Originally posted by Emacs f2.4? Is it fast?
Yes.
It's well-known classical classification of lens speed:
Superfast till f/1.2 (included) (for normal lenses)
Fast - from f/1.4 till f/2.8 (for normal lenses)
Above f2.8 is not fast (for normal lenses).
For different focal range could be used different classification of lens speed.
It's one of classification of lens focal range:
Ultra-Wide - from 8 till 24 mm
Wide - from 25 till 35 mm
Normal - from 36 till 70 mm
Short telephoto, normal telephoto extreme telephoto - from 70 mm and above...
For 35 mm lenses - f1.4 is SUPERFAST lens
f/2-f/2.8 are fast
et.c.
For 200 mm lens - f/1.8 is SUPERFAST, f2.8 is FAST
For 300 mm lens - f2.8 is FAST
For 400 mm lens - f/2.8 and f4 are FAST.
For 600 mm lens - f/4 is FAST
For ZOOM lenses - f2.8 is FAST