I have this under the Quantaray badge. Quantaray's version comes in 67mm and 72mm front element diameters. The 67mm is the Tamron version. (The 72mm is the Sigma.)
This is my most-used lens simply because I use it for my company softball games. Were it not for softball, I'd only use it about once a year for zoo trips. I find it to be most sharp from f8 to 10 throughout the range. My personal preference is to shoot this lens almost exclusively at f8. Any faster once you shoot past 100mm and the CA is unbearable -- you'll only want to take shots for monochrome conversion. Past f11, the diffraction seems to enter in and take over rapidly. So knowing that this lens has a limited range can help. The results within its limited range can be quite spectacular. But for as high as this lens' ceiling is, the floor is even lower -- when an image is bad, it's VERY bad.
This lens is easy to use in manual focus, however, with an easy turn, short draw, and nice focus pop. However, it can very easy slide out of focus as the focusing helicoil is too easy to turn for it to be a practical manual focus lens.
But understand this lens' biggest limitation -- you NEED to fill the frame to have an image that stands up to scrutiny. This is an inherently soft lens when images are cropped. So leave about 5% on the edges to crop out to eliminate some CA, but keep the rest of your subject filling the frame entirely. Do that, and you will set the lens up for success and have pleasing results. Most of the images below are 100% or cropped no more than about 8-10%.
Originally posted by snappychappy 
Post #2 by snappychappy
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