| what does a fast lens actually mean
Hi, I wonder if you can help me understand something I'm stuck on - I get the impression especially where big zooms or prime lenses are concerned, that a fast lens, in other words having a bigger aperture/ower f-number i.e f2.8 are considered to be the best and most expensive lenses
1) what does the 'fast' bit actually mean - what is 'fast' referring to ?
I know it's not focus speed, but why is a lower f-number 'fast' ? is it the speed of the light going into the lens ? (although speed of light is a constant isn't it ?)
2) why are f2.8 lenses sought after (in this case birding/wildlife big zooms) - wouldn't taking a photo at f2.8 only render focus on a small part of the bird/animal, from what I've seen it's much better taking shots at f5.6-f11 to get sufficient focus detail all over the subject ?
3) assuming build quality is the same (and not taking into account quality factors such as vignetting/CA/etc at different apertures) assume you have two lenses taking a pic of the same subject at 300mm focal length, one is an f2.8 lens minimum, and the other is f5.6 minimum, they are both set to f8 and a photo taken
will the f2.8 lens have a better quality picture ?
thanks in advance and hope I didn't confuse anybody !
Martin  |