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04-30-2013, 07:22 AM   #1
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Is a Fast Lens Faster Throughout its Aperture Range?

This is going to be a question that I'll probably be embarrassed about in a few days but the title says it all... for example, will a 1.7f lens set to 2.8f allow for one to shoot at higher speeds than a 2.8 lens wide open? I suspect that the aperture is a standard number across the board and not relative to each lens, thus meaning that a "fast" lens is only "faster" when you are using the larger aperture values.

04-30-2013, 07:27 AM - 1 Like   #2
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You are essentially correct. A fast lens is only fast when wide open. At smaller apertures it is the same speed as any other lens at the same aperture, but it may be sharper because of better optics.
04-30-2013, 07:34 AM   #3
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In theory, a specific F-stop value is consistent across all lenses. 2.8 is 2.8 is 2.8 .... In practice, the tolerances are not quite that good, and things like sharpness and contrast differences between lenses impact how our brains see exposure. But on whole, a lens capable of 1.7, set to 2.8 should admit the same amount of light as a lens that is wide open at 2.8.
04-30-2013, 07:48 AM   #4
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Perfect. Thank you for the replies.

04-30-2013, 09:02 AM   #5
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You will, however, have the benefit of a much brighter viewfinder, a big help when manually focusing.
04-30-2013, 03:26 PM   #6
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There is also the issue of T-Stops vs F-Stops. f stops are the ratio of focal length / aperture , I.e a geometric ratio,

T-Stops are true equal value of light transmission.

In the old movie days, that amounted to an important issue with respect to exposure, but today, modern coatings, lens transmission factors are generally quite high and people tend today to ignorenit
04-30-2013, 08:29 PM   #7
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If you just confine it to what shutter speed you can use, f2.8 is always the same. But although the aperture and shutter speed numbers look very precise, the actual light measured can vary a lot. A full stop (from say f2.8 to f2) is double the light, and the camera only does 1/3 of a stop at best. There's a lot of room there. The light meter usually doesn't care about corners and edges, and some lenses have vignetting, darkening by a full stop out there when wide open. So images could look very different with extremely different lenses (price or design or purpose).

After you go crazy with lens buying, you may have the lenses to try this out yourself, which is interesting. You might have a Pentax-M 50mm f1.7, a Sigma 50mm f2.8 macro and a Tamron 17-50mm f2.8 zoom. The zoom is likely to produce the worst image. It's at one of its extreme focal lengths and it's wide open. The macro might suffer a little because it's wide open, but macros are so well-corrected for aberrations, it might be already great wide open. (Even slightly stopped down, it should be sharper than the M50/1.7, by a lot.) The M50/1.7 should be pretty good at f2.8, well away from wide open, only 6 elements so less opportunity to lose light.
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