"Here's the two lenses aperture openings/focal length data to compare:
OLD --- APERTURE --- NEW
14-17mm - f=2.8 - (14-18mm)
18-21mm - f=2.9 - (19-25mm)
22-25mm - f=3.0 - (26-34mm)
26-28mm - f=3.1 - (35-37mm)
29-35mm - f=3.2 - (38-43mm)
36-44mm - f=3.3 - (44-48mm)
45-50mm - f=3.4 - (49-51mm)
50-54mm - f=3.5 - (52-54mm)
It's plain to see the newer version holds its brightness better especially in the mid-ranges. This must be one of the ways the engineers improve the contrast allowing the sensor to perform contrast detect AF."