[mart_d50]

many thanks for your advice !

so to summarise:

1) 'fast' refers to the lenses ability to allow a higher shutter speed

2) f2.8 are useful in low light situations allowing a higher shutter speed and a brighter viewfinder but limited DOF; even in bright conditions f2.8 is still going to yield limited DOF so a shot taken in any conditions at f2.8 is still just a compromise ?

which brings me to the last point I'm not sure about...

3) assume you have two different lenses taking a pic of the same subject at the same focal length ie.
sigma 50-500 f4-6.3 set to 300mm f8
sigma 120-300 f2.8 set to 300mm f8

ignoring the individual characteristics and quality differences of each lens will the picture quality theoretically be roughly the same at the same settings ?

thanks
Martin