Hi Roy.
I think the review is slightly confusing but does make a recovery.
The new Sport lens has two FLD coated optics and one SLD coated optic the latter not being mentioned in the review. The former OS lens also had two FLD and one SLD coated optic(s).This can still be checked and confirmed by reference to internet reviews.
Several weeks ago I compared Sigma's MTF charts and for the two lenses they presented congruently. I also telephoned Sigma's technical department and was informed unequivocally that optically the new lens remains unchanged. The MTF chart for the older lens is now unavailable unless of course there is a review out there that has copied and pasted it. I can't find one.
However, I do not think that I would be misrepresenting Sigma if I say a larger proportion of Sigma's customers were unhappy with various aspects of the former lens and were returning them. I think the leading lens rental supplier had the lens as the one they had to return for fixing more than any other lens in their catalogue. Clearly there were problems with focusing/electronics which would of course lead to sharpness issues when trying to achieve critical focus. These problems were not universal and there are many who have lenses which they are happy with and seem to function perfectly well. The problems referred to may be the reasons which causes the reviewer to think that the optics have been upgraded.
Sigma has now indicated that each of their lenses will be put through a rigorous test procedure in the hope that each lens sent out performs as expected. In other words they are addressing quality control issues which previously have been the subject of criticism.
It seems that most of the in depth reviews indicate that the 120-300 Sports is very sharp but not quite as sharp as the new Canon (I would not expect it to be). The cost difference in terms of the added sharpness of the Canon Lens is it seems not justified. Thus making the Sigma Sport a very attractive consideration.
Unfortunately the reviews do not seem to cover sufficiently well the needs of birders when dealing with aspects of IQ and focus performance across the zoom range when a 2X converter is used. I think most bird photographers whilst accepting that at 600mm FL they will not get the same results as a 600 prime in terms of IQ and lens performance: they still want to know what might be reasonably expected for near and medium distance birds when a 2X converter is used between say 400mm and 600mm. I'm sure that many 300mm Canon shooters have their 2X permanently attached and they seem to be generally very happy with performance and IQ. Sigma shooters will want to know can they do the same?
For me, if I had the Sigma zoom I would want to make use of the full zoom range, that's what I find so attractive about the lens, but since I usually spend 4 months of the year shooting peregrines the lens would need to be able to perform pretty well when using longer focal lengths for flight shots.
I eagerly await your review when you get one
