You bought a camera and it was bundled with some cool stuff like a memory card, a bag and a 'standard lens' (a kit lens). What should you look for in expanding your gear? What are you going to be shooting? There are plenty of choices between the branded and third-party offerings out there but with a basic idea of what to look for you should be better equipped to make that decision.
CONSTANT APERTURE: Kit lenses are designed as a kind of Swiss army knife - the apertures will generally range from around f/3.5 to f/5.6. This is fine for general use but not so great when you want to achieve a particular outcome (shallow depth of field or a faster shutter speed in poor light). For this reason, a lens with a fixed (or constant) aperture - one that offers the same maximum aperture at both end of the focal range - should be on your shortlist. To find out whether it offers a fixed aperture, the clue will be in the name. Zoom lenses with constant maximum aperture will simply have one aperture after the focal lengths, such as, "17-55mm f/2.8", instead of, "18-55mm f/3.5-5.6".
IMAGE STABILISATION: If your camera doesn’t offer an image stabilisation feature then don't stress, check that any lens you're considering does have the feature - you'll notice a price difference between lenses that do and do not have the feature. Any manufacturers website is a good resource to bookmark in an effort to understand the system and discover to what extent it can be relied upon.
WEATHER SEALING: If your camera was designed with some form of weather resistance (mine has a weather sealed body), it’s a good idea to pair it with a lens that has a similar degree of protection. Make sure to check the manufacturer's notes to avoid using it in conditions that cause adverse effects.
AF MOTOR: There are different names for this feature such as Ultrasonic Motor, Silent Wave Motor (SWM), Supersonic Wave Drive and Super Sonic Wave Motor. A quick glance at the specifications should make it obvious to see if this is a feature of the lens. If you capture a lot of video and plan on using auto-focus (I can hear you cringing), you would be wise to consider a lens designed with this in mind or you'll be capturing rather obtrusive operational sounds while recording (you need only need to do this once to understand).
VERSION: Occasionally popular lenses get replaced as technology evolves. It’s worth checking to see which version of an optic you’re committing to. There will be a demarcation of ‘II’ or ‘III’ showing it to be a second, third, or further generation respectively. Typically, these are more advanced incarnations than the preceding models that they replace but their price will also reflect this.
OPTICAL CONSTRUCTION: Here's the jargon - a lens with many extra-low dispersion elements and special coatings is designed to control chromatic aberration, distortion and spherical aberration better than a standard (kit) lens. Check the manufacturer's resource to see what they're offering in this regard - reducing aberrations will save you time spent in post-production trying to correct it.
MANUAL FOCUS OVERRIDE: Sometimes you want to be able to tweak your lens auto-focus system manually and many lenses now offer this kind of manual-focus override. If you find yourself wanting to do this often, check the features to make sure if it's possible.
DIAPHRAGM BLADES: You like bokeh? Everyone likes bokeh. Bokeh. Bokeh. Bokeh is the aesthetic quality of the blur produced in the out-of-focus parts of an image. The general rule: more diaphragm blades within a lens make it easier for the lens to render the out-of-focus points of light. So dreamy.
Anything you want to ask?