fast lens definition

Rachelsne · Jun 18, 2008

so I am probably going to be nominated for the silliest question of the year but here goes anyway...

the term fast lens confuses me. because I figure that fast = speed which comes fromshutter speed in camera not the lens, so what exactly does it mean and how do I know if its a 'fast lens'

All I can think of is its talking about focussing speed-which is obselete if you use manual focus (which I dont)

Thanks:blushing:

Mav · Jun 18, 2008

A fast lens means fast as in more light gathering ability (larger aperture), which allows for the faster shutter speeds. An f/4 is one stop faster than an f/5.6, and an f/2.8 is two stops faster. On Canon and Nikon, an f/2.8 zoom is a fast zoom. At a given ISO shooting sports with an f/5.6 zoom, maybe you're only getting 1/125s shutter speed. With an f/4 now you can get 1/250s at the same ISO, and an f/2.8 gets you to 1/500s.

For prime lenses, f/2 or smaller (f/2.8) is "slow" but still faster than zooms. Truly "fast" primes are f/1.4 or f/1.2. There's even some highly specialized f/1.0 and f/0.9 glass out there.

Big Mike · Jun 18, 2008

Yes.

Faster means a bigger max aperture...which gives the option of a faster shutter speed.

ANDS! · Jun 18, 2008

All I can think of is its talking about focussing speed-which is obselete if you use manual focus (which I dont)

On the right track. Mav summed it up - "faster" lenses allow you do use a wider ap. setting, and thus "compensate" with a faster shutter speed. In settings where your subject isn't moving, but theres not a whole lot of light (and you're not using a flash), you can tripod the camera, use a slow shutter speed and wide ap, and compensate for the lack of light (ie "low light" situations).

The oft pimped "fast" prime lens is the 50MM f/1.8 (Sigma, Tamron, Nikon, Canon, et. al). Its just a dirty versatile lens, and ridiculous for the cost. 100 bucks. Can't beat that.

And its not a stupid question as the terminology doesn't really match whats being talked about (at least not intuitively).