I see everyone comparing lenses and talking about how 'fast' a lens is.. this is in regards to be able to focus quickly on an object, correct? As in, for sports photography. I find my nikon 18-55 mm dx to be pretty 'fast' when taking sports pictures.. and my 85 mm f/1.8 lens seems to have a little 'lag' when focusing. Isn't my 85mm f/1.8 supposed to be 'faster'? Newb question..