Superfast Glass

The faster the lens, the more limited the depth of field when the lens is used wide open. With the superfast ƒ/0.95 lenses, you can concentrate the viewer’s attention on a specific portion of a subject through choice of focus point, while everything else in the image is thrown out of focus.

Fast lenses used wide open also produce interesting bokeh, or background (and foreground) blur. The very wide aperture combines with the lens’ optics and the number and shape of the aperture blades to produce the bokeh effect.

Something to consider is that on a DSLR or mirrorless digital camera, your superfast lens may not be as fast as you expect. At wider apertures—ƒ/2 and faster—the image sensor "wells" block some of the light, resulting in less light reaching the photodiodes than would be expected. (For full details on this phenomenon, go to dxomark.com/Reviews/F-stop-blues.) Also, the thickness of the filter stack on the sensor can adversely affect sharpness if it’s different than what the lens was designed for, and fast lenses are especially susceptible. (Film cameras, of course, have no filter stacks; Micro Four Thirds mirrorless digital cameras have filter stacks of around 4mm, and APS-C and full-frame digital camera stacks are somewhere in between.) For more information about this from those who researched it, go to lensrentals.com and look for the blog entries about "sensor stack thickness."

Why Are The Fastest Lenses Primes?

You’ll notice there are no zooms in our Superfast Lens chart. That’s because as the focal length increases, the diameter of the aperture must increase, too, to maintain lens speed. A 24-105mm ƒ/1.0 zoom would need an effective aperture diameter of 105mm at 70mm—that’s about four inches (not counting the lens structure surrounding that opening)—and would make for a bulky lens. And, if the designer went for a variable-aperture design, it would get slower as you zoomed to longer focal lengths; it wouldn’t be superfast at the long end. An ƒ-stop of 2.8 is a fast zoom, regardless of focal-length range (making that Sigma 200-500mm ƒ/2.8 all the more impressive—all 34.6 pounds of it!).

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