Sigma In The Theatre
a. Permission. You need it - you can't just lob out with the DSLR at the next performance of "Die Fledermaus" and stand up in the middle of Row B and start blazing away. They will grasp you by the collar and throw you into the street - a particularly awkward thing if the theatre is up a flight of stairs...
b. Position. You can't stand up in Row B because the people in rows C, D, and E will draw pistols on you. You'll be restricted to a position further aft or down the sides of the venue. If the performance is so poorly attended that you can indeed stand up in Row B with nothing but silence behind you, is it worth recording the stage? Decide that for yourself...
c. Silence. While this is primarily the province of the camera, we have still heard some other lenses that focus with the sound of a battleship turret traversing. No good. You need to be quiet and you need to have a lens that will be so too. This does not apply during performances of Arab dance music, rock bands, and Wagner. In these cases you are prohibited from firing off siege mortars but pretty much everything else is masked by the din from the stage.
d. Light. This is necessary to see what is happening on stage, but Perth stage designers and lighting technicians are frugal people - they only use as much as is necessary. If there are pretty girls or Disney characters on stage you might get some colour but if it is just Ibsen or Shaw these can be played pretty much in the dark and they pretty much are.*
Thus you need to gather all the light you can for photography - and you need a camera that will get the ISO numbers up into the nosebleed range as well. Fortunately these are becoming more common these days and Canon and Nikon buyers can get some pretty hot cameras right now. And hotter ones coming, too.
Still, it is nice to have a big aperture lens, and here is where the f:1.8 of this lens comes in. The 50mm to 100mm range is also just perfect for side and back-of-house shots. You may want to attach a tripod - there is a rotating tripod mount ring for this purpose and thankfully it has smooth click stops at the horizontal and vertical points.
Please also note that the new designs and finishes of Sigma lenses are really top-notch. There is no reason to regard them as anything but truly professional optics - backed with a two-year warranty.
* I'm not really complaining. It's restful there in the dark.