Tamron 28-75 f2.8 Continuous
Purple Fringing is well controlled, even wide open. At f2.8 there's a hint of a red fringe in OOF areas, but it's not distracting like purple fringe is. The lens is sadly pretty soft at f2.8. You can fix it to a point in post, but it's soft enough that significant detail is lost. At f2.8, the lens is significantly better at the tele end than it is at the wide. There's a little more of the red fringe at 28mm, it's softer, and strangely bright objects have a glow about them. This is gone by F4.
At F4 the lens performs very well, it's an amazing difference between 2.8 and 4, things are now reasonably sharp, and significant detail that was a smudgy blur at f2.8 can now be seen. There's no red fringing at either end. In fact, both wide and tele look excellent.
At f5.6 this lens is pretty damn amazing. Very sharp with lots of detail. Detail that was hinted at at f2.8 and shown at f4 is now extremely clear. But it's completely usable at f4. In fact, there's a stop between f4 and f5.6, but I didn't test there.
The only real downside is the lens has a very short focus throw, which makes it difficult to nail the focus perfectly, especially when hand holding.
Overall, a great lens.
When I compare the telephoto of the Tamron to my 100mm f3.5 macro, the Tamron is significantly sharper at f4 than the 100mm macro is at f3.5. The 100mm has significant purple fringing at f3.5 where the Tamron has none. However, one full stop up to f5.6 and the 100mm macro wins by a hair.
28mm f2.8:
28mm f4:
28mm f5.6
75mm f2.8
75mm f4
75mm f5.6
Charles.