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.