Working at ISO100 on my Pentax K20D, I focused as closely as possible. On this lens that is very close indeed. At this extreme the barrel is marked .23m distance and 1:5 magnification. I do believe this is the shortest of all focal distances in the 28mm Vivitars, though several other variants manage 30cm. The difference may not matter too much, but if you're into pseudo-macros then every little bit helps.
I found a patch of daisy plants as a torture test. The extreme yellow and white are difficult to capture. Plus the grass in the background will play havoc with the bokeh. I hand-held a bunch of similar shots; a tripod would have guaranteed greater consistency. Setting the lens wide open I used the green button to get a shutter speed. I believe I increased this half a stop. Then, as I stopped down for each shot I manually decreased the shutter speed in the same proportion; I did not re-meter and the light remained consistent. Yes, I know it's hard to believe: a sunny day in Ireland!
Here follow thumbnails of the shots. Click through for larger Flickr images that are reduced about 50% from a 50% center image crop (but still large). These have been processed using my standard steps, with ACR and Photoshop used to apply curves and a slight sharpening. I don't think there's any point looking at shots directly out of the camera, as these are designed to be tweaked and (in the case of Pentax at least) are undercooked. A good thing!
On the other hand, I would normally do more to prepare photos than I have here. I have compromised my PP so the test is meaningful.
The bokeh is busy but not offensive. Absolute sharpness is lacking, but there was a certain breeze, so this is to be expected. No highlights are blown though some detail has been lost. Given the direct sun this is inevitable.Stopping down from f/2.8 to f/4 increases sharpness visibly, but nonetheless f/2.8 is totally usable. This is not the same behaviour as other Vivitar variants, where I have found the widest aperture to be usable only in a pinch. There might indeed be something special about this lens. I will do some comparison shots in a more controlled environment eventually, but for now I am quite satisfied. More than that; I am impressed.
More shots with this lens coming soon.