just wondering

*gokenin* · 08-20-2009, 01:45 PM

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

Lowell Goudge · 08-20-2009, 01:48 PM

you have to remember that many zooms used to be in the F4-5.6 range, especially telephoto zooms

jsherman999 · 08-20-2009, 01:51 PM

Originally Posted by gokenin

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.

MattGunn · 08-20-2009, 01:57 PM

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

**Marc Sabatella** · 08-20-2009, 02:22 PM

Originally Posted by gokenin

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.

little laker · 08-20-2009, 02:51 PM

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.

*blind-bat* · 08-20-2009, 02:57 PM

* 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....

MattGunn · 08-20-2009, 03:10 PM

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.