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Wednesday, December 28, 2005

Variable Aperture Zooms: Fear No Longer

When variable aperture lenses were first introduced, photographers learned that they were quite unsuitable for use with studio flash lighting, or hand-held meters generally, because you didn’t really know how much light was getting to the film except at the shortest focal length. With the mechanical aperture ring of the lens set to ƒ/8, the amount of light getting to the film would be ƒ/8 at the lens’s shortest setting, but might drop to ƒ/13 by the time you got to the longest setting. Exposure value for intermediate settings was guesswork. The problem of exposing with a variable aperture could be dealt with directly by the camera’s built-in meter, but if you didn’t want to use the built-in meter, you had a real problem. This led to the general opinion that variable aperture zooms were as a class not suitable for professional work. The judgment was seemingly confirmed by the fact that such lenses offered by the major manufacturers usually were cheaper stablemates of fixed-aperture zooms with similar range.

Things have changed. With modern lenses for digital cameras this distinction can become trivial. If the aperture is set by the camera’s electronic controls (no aperture ring on the lens) the system can be designed so the actual exposure reaching the sensor varies only if you are trying to use the lens wide open. At smaller stops, the camera is clever enough to keep things constant so, if your flash meter said to use ƒ/8, you can set ƒ/8 and you’ll get the same exposure at any focal length.

You can test this. I just stepped out and tried it with an Olympus E-1 and 14–54mm digital Zuiko, which has an aperture that varies from ƒ/2.8 to 3.5 wide open. Set the camera to Aperture priority, open the aperture all the way (ƒ/2.8 in my case) with the lens at it’s shortest setting.* Aim at a nice big, evenly lit subject like a lawn, and zoom the lens out longer. The aperture will fall, from ƒ/2.8 to 3.0 to 3.5 in my test, and to compensate, the shutter will fall to slower speeds. So far, just what you’d expect from any variable aperture zoom. Now set the aperture to ƒ/8 and repeat the test. The aperture readout will remain at ƒ/8, but the shutter speed will also remain constant. The camera is cleverly keeping the amount of transmitted light the same, which lets the shutter speed remain constant. So if you determine your f-stop setting with a separate light meter—essential with studio flash and sometimes desirable in other situations—you are safe unless you need the widest aperture on the lens.

With camera systems designed this way, the rule that variable aperture lenses are by definition inferior to fixed aperture ones has lost whatever legitimacy it may once have had. Of course it can be an advantage for the lens to maintain a fast maximum opening at longer focal lengths, but since this will almost surely make the lens bigger, heavier, and more expensive, don’t let this one issue be a “deal breaker” when selecting a piece of glass for your computer-that-takes-pictures.

Posted by: CARL WEESE

*Most variable aperture lenses go from wide/fastest to longest/slowest, but some are more devious. You can still use the above test to see whether the set aperture can be trusted.


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