[Ali]

I often see people making reference to a “fast lens” and I wondered if you could tell me the definition of “fast” and what deems a lens to be classed as such.

I’m sure it’s a really silly question, but unless I ask I won’t learn… so please don’t laugh too hard at me

Thank you

[orangepeel]

I think of it as shutter speed. A larger aperture lens allows a faster shutter speed for the given light conditions.

Dunno if that's right mind.

[silkstone]

It means one with a wide maximum aperture. The lower the f-number, the wider the aperture because it's the ratio of iris diameter to focal length.

For example, these days f/1.4 would be considered very fast. f/2.8 is fast for a zoom lens and many prime (non-zoom) lenses of longer focal length. f/4 is pretty standard and anything from f/5.6 onwards is rated as being rather slow.

The aperture does two things - it lets more light in so you can take shots in lower light levels, and it affects the depth of field - i.e. how much is in focus. Wider apertures (smaller f-numbers) give smaller depth of field.

These days, with modern cameras having such good high-ISO performance, the light-gathering abilities of fast lenses are not quite as vital as they used to be - IMO of course.

[Dabhand16]

Fast lenses are those that allow a greater amount of light to pass through them than 'ordinary' lenses. The lower the f number, (that is the bigger the apature) the faster the lens.

The actual f number will vary depending on the focal length of the lens, and if it is a prime or zoom lens. Fast primes will be f2.8 or under up to about 85mm, around f4 at 300mm. Standard or wide angle zooms can get closer to the primes than telephoto zooms.

The downside is that fast lenses are physically bigger and heavier than 'ordinary' lenses, which is why there are more and more lenses with stabilization available. This effectively enables an 'ordinary' lens to be used with slower shutter speeds which has the same effect as a fast lens using faster shutter speeds.

[fujitsu1522]

Lens speed - Wikipedia, the free encyclopedia

[danpen]

Bigger heavier lenses roll down the hill and into the lake "faster" when you drop them, hence "fast lens"

cheers

Dan

[stepheno]

Originally Posted by danpen

Bigger heavier lenses roll down the hill and into the lake "faster" when you drop them, hence "fast lens"

[Ali]

Thank you all, very much appreciated
It all makes sense now. I wasn't sure if it was to do with larger aperture size (or "big 'ole" as I like to think of it ) or whether it was related to focusing speed/motor.

Originally Posted by danpen

Bigger heavier lenses roll down the hill and into the lake "faster" when you drop them, hence "fast lens"

LOL - thanks I'll bear that in mind