First, it is probably a 2.8/4 lens or at least a 3.2/4 as the aperture automatically closes down from 85mm to 28mm (by a full stop to my eye). That was their way to keep quality up at 28mm. And easy f/16 metering (Well, they have not thought about this one...). By the way, Pentax-A 35-70 f4 works the same way also, but here you see a device built-in the aperture blades to do the job. On the Tokina, the compensation mechanism is not seen.
Secondly, the front optic gets into the lens barrel as it goes toward 80mm, which provides better shading. The only lens I have ever seen to use this device was the Angenieux 35-70 2.5/3.3. I wonder when Tokina began to buy Angenieux's savoir faire...
Andre --
