I have no intention to knock Sigma, Tokina, or any other brand but in my experience, the facts are as follows:
My frst two lenses from 1996 were Tokina 28-70 and Sigma 70-300;
Purchased a 100-500 manual focus Vivitar lens (what did I know about photography??);
Forth lens in 1998 was the Sigma 170-500 (to replace the manual Vivitar);
all lenses for Nikon mount for use on N50 Film camera
While the pic's were OK, few were great. Second Hand body (F100) added to collection, which came with a 24-120 Nikon lens and the improvements were startling (which I put down to the improvement in the new body).
Moved to a D70 digital body (6.1mp) and the 18-70 kit lens and retained all the other lenses. Backwards move for me as the D70, while an iconic camera, could not match the F100 film camera in any respect (shooting Velvia slide and ASA 100/200/400 film, depending). Thought I had a dud D70, sent in for calibration, returned with no difference in quality. Finally put the output down to my personal photographic skills (or rather lack thereof) and not the equipment.
Purchased a Nikon 50mm f1.4 and got stunnning results with all my (both digital and film) bodies and put that down to the fact that it was an f1.4, high quality Nikon lens. However, this did not point out to me that the other lenses I had were of lower quality, but rather that they were just slower lenses.
Earlier this year, I finally took the plunge and upgraded my body to the new D7000, when the whole federal reserve bank dropped (ie. Not just the penny
). The cause of all of my poor quality pictures had been the lower quality lenses all along. The D7000 is just so damn good that all the flaws in all my old equipment bubbled to the surface and became exposed once and for all.
So again I took the plunge and replaced my (trusty?) 70-300 Sigma, now 15 years old, with the new 70-300 Nikon VR lens and again, the improvement was dramatic and immediate. Cropped pictures with the new Nikon glass are superior to the uncropped pictures taken with the Sigma 170-500.
Equipment-wise, I have what I need. Sure, there may be lots that I want, but I have learned the hard way, to save for what I want rather than compromise on quality and rush into buying an interim product and then having to forego on possible quality of the output. Personally, given my history, I can do without....
(PS.. I still question what I know about photography)
(PPS.. My D70 is now taking the picture quality it was originally designed to do from the start....IT'S ALL ABOUT THE GLASS)