The ramping starts as soon as you leave the widest angle setting Daniel. What this means is that the lens is only f/1.6 at the very shortest focal length, and as soon as you zoom even a fraction away from this focal length you start to lose speed.

Most aperture readouts are to the nearest half stop, so it may well appear otherwise. Also note that the f/1.6 is a theoretical aperture, not a T stop (T for True Transmission). When a common 12x zoom has 13 elements and three beam splitters and when production tolerances are taken into account as well, you could well be setting off with an f/1.8 lens and yet never know it.

tom.