“ And I found that on the Sigma (sport) the focusing wheel was stiff and uncomfortable to use (too close to mount) and because of the weight of the lens (much heavier than Tamron due to the optical elements/group), didnt handle well when focusing manually. The zoom ring I also found very stiff. I found AF similar on both models. The Tamron was much lighter, had smoother rings and better balanced, although the Sigma should have better optical performance. Took photos with both at 600 and I couldnt tell the difference, bith looked very sharp, perhaps slightly more contrasty on Sigma............. Ho trovato questa opinione googleggiando! „
Dopo questa impressione di chi lo ha provato (potrebbe lasciare il tempo che trova comunque) è meglio non cimentarsi in affrettati betatester! Concordo Otto, la rapidità fulminea di messa a fuoco è prerogativa di lenti di un certo livello! 
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