Do cameras secretly raise ISO with very fast lenses like f/1.4 or f/1.2?

I am also very skeptical about this article. If that was true, then opening the aperture past a certain point should not make any difference in the defocusing ability of the lens.

I tried a small experiment: these are pictures of a couple of street lights close to my home. I set everything to manual and used the exact same settings for all the pictures: same ISO, shutter speed and defocusing. Only the aperture was different from shot to shot.

As you can see, the blur discs increase in size all the way to 1.4. Additionally, the surface brightness is about constant, which would not be the case if the ISO was changing.

Update 1: To address che's point, I tried the same experiment, but this time with the blur circles near the corner of the picture, instead of at the center. This is intended to maximize the light ray's angle of incidence. Here is a composite at f/1.4:

The angle of incidence is maximized in the far corner, because those light rays come from the top-right edge of the aperture and fall on the top-left corner of the sensor.

There seems to be a slightly lower brightness in the corner compared to the center, but it is hard to say whether this comes from the sensor or the lens (or the classical cos^4 illuminance law). Dubovoy's article sounded like the sensor would be completely blind past some angle. I cannot assert from my experiments that there is no angle-dependent sensitivity in the sensor, but if there is, then it is far from being as strong as the article suggests. At least the claim that “the marginal light rays just don’t hit the sensor” seems to be a gross overstatement.

Update 2: I had some correspondence with the author of the article, Mark Dubovoy (not Michael Reichmann, my mistake). After trying to dismiss my evidence with bad arguments (and after my lecturing him on geometrical optics, which got him upset), he now barely acknowledges that “It may very well be that with your camera and with your lens the issue is negligible.” But he still stands by his position, believing this issue may still affect “a significant number of camera/lens combinations.”

For those of you who would like to know whether their camera and lens is among this “significant number”, here is the the way to do a quick test:

• Look for a strong light source that is small and distant. A street light can do.
• Defocus the lens all the way to the minimum focusing distance. The important point is that the blur disc must be a lot larger than the focused image of the source.
• Take a series of pictures at different apertures, keeping the exact same settings of focus (most important!), shutter speed and ISO.

If the blur discs increase in size with increasing aperture, then you are fine. You should then notice that the discs have the shape of the aperture (you can count the number of blades). If the size of the blur discs stops increasing past a given aperture, then Mr. Dubovoy is right, at least for your camera and your lens.