The argument that the ring marking somehow means it is not a true f/2 is
specious. That is the index mark to match the index marking on the lens
barrel. There is no requirement that the ring turn just as far (the same
number of degrees) to reach f/2 from f/2.8 as it did from f/4 to f/2.8 on
any lens. How many degrees there are between stops on the aperture ring is
determined by the mechanical linkage inside the lens and what exact
diaphragm diameter will result in an accurate _effective_ f-stop number.
Nearly as specious is the argument that any particular camera body's TTL
metering can somehow determine the accuracy of any f-stop number relative
to any other f-stop number on the aperture ring. If you look at the lens,
you will note that turning the aperture ring also moves a biasing lever
that links to the camera body. This biasing lever adjusts the metering.
There may be inherent inaccuracies in how accurately a specific camera
body's TTL metering and this biasing linkage is working. If it's on the
lens end, then there is a flaw with the lens biasing lever, but not
necessarily an inaccuracy in the actual _effective_ aperture.
Note also that an attempt to determine the aperture f-number by measuring
the diaphragm diameter from inside (by disassembling the lens) or
especially outside the lens is also inaccurate. That is why I emphasised
the _effective_ f-stop number. Theoretically the f-stop number is the
focal length divided by the diaphragm diameter. However, in practical lens
design this must be adjusted some for light absorption inside the lens, and
other design considerations that work to reduce or eliminate effects such
as cos^4 falloff.
The only method for accurately measuring the actual _effective_ aperture
f-stop numbers is to bench test the lens using a known uniform light source
and stop down the lens at various aperture settings using the stop-down
button (a.k.a. DOF preview button). My burning question is did he do this
in an optical laboratory using proper equipment and accepted methods with a
proper sample of each lens type, or is this pure conjecture based on
unscientific anecdotal observation with a single lens of each type on a
single camera body?
-- John
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