The 50mm focal length, also known as the "nifty fifty", is the lens I recommend above all others (see a more detailed lens recommendation at Phat Baby Photography). It's called a standard lens because it perceives things much as the human eye does (not too wide or too close, but just right). My camera also has this lens attached at least 50% of the time since it's light, sharp and versatile. The following two photographs show just how versatile the 50mm is: toddler head shots to full body (all 30 inches of Zoe), low light to daylight, and blurring distracting backgrounds to razor sharp, distortion free back drops.
For Canon users, there are at least three flavors, a 50mm f1.8, f1.4 and the ludicrously expensive f1.2 L lens. I don't recommend the first since it's all plastic construction is unforgiving, at least with my children, but at $80 it is a steal and easily replaceable if broken. The f1.4 lens has been my bread and butter lens. It's a great lens but the photos taken wide open (i.e. at f1.4) are soft and flat which is okay for some baby photos when you want a romantic feel, but I prefer to have sharp photos and add the softness later if it suits the subject. So I sucked it up and bought a used f1.2 lens and despite it being about $1000, all my regrets melted away after taking and seeing the first few photos. The one thing no review emphasizes enough is that if you like to manual focus, the other two lenses are next to worthless (there is just too much play for quick, consistent, candid children's photography). The Canon 50mm f1.2 is not for every parent (or even most) since the lens has a high learning curve, weighs more than Zoe's wet overnight diaper (enough that I still carry the f1.4 lens for goofing around with the kids), only stops down to f16 and costs as much as most parent's spend on their dSLR.
It is however, weatherproof which didn't mean much to me till last Sunday when A and Z got a hold of some spray bottles. In conjunction with the the previous owner's UV filter (piece of glass that screws onto the front of the lens), the feature probably saved my investment.
Is it me, or does it look like Zoe ran into the wall?