Sensor compensation.
The 70/30 ratio of light loss is 1/2 stop, almost exactly, not 1/3 stop - always has been.
The difference between 1555 and 2980 is almost one full stop.
DXOmark's rating favors sensors that are tweaked to expose to the right, saturate early and consequently have a higher DXO ISO rating which ultimately affects low light score, and every other score. A consequence to it is that, with quick saturation sensors, chance of blowing highlights goes up. This is also a reason why A900 appears to be as good as A99 but in fact it isn't.
You mean both cameras produced identical exposures with these settings? If so, that discrepancy doesn't mean much (ISO 'accuracy' can vary even within the same brand) and it has nothing to do with the SLT mirror. The SLT cameras already compensate internally for the 1/2 stop light loss by boosting the sensor gain behind the scenes.
That is primarily because of the way DXO measures ISO and then uses it for other measurements. And this applies differently to RX1 than to A99.
Dxomark's noise measurements are more or less ISO independent. The SNR graphs show how much noise you will get (vertical axis) a given light intensity on the sensor (horizontal axis).
But, DXO quotes its other ratings based on sensor ISO. So, if a sensor that can absorb a lot of white light before reaching saturation, will have lower sensitivity to white light, which gets it lower sensor ISO which affects its low-light noise rating because, not exposure but sensor ISO is used to quote the measurements.
It's quite an easy exercise to test for yourself. Just go over to the dp test pages and setting the A99 to 3200 and D600 at 6400. And you will see that the noise is very comparable.
Closer to half a stop, since the D600, 5DMKIII and D800 are clearly noisier at ISO 6400 than the A99 at ISO 3200. Or ISO 12800 vs 6400.
Nope. In fact, try ISO 6400 for all four (A99, D600, 5DIII and D800) and looking at Kodak Gray Scale, shifted to the right (dark chips). D600 is about half stop better. However, 5DIII and D800 have more chroma noise.
Yes. But think of those ratings as based on light intensity and not ISO, and suddenly everything makes sense.
Exactly. And that is why DXO measurements make so much sense. That person only has a given amount of light at his disposal if he wants to avoid motion blur and opening up the lens too much, and he wants to make the most out of that amount of light.
Those ratings are based on saturation points reached with white light. The sooner the pixels saturate, the higher the sensitivity, the higher the ISO (relative to the exposure ISO) and you end up with a higher rating for low light noise.
It is what exposure ISO is for.
Try this thought experiment:
Please elaborate on that. I have no idea what you are trying to say.
