Originally posted by Unsinkable II Losing nearly 1.5 stops of light that you thought you were paying for is an issue.
Also, keeping with the Sigma 70mm "F2.8" as an example, it's not even F2.8 until 10 feet, meaning you don't get the full DOF advantage with headshot portraits.
You don't seem to either understand the real issue or want to accept the facts.
NO LENS actually performs the way you would like, i.e. a constant F stop (i.e. no light fall off) as you focus close in. All lenses perfrom the same way, and all lenses are tested for aperture at infinity regardless of manufacturer.
If you do enough research on the subject, you will probably fond some form of international standard on the issue, which details exactly the tests made.
As for light fall off in macro mode, that is a published issue, It is unfortunate that you seem to have been caught unaware of the problem, but we can't fix that on this forum. The same holds true of lenses and depth of field, they all work the same way so you are not being "cheated" by getting a "smaller" aperture than what you expected.
I appologize if I am sounding a little abrupt here but you have to accept the realities of physics.
As for the sigma, how are you determining the apertures it is providing? Is this in the EXIF data, or displayed on the camera display in any way? or are you calculating it from the change in shutter speed with a "preset" aperture on the lens?