[DnA Diesel]

While the effect of good-looking defocused backgrounds is primarily a function of large aperture setting and longer focal length, there are additional factors that come into play as well, including diaphragm configuration (e.g. a 9-blade diaphragm will generally provide smoother defocused background than fewer blades) and lens quality/design (more lens elements in the groupings generally provides better background), both coming into play when there are light sources or high contrasting shapes/colours in the background.

Since you have the D3100, may I suggest the "NIKKOR AF-S 35mm F1.8G", a very nice prime lens for a 1.5x format camera (your D3100 is a "DX-format" camera, it's sensor is 2/3 the size of a full-frame 35mm camera, so any lens you use on it will appear 1.5 (or 3/2, the inverse of the 2/3 sensor ratio) times 'larger' (longer focal length) than if you used that very same lens on a full-frame FX (35mm eqvt) camera (like a Nikon D700 or D3s).

50mm for a full frame camera is roughly equivalent to the unaided human eye -- and a 35mm on a DX body is equivalent to 52.5mm on an FX body. Every photographer should have such a prime lens like this. Yes, you have an 18-55mm lens, so you're asking yourself, "Hey, why have a single focal length lens?"

Good photographs generally need light...the more light the lens provides, the better dynamic range the sensor is going to have and the higher quality a picture will be. The 35mm DX F1.8G is what we call a 'fast' lens...it has a relatively large aperture...in fact, by definition, it's aperture is 19.4mm (35mm÷1.8) and that's fairly big. Compare that to your kit lens which has a relatively small diaphragm...between 5.1mm at 18mm focal length and 9.8mm at 55mm focal length. That fixed focal length 35mm F1.8 lens has a diaphragm between two and four times larger than that mid-range zoom you have, and will bring in a lot more light for given conditions. There are only so many situations where you want to have to use a flash to compensate for less light passing through a lens...it doesn't work well and makes the photograph unnatural (and only works when the subject is relatively close to you.)

Later on, you can worry about a zoom-telephoto lens, but for now, I would stick with the addition on that 35mm DX F1.8G lens. Your wife and you will get more out of learning to work with a moderate mid-zoom and the nice, 'fast' (read grabs lots of light) prime lens.

Enjoy!

Regards
D.