Get The Most Out Of Variable Aperture Lenses

Often dismissed by “serious” photographers, these lenses offer some significant advantages

By Mike Stensvold

Labels: Lenses, Gear

This Article Features Photo Zoom

Olympus Zuiko 35-100mm ƒ/2.0 PRO ED (equivalent to a 70-200mm on a 35mm SLR due to the Four Thirds System sensor size).

In a variable-aperture zoom (here, Canon’s EF 70-300mm ƒ/4-5.6 IS USM), elements in front of and behind the diaphragm move (and the diaphragm itself moves), so the entrance pupil doesn’t vary in proportion to the magnification, and the ƒ-number changes as you zoom the lens. (Note: Some zooms also may change the physical aperture diameter during zooming, as well.)

You also can see from the diagrams that the 70-200mm ƒ/2.8 has internal zooming—the physical length doesn’t change as you zoom it, making for a better-balanced package at long focal lengths. The 70-300mm ƒ/4-5.6 does increase in physical length as you zoom it to longer focal lengths. Some fixed-aperture zooms do increase in physical length when zoomed (Canon’s wide-to-tele EF 24-105mm ƒ/4L IS USM, for example), but all variable-aperture zooms do.

Zoom lenses come in two varieties: fixed-aperture (70-200mm ƒ/2.8, for example, where the maximum aperture is ƒ/2.8 at all focal lengths) and variable-aperture (70-300mm ƒ/4-5.6, for example, where the maximum aperture decreases from ƒ/4 at the 70mm setting to ƒ/5.6 at the 300mm setting). While fixed-aperture zooms are thought of as being the best of the best, variable-aperture models have some real benefits for nature photographers.

Most lower-priced zooms are of the variable-aperture type. Allowing the effective aperture to change with the focal length permits lens designers to keep cost and bulk down. A typical 70-300mm ƒ/4-5.6 zoom with built-in stabilization measures around 3.0x5.6 inches, weighs about 24 ounces and costs around $600. A typical 70-200mm ƒ/2.8 zoom with stabilization measures about 3.5x7.8 inches, weighs around 53 ounces and costs about $2,400. Even a comparable-speed 70-200mm ƒ/4 zoom with stabilization measures around 3.0x6.8 inches, weighs about 27 ounces and costs around $1,300—and the 70-300mm variable-aperture lens provides 50% more “reach.” You could add a 1.4x teleconverter to the 70-200mm ƒ/2.8 to get a 280mm ƒ/4 or a 2x extender to get a 400mm ƒ/5.6, but that would make an already bulky lens even bulkier, while further increasing the cost; plus, you have the inconvenience of having to add or remove teleconverters each time you want to change focal-length range. With these facts in mind, it’s clear that choosing a variable-aperture lens isn’t just about cost savings.

Sigma 70-300mm ƒ/4-5.6 DG OS

Why The Aperture Varies
The ƒ-number is a ratio between the lens’ focal length and the diameter of the diaphragm opening (actually, of the entrance pupil) at that setting. For example, a 70-300mm ƒ/4-5.6 zoom wide-open at 70mm has an aperture of ƒ/4. This means the effective diameter of the opening through which the light passes to the sensor or film is one-quarter the focal length, or 70/4 = 17.5mm (about two-thirds of an inch). For the lens to maintain that ƒ/4 maximum aperture at 300mm, the effective opening diameter would have to be 300/4 = 75mm (about three inches). To produce the ƒ/5.6 maximum aperture at 300mm, the opening needs to be only 300/5.6 = 53.6mm (about two inches), allowing for a much smaller lens. Larger-diameter lens elements (and the required supporting structure) weigh more, so a fixed-aperture zoom also would be heavier. And larger lenses with larger elements also cost more.

With telephoto zooms, the aperture change from widest to longest focal length is generally a stop or less. For the wide-range “superzooms” (18-200mm, 28-300mm, etc.), the difference is nearly two stops. Most superzooms have a maximum aperture of ƒ/3.5 at the widest setting, narrowing to ƒ/6.3 at the longest focal length. Most lower-priced wide-angle zooms also have variable maximum apertures, generally with a stop or less of difference from wide to tele. The 18-55mm “kit” zooms are generally ƒ/3.5-5.6, a little over a stop difference.

Tamron SP70-300mm F/4-5.6 Di VC USD

Back in the day, variable-aperture zooms were a bit awkward to use because exposure was manual, and the aperture would change each time you changed the focal length, thus changing the exposure. If you set an exposure of 1⁄1000 sec. at ƒ/4 at the wide setting, then zoomed to the long setting, the aperture would change to ƒ/5.6, and your image would be underexposed by a stop.

Today’s DSLRs automatically adjust the lens as you zoom it to maintain your selected ƒ-stop, thus eliminating this problem (assuming your selected ƒ-stop is no wider than the long-end maximum aperture, i.e., ƒ/5.6 with an ƒ/4-5.6 zoom). Of course, in automatic exposure modes, the metering system also keeps the exposure correct at any focal-length setting automatically. So if you want to shoot at ƒ/8 for a particular shot, you can rest assured that the system will give you ƒ/8, regardless of the focal length you choose.

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