what's a fast lens?

[tenglw86]

hi all, i've been browsing through the forums and i've come across the term "fast lens" quite often (mainly in the forums to do with travelling). so, what makes a lens "fast"? how do i tell if it is "fast"? what are they most commonly used for? and most importantly, how much do they normally cost?
thanks for any assists on the topic, forgive this noob for his ignorance

 

[kcuf2]

mmm...to answer ur question, got to ask u some question first.

wat is the current camera and lens that u used?

 

[zac08]

Essentially used to refer to lenses with large maximum apertures.

f2.8 for zooms or f1.8 and wider for primes...

Can also be used to mean fast focusing lenses e.g. SWM from Nikon, HSM from Sigma, USM from Canon.

 

[StrifeYun]

hmmm think called fast lens cause of the apature size the lens can set. which will allow more light to enter. and thus a faster shutter speed can be used to capture fast objects.

think so

 

[SnapJaX]

hi, i oso newbie here so seniors pls correct me if i'm wrong.

Lenses have aperatures, eg: F/3.5-5.6, these "holes" are equiv to the pupils in our eyes, contracting to let lesser light in when it's bright, widening to let more light in when it's dark.

Lenses work in almost the same way, and fast lenses are those usually F/4 and below, but they usually come with a heavy price tag...

for example:
a slow lens: 18-200mm f/3.5-6.3 DC OS

at 18mm, aperture is at 3.5
at 200mm aperture is at 6.3 (which usually other lenses are at 5.6)