This graph clearly shows that camera manufacturers “game the system” by increasing the ISO without the photographer's knowledge.
Personally, I think this brings up several issues.
1. When I select a specific ISO, I have a reason to do so. I find it unacceptable for the camera to change it without my knowledge.
2. If the camera is automatically going to increase the ISO due to a significant light loss at the sensor, does it make sense to buy bigger, heavier and much more expensive large aperture lenses?
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.
DxO is currently performing thorough focus measurements to support this argument. We will keep you updated as soon as the data becomes available.
Bottom line: Due to the complexity of design and manufacture (let alone the high cost and weight) of large aperture lenses, one may actually end up with better results at virtually the same ISO and depth of field using lenses with more modest maximum apertures.
3. While increasing the ISO without the photographer's knowledge is a pretty neat engineering trick, there is the issue of full and accurate disclosure to customers.