I am actually half decided to recommend or not recommend this product. Most people say this is cheap but I'd say this is priced not affordable(Since I am a student and don't really have income-imagine how hard I saved for this bloody lens)...
The quality of the glass against CA is good; lens hood to prevent lens flare reasonably; AF speed is not superbly fast but neither is it exceedingly slow; lens build is solid-Sigma's famous matte black finish; mass of lens although heavy but still acceptable; the OS is reasonable well to say it at least it beats the "SteadyShot Inside" of my Sony a55v body...
The deciding factor is most probably the IQ(Sharpness).... I'd like to say at 120-200mm the lens is pretty sharp wide open(who the heck uses this to shoot birds far away?) but anything beyond that prepare to stop down... sharpness is still acceptable at 300mm wide open(note that its already f5.6)
I have seen tons of reviews saying sharpness wide open at 400mm is acceptable but I beg to differ... Its worse than my (Tamron DI 70-300 f4-5.6 macro) at 300mm wide open. Please mind the price difference. The Tamron although older, is 5times cheaper than the Sigma(Although it lacks the 400mm range and has noisy AF). The sharpness of the Sigma can't even be recovered even using lightroom 4(You just create ultra pixelated pictures) so if you are looking for good IQ you have to stop down but I don't like the idea of stopping down. Who in the right mind buys a lens already at f5.6(very slow) wants to stop down more(f8)? Moreover if you are going to shoot at forested area(like me) I don't think this is going to work out.
Yes... yes... most of you would tell me to use a tripod but I don't want to lose mobility for stability...Moreover the tripod only steadies the camera and lens but not the subject so.... shooting at 1/50 isn't a very good idea is it?
As you all are already told, I use Sony (a55v) and that means my High ISO capability isn't good so basically my ISO threshold is ISO1600-3200.
In conclusion, I must say this lens will work better for High ISO performer cameras like those that can perform at above ISO3200 but not for mid-range or entry-level cameras. If you are starting Bird Photography go get the kit lens... It is more than sufficient for your birding needs till you mature into a full fledged hobbyist bird photographer where you should most likely get manufacturer's own lens in my case the Sony 70-400; Canon users, Canon's very own 100-400 push-pull and for Nikon users, 200-400... Thats why I am half-decided on whether to recommend or not to recommend since I don't really want to make people think to highly of this lens and in the end be disappointed that it does not perform up to their standard...
$1500 is a great deal of money even to those with middle income...
So do consider and weigh your options...
To save more or to buy cheap equipment? Thats your choice....
Do visit my profile on facebook and comment on my photos..
Sincerely,
Darren(facebook name: Dlkillwaterbear)
A hobbyist Bird photographer stuck in Singapore.