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"Fast lens" What is the exact definition?

Matt White , May 18, 2007; 01:35 p.m.

I've seen the word usage "fast lens" many times when referring to aperture. Is this word usage due to the lens having a larger aperture opening therefor making it possible to use a faster shutter speed to capture the same image (light) it would take with a lens with a smaller aperture?


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Evan Goulet , May 18, 2007; 01:38 p.m.

Essentially, yes.

Matt Laur , May 18, 2007; 01:39 p.m.

Right! If it can get wider (open), you can shoot faster, everything else being the same. Also, money tends to leave your wallet faster when you buy such lenses.

Todd Peach , May 18, 2007; 01:48 p.m.

'fast' is also relative. Most folks think f/2.8 is fast in a zoom lens. Prime shooters think f/2.8 is slow, f/1.4 is fast.

F Stop , May 18, 2007; 01:49 p.m.

Having a "fast lens" is 10X more convenient when you're in the field, because you always seem to need that extra stop or two.

-my 2 cents

Matt White , May 18, 2007; 02:08 p.m.

Is there are reason why a 'fast lens' costs more? Is the cost of construction that much more?

Gerry Siegel (Honolulu) , May 18, 2007; 02:15 p.m.

One nitpic.Just to be more optically precise, I suggest a change in wording is better in this definition. I would not say "aperture" and stick to "f stop," as in f1.4 etc. (Aperture in relationship to focal length.) Regards,GS.

Matt Laur , May 18, 2007; 02:16 p.m.

Essentally, to keep the light consistently focused through a larger aperture, you need more (and better) glass. That means more time consuming manufacturing, quality control, and physically larger parts. If it's an autofocus lens, there are larger parts being moved, so the autofocus mechanics need to be better. The demand for faster lenses also tends to come from more professional/serious shooters, which means they're also usually looking for more robust all-around construction (more metal, less plastic, more nimble A/F, etc). It all adds up. And of course, the golden rule: if there's less demand for it, there is less of a supply built... and that means you lose some economies of scale. Nikon probably ships hundreds of 50mm f/1.8 lenses for every single f/1.4 they sell.

Matt White , May 18, 2007; 02:21 p.m.

Ok, that all makes sense. Thank you for explaining things Matt.

Gerry - Can you explain why my wording is incorrect? From what I understand, a lens rated at F1.4 has a larger aperture than a lens rated at F2.8, correct?

Evan Goulet , May 18, 2007; 02:45 p.m.

Well f/1.4 does not refer to the actual aperture. It is a way of standardizing the relationship of aperture to focal length. An f-stop of f/1.4 uses a larger diameter aperture on a 50 mm lens than on a 28 mm lens.

F-stop # is given as N = f / D, where f = focal length of the lens, and D = diameter of aperture.

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