![]() ![]() 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). 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. The 70-300mm ƒ/4-5.6 does increase in physical length as you zoom it to longer focal lengths. 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. ![]() (Note: Some zooms also may change the physical aperture diameter during zooming, as well.) 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. 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). ![]()
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