Sigma 105 mm F2.8 EX DG Macro looking at this lens as opposed to the 18mm -105mm F3.5 - 5.6 VR that comes with the kit. Now I have limited experience with lenses but plan on doing of course portrait work with my kids etc since I am a MWAC lol. Is this lens something better to start out with as opposed to one with some zoom capabality. Please excuse any ignorance on my part. I keep tossing the idea of the kit lens with either a decent quality zoom plus prime 50mm and stumbled across this.