full-frame sensors, as well as APS-C. We just received an evaluation lens from Tamron, and we'll report on our experience with it at www.outdoorphotographer.com. Find the specs in the chart on page 5.
Variable Vs. Constant Aperture
Some zoom lenses (generally, the higher-priced ones) maintain a constant aperture throughout their zoom range. For example, a 70-200mm ƒ/2.8 zoom has a maximum aperture of ƒ/2.8 at 70mm and at 200mm, and everywhere in between. The aperture doesn't change as you zoom the lens. With variable-aperture zooms, the maximum aperture does change as you zoom, becoming "slower" at the longer focal lengths. For example, a 70-300mm ƒ/4-5.6 zoom has a maximum aperture of ƒ/4 at 70mm and a maximum aperture of ƒ/5.6 at 300mm. Just how quickly the aperture "slows" as you zoom varies from lens design to lens design; with most, you can assume halfway through the zoom range that the maximum aperture is close to the slower end of the range.
If you use the camera's built-in TTL exposure meter, it doesn't really matter which type of zoom you use. The TTL meter automatically will compensate for the change in aperture as you zoom. If you determine exposure manually with a handheld meter (or using the Sunny 16 Rule), you'll have to compensate manually for the slower apertures at the longer focal lengths.
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