I've worked up a direct comparison using various image crops from the four telephoto lenses listed
below. The crops used here are the same as the crops used in each corresponding lens review. This comparison is
just for fun as the real differences between each lens wouldn't be noticeable (in most cases, but not all) unless displayed
side-by-side as I've done here. The distances from camera to subject are; at 70mm, 14' (4.3m), at 200mm, 27' (8.2m),
which might be the same distances for bird and small animal shots. The Sony A900 was used for this comparison, along
with a tripod, remote shutter and 3200k hot lights. Bracketed manual focusing was used, with the resulting absolute
sharpest shot used as the base for the entire set, at each length.
Sony and Sigma OS.
Best control of flare and ghosting; tough call, but maybe the Sony and Sigma HSM II by a hair.
Best focusing in speed, all are about the same except the Tamron, which is last.
Best value for image
quality alone; Tamron.
Best value all around; Tamron, or Sigma HSM II, but Sigma quality is bad, so this may not
be a bargain at all.
If money is not a factor, the Sony.
All the crops here look pretty much the same. If you nit-pick, the Sony looks good wide open, and
the other lenses look good stopped down a little.
The Sony corners are slightly soft from F/2.8-4, but catch up to the other lenses at smaller apertures.
The Sony and Tamron look very sharp in the far corners at all apertures, while the two Sigmas struggle
for resolution at wider apertures, especially the Sigma OS, which looks awful compared to the rest.