Not sure if this has been discussed already, apologies if so, but have people here on PF discussed this open letter to the industry from Luminous Landscape?
An Open Letter To The Major Camera Manufacturers
In it, with the empirical help of DxOMark labs, they discover that CMOS sensors are physically limited in accepting the light given by large aperture lenses, and consequentially, digital camera manufacturers stealthily increase the sensor ISO over the set ISO to compensate. worse than any noise gain of this sensitivity boosting is that you don't gain any DoF blur by using these extreme aperture lenses, the light rays are just too off-angle to reach modern CMOS sensor wells.
They show a graph where the 'secret' ISO compensation kicks in below F2.8, slowly gaining the sensor sensitivity up as the aperture size increases. the true sensor ISO becomes 1:1 by F2.8 in these tests, but they were testing Canon and Nikon cameras which have no F2.4 lenses.
as many noted, the optical formula of the new DA 35mm superficially looked identical to the old F2 FA 35mm. many wondered why it "lost" a half stop of light. I'm betting that in making this lens a truly "Digitally Optimized" lens, Pentax tuned the aperture and back focus to get the most direct CMOS compatible light without any background sensor ISO increase. In other words, at F2.4 the given ISO is the true ISO of the sensor. anything lower in number than that is effectively lost and compensated by ISO gain.