Next silly question

This is the "problem" on this site - someone asks a simple question, and instead of giving a simple answer there are always people who want to elaborate way beyond what's necessary for the OP. So

Q. What is a fast lens?
A. A lens with a large aperture (low f-number).

Normally, a lens in the range f1/1.4 - 1/2.8 is regarded as a fast lens.

Specifically there were two models, the SMC Pentax 50mm f/1.2, known as the "K" series and the SMC Pentax A 50mm f/1.2. There is also a Cosina 55/1.2 for K mount.

Yes, though cine lenses are usually specified in t-stops rather than f-stops.

Yes, one should write 1:2.0 or 1/2.0 or simply f/2.0. The slash shows that the f-stop number is actually the denominator of a fraction. Thus, the smaller the number the larger the aperture. Which is why the 50mm f/1.2 is the largest aperture available from Pentax.

The trouble with simple answers is that they can oversimplify. While any lens of f1/1.4 is certainly fast, a 50mm f1/2.8 would be regarded by many as quite slow, and conversely a 400mm f1/4.0 would be regarded as pretty fast... Whether a lens is 'slow' or 'fast' is itself a relative question depending on wht it is compared with. So a fast lens is one with a larger maximum aperture than whatever you are considering to be normal... So the various 50 mm f1/2.0 lenses are relatively slow as 50mm primes, but pretty fast compared to an average kit zoom...

It's just that I think it is not necessary to answer a simple question by explaining "everything" one knows about the theme. If the person seeking information is not satisfied with the simple (short) answer, he will soon be asking more questions. Then you will know he is ready to receive further information.

Certainly, for my purposes, the early answer linking fast glass to the quoted aperture for the lenses was what was needed - but then that's just me .

I'm trying to fill in a lot of early gaps , and I prefer to take things in slow stages so that I can become more confident .

Terminology is something that you need to learn and unfortunately advanced folk can [ please note 'can' -- not all are guilty of this ] forget that learners need to take things in stages .

Indigestion can be a very uncomfortable problem

Anything f2 or better (f1.8, f1.4, f0.95) is normally considered 'fast' in absolute terms. However for zooms, f2.8 is sometimes described as 'fast' in relative terms (i.e. a 70-200mm f2.8 could be described as 'fast' because the alternative is a 50-200mm f4-5.6).

It's also relative to focal length. An f2.8 prime isn't very fast if it's 35mm (e.g. the Pentax DA35) but at 400mm, f2.8 is extremely fast and extremely expensive (Nikon charge £6518 for their 400mm f2.8 prime).

£2518 for the Lens, an extra £3000 for the name and £1000 to the fund for keeping the reviewers happy!

Actually it was a still camera lens, so it is f-stops.

There's an interesting page devoted to the story of the lens and Kubrick here.

You must also consider the smaller the "F" number the shallower the depth of field is!.......