Cost: Variable-aperture telephoto zooms cost far less than fixed-aperture zooms or prime lenses of equivalent (or even shorter) focal length, and are the lowest-cost way to get really long focal lengths.
Bulk: Variable-aperture telezooms are much smaller and lighter than fixed-aperture telezooms.
Range Of Focal Lengths: Fixed-aperture telezooms generally have zoom ratios of less than 3:1, while variable-aperture telezooms can go well beyond that. For example, the 70-300s have a zoom ratio of 4.28:1, giving you more compositional flexibility when you can’t easily move closer to or farther from a subject.
(Drawbacks) Lesser Optics: While many variable-aperture zooms contain ED elements and optical performance can be quite good, they just aren’t in the class of the much more costly pro fixed-aperture zooms in terms of correction for aberrations and distortion.
Slower Performance: As higher-end pro lenses, the fixed-aperture zooms tend to have their manufacturers’ best AF motors, and thus provide quicker and more accurate autofocusing than the variable-aperture zooms. The slower long-end maximum apertures of variable-aperture zooms also slow AF performance, and make them less well suited for shooting in low-light levels.
Extending Lens Barrel: While variable-aperture telezooms are quite compact at their shortest focal length, they extend considerably when zoomed to their longest. The barrels of most fixed-aperture telezooms don’t extend when the lens is zoomed.
Not As Rugged: As pro lenses, fixed-aperture zooms tend to be more ruggedly constructed, with better dust- and weatherproofing. (There are exceptions: Pentax’s WR variable-aperture zooms are weather-resistant.)
Popular 70-300mm Variable-Aperture Lenses
CLOCKWISE FROM TOP:
Pentax DA 55-300mm ƒ/4-5.8 ED; Sigma 70-300mm ƒ/4-5.6 DG OS; Tamron SP70-300mm F/4-5.6 Di VC USD; Sony 70-300mm ƒ/4.5-5.6 G; Olympus Zuiko Digital 50-200mm ƒ/2.8-3.5; Canon EF 70-300mm ƒ/4-5.6 IS USM; AF-S VR NIKKOR 70-300mm ƒ/4.5-5.6G IF-ED
If you’re looking for an affordable wildlife lens, the 70-300mm variable-aperture zoom might be ideal. The fixed-aperture zooms are “better” in terms of optical performance, rugged construction and AF performance. But they’re also considerably more costly and bulky. While not equal to that of the pro lenses, the AF performance of the stabilized 70-300mm variable-aperture zooms is quite good: the Canon EF 70-300mm ƒ/4-5.6 IS USM, Nikon AF-S VR NIKKOR 70-300mm ƒ/4.5-5.6G IF-ED, Sigma 70-300mm ƒ/4-5.6 DG OS and Tamron SP70-300mm F/4-5.6 Di VC USD. Olympus, Pentax and Sony don’t make stabilized lenses because their DSLRs incorporate sensor-shift stabilization that works with all lenses; the Olympus Zuiko Digital 40-150mm ƒ/4.0-5.6, Pentax DA 55-300mm ƒ/4-5.8 ED and Sony 70-300mm ƒ/4.5-5.6 G also make good variable-aperture “starter” wildlife zooms, as does the Tokina AT-X840 AF80-400mm ƒ/4.5-5.6 (longer than the others, but in the same price range).
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