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Because most digital SLR camera owners demand compact, lightweight lenses, the vast majority of telephoto zooms feature a small maximum aperture. A typical 55-200mm or 70-300mm zoom is designated as f/3.5-5.6 or f/4-5.6 indicating that the maximum aperture is acceptably wide at the short end but small at longer focal lengths. In practical terms, that translates to moderate light gathering ability except at shorter focal lengths.

That's fine for some shooters but serious enthusiasts and pros prefer a wider maximum aperture such as f/2.8 with a tele-zoom lens. That's because f/2.8 can transmit more light to the sensor (or the film) providing several benefits, particularly at longer focal lengths. When testing a DSLR of any brand, I always ask for a 70-200mm f/2.8 model (if available) because a lens of this type offers several advantages. 

Note: Canon also makes two 70-200mm f/4 lenses to be discussed later. These offer the same benefits as the f/2.8 models but to a lesser extent.

Low Light Benefits at f/2.8

The primary reason you might want an f/2.8 telephoto is because it allows for shooting at faster shutter speeds. Granted, this may not be an issue on bright, sunny days even at f/5.6 when using ISO 200 for example. In much darker locations however, a wide aperture or "fast" lens can be a blessing. Let's say you're shooting a sport event in an indoor arena. If a lens' widest aperture is f/5.6, you might need to use ISO 6400 to freeze the competitors' motion. Switch to a 70-200mm f/2.8 lens and you could achieve the same action-stopping effect at ISO 1600.

Low ISO Benefits: An image made at a lower ISO level will exhibit less digital noise: the "grainy" pattern of random, coloured specks that can obliterate intricate detail. While Noise Reduction processing (in the camera or in a computer) can provide smoother high ISO images, the effect is achieved by blurring the digital noise  pattern. That also smudges fine details, reducing resolution. On the other hand, a low ISO image be "cleaner" -- with less obvious coloured speckles -- and more richly detailed.

Non-Variable Aperture: Note too that the f/2.8 aperture is constant: available at any focal length from 70mm to 200mm. That's a definite benefit over the more typical variable-aperture zooms. With those types of lenses, the maximum aperture becomes smaller as you shift to longer focal lengths. Hence, the shutter speed becomes longer exactly when the higher magnification is most likely to produce greater image blurring.  

Extender Compatibility: The 70-200mm f/2.8 lenses also accept teleconverters for greater effective "reach". Add a 1.4x extender and the effective maximum aperture will be f/4 because there's a one stop loss of light transmission. That's still a wide effective aperture and autofocus should remain fast and reliable. A 2x extender can be used too but that causes a two stop loss of light transmission for an effective maximum aperture of f/5.6. While autofocus is maintained it may be slower; more importantly f/5.6 is not a wide aperture. For these reasons, I recommend using only a 1.4x accessory with any f/2.8 lens.

Note: When a 1.4x converter is used with a 70-200mm f/4 lens, the effective maximum aperture is smaller: f/5.6. Autofocus remains available but may not be quite as super fast in dark locations.

Optical & Build Quality

Because wide aperture lenses are prosumer or professional grade products, they usually feature high-grade optical elements for better image quality. This definitely applies to the 70-200mm f/2.8 models. All such current lenses employ at least several large pieces of low dispersion glass for optimal image quality at wide aperture.

Superior Glass: The high-tech elements are designed to correct chromatic aberration: the ttendency for glass to disperse (separate) light into its various colours which do not focus on the same plane. Low Dispersion glass is highly effective in causing all wavelengths to focus more accurately on the digital sensor (or the film). That minimizes "purple fringing", the halo of colourful blurring (not always purple) around distinct edges in an image. It also provides other advantages: higher sharpness, particularly noticeable at the edges of the frame, better color rendition and higher contrast.

 Many inexpensive tele zooms also use low dispersion glass, but typically, only one or two pieces of a small size. But the 70-200mm f/2.8 lenses are more impressive since they employ larger elements and often benefit from superior types of glass. For example, the Canon EF 70-200mm f/2.8 L IS II USM zoom benefits from a very expensive fluorite element plus five pieces of UD (ultra-low dispersion) glass. And the Nikon AF-S 70-200mm f/2.8G II ED VR boasts a full 7 ED (extra-low dispersion) elements. Both provide stunning image quality, even at f/2.8, at all focal lengths.

Rugged but Hefty: Because families often want very portable lenses, light weight is a primary consideration when designing a mass-market zoom. But serious outdoor photographers often demand better construction and superior mechanisms. Hence, all of the 70-200mm f/2.8 lenses are very  rugged, usually with a metal barrel. Some are even weather and dust resistant, a bonus especially when used with a prosumer DSLR that's also well sealed against the elements. In addition to durability, the combination is ideal in inclement weather or in harsh environments such as rainforests and deserts.

Focus and Depth of Field  

A fast lens transmits a great deal of light to the camera's viewfinder and to the autofocus sensor, a valuable feature in low light. Manual focusing is less problematic than it would be with a small aperture lens because the view is brighter. That can be very useful when shooting inside a dark home, a theatre, a cathedral or a castle. Autofocus is quicker and more reliable in dark locations as well. It can make AF possible in situations where the camera would balk if you were using a more typical f/4-5.6 zoom. 

 Internal Focusing (IF): All of the 70-200mm f/2.8 zooms (like other high end lenses) benefit from a design that shifts only a few elements instead of a dozen or more pieces of glass. As a result, autofocus speed is increased. As a bonus, all of the lenses provide internal zooming so the barrel maintains a constant size; the center of gravity does not shift when zooming. That's definitely appreciated especially when using a heavy lens on a tripod.

 Defocus Control: There’s a secondary benefit to using an f/2.8 lens especially at focal lengths of 100mm or longer: the depth of field is shallower at the wider aperture.  The narrower range of acceptably sharp focus can be useful for defocusing a cluttered background, rendering it as a soft wash of colours. (The effect is most obvious at longer focal lengths when the subject is not too far from the camera.) Frame a person or an animal against a blurred backdrop and it will immediately attract and hold the viewer’s attention.

Depth of Field Issues: When using a very wide aperture, only subjects in the focused zone will be very sharp. Do not interpret a "defocused" foreground or background as a sign of poor optical performance. Remember too that you must focus with great accuracy, on the most important subject element. In a portrait for instance, we usually want at least the eyes to be in perfectly sharp focus. The ears may be blurred slightly due to shallow depth of field when using a 200mm focal length at  f/2.8 so you may want to stop down f/5.6 for greater depth of field in that situation.  

Fast Lens Drawbacks

As they say, “there’s no free lunch” and the 70-200mm f/2.8 lenses are not ideal in all respects. First, they rarely focus as close as the 70-300mm "macro" designated zooms. And they require a much larger diaphragm mechanism to accommodate an oversized aperture. Hence, the (metal) barrel and  the numerous optical elements must also be larger. All of that increases  the manufacturing cost and the overall size and weight. Hence, the 70-200mm lenses are not as portable or as affordable as the family-oriented telephoto zooms.

The Bottom Line

Though not intended for everyone, a 70-200mm f/2.8 zoom should certainly be of interest to demanding photographers. In addition to the benefits of a wide maximum aperture, these lenses provide image quality ranging from excellent to superlative. Admittedly, the most expensive models are the best at f/2.8 especially in edge sharpness, but by f/4 they're all fine or very fine performers.  If your camera tends to produce soft images -- as many DSLRs do -- set a +1 sharpening level or use advanced software utilities such as Smart Sharpen in Photoshop and Elements.      

Check out a couple of the available models in person to determine whether you’ll be comfortable with a lens weighing around 3 pounds. Frankly, some shooters will prefer a rugged,  high-grade 70-300mm f/4-5.6 model with several low dispersion elements. Owners of an EOS system might also want to ask for a demo of the EF 70-200mm f/4 zoom listed in the next section. Of course, an f/2.8 zoom is preferable in some respects as discussed earlier, making the size/weight more tolerable. Used with the right shooting techniques, any of the high-grade 70-200mm lenses can pay dividends in terms of professional calibre images.

The Fast 70-200mm Lenses

Most lens manufacturers market a wide aperture tele-zoom lens that's suitable for use with a 35mm SLR and with digital SLRs regardless of the sensor size. Regardless of the brand, they're similar in some aspects but differ in others: size, weight, construction and the number and type of high tech optical elements. All of the f/2.8 models include a removable tripod mounting collar but not all are equipped with an image stabilizer or an ultrasonic focus motor. But all benefit from each manufacturer's best technology for providing high edge sharpness/brightness and resistance to flare in bright light.

Note: Canon markets a couple of 70-200mm f/4 lenses; these are smaller, lighter and more affordable than the f/2.8 models. While they do not offer all of the benefits, they're preferred by some serious photographers so they're included in the chart below.

Canon EF 70-200mm f/2.8L IS II USM : The latest version of Canon's fast water-resistant lens with very fast ultrasonic AF, this one boasts the most effective Image Stabilizer II system, an incredibly strong magnesium alloy barrel plus five UD and one Fluorite elements for outstanding image quality. The diaphragm provides a circular aperture so defocused highlights in the background will be round and not octagonal even at f/2.8 to f/5.6.

Image Quality Rating: 9.8

Specs: Minimum focus to 3.94 feet, 0.21x maximum magnification. Size/weight: 3.5x7.8 inches (77mm filter); 3.29 lbs. Compatible with all Canon EOS cameras. Average Price: $2500 in the US. (The previous f/2.8L USM model, with four UD elements and without the Image Stabilizer, also remains available, a bargain at $1420).

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Canon EF 70-200mm f/4L IS USM: This model offers only a (constant) f/4 aperture so it does not provide all the benefits of an f/2.8 lens but it is much smaller/lighter and maintains autofocus with a 1.4x extender. Its metal barrel is well sealed and includes one fluorite and two UD glass elements for impressive  image quality. However, the removable tripod mounting ring is an optional extra ($140) with this lens.  

Image Quality Rating: 9.7

Specs: Minimum focus to 3.9', 0.21x max. magnification. Size/weight: 3x6.8" (67mm filter); 1.7 lbs without tripod collar. Compatible with all Canon EOS cameras. Average Price: $1329 in the US. (The previous f/4L USM model, without the Image Stabilizer or the weatherproof seals, remains available too at $700.)

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Nikon AF-S 70-200mm f/2.8G ED VR II: This weather-resistant lens with a tank-like magnesium alloy barrel employs a very fast Silent Wave AF motor and includes the most effective VR II stabilizer, a circular diaphragm aperture and a full seven oversized ED elements for stunning image quality. 

Image Quality Rating: 9.8

Specs: Minimum focus to 4.6'; 0.12x max. magnification. Size/weight:  3.4x8.1" (77mm filter); 3.2 lbs. Compatible with all Nikon SLR cameras. Average Price: $2400 in the US. (The older Nikon 80-200mm f/2.8D ED AF, without the VR stabilizer or the fast Silent Wave AF, remains available at $1120.)

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Sony 70-200mm f/2.8 SSM G: Very rugged (but not weather-sealed) this white lens in a metal barrel features a Supersonic wave focus motor, four large elements of ED glass for superb image quality, closer focusing than average and a circular diaphragm aperture. 

Image Quality Rating: 9.2

Specs: Minimum focus to 4', 0.21x max. magnification. Size/weight:  3.4x7.7" (77mm filter); 3 lbs. Compatible with all Sony and most (discontinued) Maxxum/Dynax SLR cameras. Average Price: $1800 in the US.

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Sigma 70-200mm f/2.8 EX DG APO Macro HSM II: The latest version of Sigma's f/2.8 lens with the very fast Hypersonic AF motor, this one employs five pieces of low dispersion glass (both SLD and ELD) for very fine image quality. It's rugged but not as weather-resistant as some competitors, features a circular diaphragm aperture and a shorter minimum focus distance than average for the category.  

Image Quality Rating: 8.5

Specs: Minimum focus to 3.3', 0.285x max. magnification. Size/weight: 3.4x7.3" (77mm  filter); 3 lbs. Available in Canon, Nikon, Pentax, Sigma and Sony mount. Average Price: $950 in the US. (The older model, without the II designation, remains available for a surprisingly high $1400.)

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Tamron 70-200mm f/2.8 DI LD (IF) Macro: This rugged (but not weather-resistant) lens employs an older design, with conventional (not ultrasonic) AF, but offers a short minimum focus distance than average, a circular diaphragm aperture and three LD glass elements for impressive image quality.   It's not quite as heavy as competing f/2.8 models.

Image Quality Rating: 8.6

Specs: Minimum focus to 3.1', 0.32x max. magnification. Size/weight:  3.5x7.6" (77mm filter); 2.5 lbs. Available in Canon, Nikon, Pentax and Sony mount. Average price: $769 in the US.

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