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Get The Most Out Of Variable Aperture Lenses
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 3.0×5.6 inches, weighs about 24 ounces and costs around $600. A typical 70-200mm ƒ/2.8 zoom with stabilization measures about 3.5×7.8 inches, weighs around 53 ounces and costs about $2,400. Even a comparable-speed 70-200mm ƒ/4 zoom with stabilization measures around 3.0×6.8 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.
Sigma APO 70-200mm ƒ/2.8 EX DG Macro HSM
Tamron SP AF70-200mm F/2.8 Di LD (IF) Macro
AF-S NIKKOR 70-200mm ƒ/2.8G ED VR II
There are a number of zoom-lens “formulas,” but basically a telezoom changes the magnification (focal length) by moving elements in front of the aperture diaphragm.
A photographic lens’ ƒ-number is the ratio between its focal length and the diameter of the entrance pupil (the apparent size of the aperture as viewed from the front of the lens, not the physical diameter of the diaphragm opening). In a constant-aperture zoom (here, Canon’s EF 70-200mm ƒ/2.8L IS II USM), the front elements change the magnification of the image and the entrance pupil proportionately, so the ƒ-number remains constant throughout the focal-length range (the entrance pupil diameter must be 70/2.8, or 25mm, to provide ƒ/2.8 at an effective focal length of 70mm, and 200/2.8, or 71.4mm, to provide ƒ/2.8 at an effective focal length of 200mm). Note that in this lens, the actual aperture diameter is 25mm at all focal lengths; the optical magnification during zooming produces both the increased focal length and the increased entrance-pupil diameter.
Canon EF 70-200mm ƒ/2.8L IS II USM
How The Aperture Varies
Okay, we know that the effective aperture on a variable-aperture zoom lens varies, but exactly how does it vary? We wondered if a relatively inexpensive 70-300mm ƒ/4-5.6 zoom can provide you with a 200mm ƒ/4 in the midst of its range, along with the wide and tele ends. In other words, can you effectively get a 200mm ƒ/4 for the much lower price of a 70-300mm variable-aperture zoom?
We asked our friend and equipment expert Mark Comon of Paul’s Photo in Torrance, Calif., to check this for us, as he has access to a number of such zooms. Using the same testing procedure we describe in the “Testing Variable Apertures” sidebar, Comon evaluated several popular 70-300mm zooms, a 55-300mm, a 120-300mm and an 80-400mm, and the results are in Chart A. Considering that ƒ/4 is the maximum aperture at 70mm and 200mm is much closer to its maximum (slowest) focal length, we suspected you can’t get something for nothing and we were right, but the results still had some surprises. As you can see in the chart, the effective aperture rapidly narrows as you zoom to longer focal lengths, reaching ƒ/5.3 to ƒ/5.6 by 200mm with all eight lenses. If you need a 200mm ƒ/4, you’ll have to spring for an actual 200mm ƒ/4 prime lens. But you also can see that the lenses don’t follow a linear path across their variable-aperture range. Some go to a slower aperture relatively quickly and some stay at the faster aperture for more of the zoom range. This information is particularly useful as you consider your shooting priorities.
Sigma 70-200mm ƒ/2.8 EX DG OS HSM
So will a low-priced variable-aperture zoom do the job for you? It depends on what you usually photograph. For example, if you’re a dedicated wildlife photographer, the biggest challenge for a wildlife lens is birds in flight. In our local group of bird photographers, we have several talented individuals who use stabilized 70-300mm variable-aperture zooms, and they have produced many sharp bird-in-flight shots right there with those produced by users of far more costly lenses. Having used both, I can say that the pro lenses autofocus noticeably more quickly, and the number of “keepers” is higher. But the budget-priced, variable-aperture zooms mentioned here can do the job, and very well. (Note that the lowest-priced 70-300mm zooms—those costing under $250, in general—are less suitable for wildlife action photography due to slower AF performance.) In situations where lightning-fast AF is less of an issue, the variable-aperture models are very well suited. Their generally lighter weight and reduced bulk make them excellent for hiking and traveling and as general-use zooms.
Does a variable-aperture zoom change its aperture in a linear manner, in a gradual but nonlinear manner, or in steps? How can you tell what that 70-300mm ƒ/4.5-5.6 zoom’s aperture will be at 200mm?
One way is to attach the lens to your SLR, set the camera for aperture-priority mode, then set the lens to its widest aperture and widest focal length. Activate the meter (by pressing the shutter button halfway down) and slowly zoom the lens toward its longest focal length. The viewfinder aperture readout should change to the maximum possible for each focal length as you zoom the lens. This method isn’t 100% precise, but it puts you in the ballpark, and you should be able to do this in the camera store before buying the lens. Doing this just now with a 70-300mm ƒ/4.5-5.6 zoom I’m testing, I see that it switches to ƒ/4.5 a little before 100mm, to ƒ/5 a little past 135mm and to ƒ/5.6 about one-third of the way between 200mm and 300mm.
You also could set the camera to manual exposure mode, move in close enough to fill the frame with a gray card or other medium-tone reference, set the exposure with the lens wide open at its shortest focal length, and make a series of shots at that exposure at each focal length setting on the zoom. You can then use Photoshop’s Eyedropper tool to read the tonal values in each shot on the Info palette. (Thanks to Canon’s Chuck Westfall for this method.)
Canon EF-S 10-22mm ƒ/3.5-4.5 USM
Telephoto zooms aren’t the only ones that come in fixed- and variable-aperture forms. With wide zooms, the variable aperture is less of a concern because you’re usually using wide zooms stopped down to increase depth of field. Telezooms are often used wide open to provide the fastest possible shutter speeds (and to isolate a subject from a busy background via limited depth of field). As with telezooms, fixed-aperture wide zooms generally are faster, bulkier and more costly than the variable-aperture zooms, but also better corrected
for aberrations and distortion.
Variable-aperture wide zooms are more compact and less costly, and the higher-end ones produce very good results.
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.
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.)
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).