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08-20-2009, 12:45 PM   #1
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just wondering

I was on the way into work this morning and a thought occured to me ( rare at 7 am) why is it that on a zoom lens f2.8 across the board is considered a fast lens but on a prime if you have anything over F2 its thought of as a slow lens? or am I just wrong? or should i not try to think before coffee

08-20-2009, 12:48 PM   #2
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you have to remember that many zooms used to be in the F4-5.6 range, especially telephoto zooms
08-20-2009, 12:51 PM   #3
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QuoteOriginally posted by gokenin Quote
I was on the way into work this morning and a thought occured to me ( rare at 7 am) why is it that on a zoom lens f2.8 across the board is considered a fast lens but on a prime if you have anything over F2 its thought of as a slow lens? or am I just wrong? or should i not try to think before coffee
.

Because it's much easier and less costly to construct a quality optic with a fixed focal length with a large maximum aperture than a zoom - also, there are very few telephoto primes over 85mm that have greater than f/2.8, so focal length plays a part, too. 28mm --> 85mm seems to be the sweet spot for large-apertures in lens design - those lenses can have f/2 and greater max apertures without being enormous or too cost prohibitive.

Olympus makes some f/2 zooms for their 4/3 systems, but they're huge and costly.
08-20-2009, 12:57 PM   #4
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Unfortunately, with optics as with everything else in life, you can't have everything at once. For the increased versatility of a variable focal length you have to sacrifice speed. It is possible to make a super fast zoom but it is likely to be so expensive that it is impractical and it would almost certainly be soft wide open.

Gokenin, How true that quote is

08-20-2009, 01:22 PM   #5
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QuoteOriginally posted by gokenin Quote
why is it that on a zoom lens f2.8 across the board is considered a fast lens but on a prime if you have anything over F2 its thought of as a slow lens? or am I just wrong?
You're not wrong - that *is* the conventional wisdom. I'd suggest that to some extent, it is that cnventional wisdom that is wrong - f/2.8 isn't fast for a prime *relatively speaking*, but it's not a slow lens in comparison to zooms, and in fact, depending on how/what you like to shoot, you can get by quite nicely with nothing faster than f/2.8.
08-20-2009, 01:51 PM   #6
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In my books a 500mm f4 is a fast lens.
So is my 200mm f2.8 Zeiss.
However since everyone makes a faster lens I don't consider my 50mm f2.0 as fast.

At least in my books it isn't JUST the f-stop that makes a lens fast. It's a combination of f-stop, and focal length combined.
08-20-2009, 01:57 PM   #7
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* F-stops, focal length and lens aperture

I'm sure there are other reference to read, but last paragragh kind of explains it. Go get some coffee....
08-20-2009, 02:10 PM   #8
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Although the ability to take photos in low light is considered important, the shallow depth of field a wide aperture provides can be more important. However long focal lengths also reduce the depth of field, so with longer lenses a smaller aperture will do. This means the impractically large bits of glass required for long fast lenses (500mm f/4 means a 125mm diameter front element) is unjustifiable.

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