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:
"One might be better off purchasing smaller aperture lenses and increasing the ISO. Since these lenses have much less light loss at the sensor, one may well end up with virtually indistinguishable results. In fact, is not even clear that large aperture lenses will deliver a shallower depth of field as intended. The DxO measurements to date prove that the marginal light rays just don’t hit the sensor. The point regarding depth of field is that these rays are also responsible for a larger blur spot when out of focus. If they are lost, they not only don’t contribute to the light intensity at the sensor, but they also don’t blur the out of focus planes as much as you would expect at wide apertures."
Update - related links from the Web:
- Do DSLRs play games with ISO when used with fast lenses?
- "Help! Same exposure at f2 and f2.8" discussion on flickr.