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With telephoto zooms, the aperture change from widest to longest focal length is generally a stop or less. And larger lenses with larger elements also cost more. Larger-diameter lens elements (and the required supporting structure) weigh more, so a fixed-aperture zoom also would be heavier. 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. 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). 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 example, a 70-300mm ƒ/4-5.6 zoom wide-open at 70mm has an aperture of ƒ/4. 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. With these facts in mind, it’s clear that choosing a variable-aperture lens isn’t just about cost savings. 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. 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. 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. Allowing the effective aperture to change with the focal length permits lens designers to keep cost and bulk down. Most lower-priced zooms are of the variable-aperture type. While fixed-aperture zooms are thought of as being the best of the best, variable-aperture models have some real benefits for nature photographers.
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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.
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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.
Lowest aperture 3.5 pro#
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).