Well.. me, a tripod, Canon 28/2.8 and 28/3.5 lenses, a camera with TTL metering and an evenly lit wall coincided to do some testing, and despite the hexagonal opening at full aperture, my 28/2.8 lens does let through almost a stop more light than the 28/3.5 lens does so I'm inclined to conclude, that the hexagonality is by design on my sample of the lens.
I also took some test shots but they were taken in such a crappy fashion that I won't make any conclusions based on those, instead I'll shoot some more.. 🙂
Oh and my serial number is 154xx.
