Luminous-landscape has a very interesting article on how camera manufacturers are silently increasing the ISO in order to compensate for light loss from the lens. The whole analysis is based on data from DxOMark:
The above graph show the light loss at the sensor level for different Canon and Nikon cameras when using a f/1.4 lens wide open. For example, the D3s has -0.30 EV light loss, the D300s has -0.52, the D40 has -0.80, etc. In order to compensate for this light loss and produce correctly exposed images, camera manufacturers are compensating by “secretly” increasing the ISO. The author Mark Dubovoy goes even further by saying that you may get better results with smaller aperture lenses, since they have less light loss and that this phenomenon is also destroying the bokeh:















