I noticed this link a few minutes ago. It’s an interesting article and I suggest a visit to check it out.
Using facts from the DxO group, the author suggests that large aperture lenses are not being fully utilized by current digital camera sensors due to a failing in their design. Light rays coming from the outer edges are not parallel enough, so they never reach the light sensing devices in the sensor. He also notes that when these large aperture lenses are being used, the cameras are increasing the ISO to compensate behind the scenes without user knowledge.
From a Pentax perspective, I wonder if this is a reason Pentax has been releasing lenses “slower than film days” to increase the overall parallelism of the light rays. For example the new DA-L 35mm f2.4 is very similar to the old FA 35mm f2 lens, so maybe the loss of 0.4 f-stop was to improve the overall ability of the lens to get as much of the light into the sensor as possible.
I also wonder how an older Pentax DSLR that used CCD sensors instead of CMOS sensors fair with larger aperture lenses. I have a K10D (CCD) and K-7 (CMOS) and a FA50mm f1.4 to test with, but the question is how would I figure out if the K-7 is changing the ISO?