I came a cross this open letter from Luminous landscape to manufactures, it is dating back to 2010, however its still quite interesting and alarming read. Boiled down, is the statement, depending on the individual camera manufacture, that you could lose between 0.75 EV to 1 EV (a full stop) of light by the time the light hits the sensor. All taking base in DxO Lab’s findings.
The cheating comes in because the manufactures changes your ISO settings behind your back to accommodate against this loss of light. This can also explain why the ASP-C sensor based cameras seems to be less good at handling low light than full frame cameras. the actual amount of light from an Samyang 14mm f/2.8, receives is around f/4. Of cause you will still get the exposure value of f/2.8, because the manufacturer raised the ISO to compensate. This means when you set the ISO to 800 when at f/2.8 on your asp-c , it’s actually f/2.8 at ISO 1600. Seen from my chair, this is the equivalent to the Volkswagen scandal. When shooting in daylight this possibly have very little significance, given today’s modern and high performing sensors, but shooting at night and other extreme situations makes this fairly important.
I can’t understand why this is kept under the hood, and why manufactures isn’t being public about this behaviour of the cameras they make. In my opinion, when the camera raises ISO to compensate for the loss of light, they should show that in the cameras display
The Golden Hour 