Most kit lenses open from f/3.5 to f/5.6, and in low light that's just not wide enough to freeze the action without bumping up the ISO too much. This is where the nifty-fifty, the plastic fantastic, the ever so popular Canon's 50mm f/1.8 lens comes in - surely one of the first lens every photographer purchases after the kit lens. Let me explain why that is:
1. It's a close replication of the human eye, so you're capturing the scene close to how you see it, so the result is pleasing and feels natural. A 35mm is probably the closest to the human eye though.
2. This lens has a very shallow depth of field. Depth of field is the zone of acceptable sharpness within a photo that will appear in focus. This is created by the aperture (1.8 in this case), the distance from the subject to the camera, and focal length of the lens on your camera - in this case, 50mm.
A shallow depth of field is what makes the background blurry. At f/1.8, it singles out the subject and cuts out distracting elements behind them. Perfect for portrait and product photography.
3. Bokeh, that smooth blurry background. With 7 aperture blades, it creates nice and round circular bokeh. The more aperture blades it has, the more circular the bokeh becomes.
4. Shooting in low light. As it opens to f/1.8, compared to f/3.5-5.6 of kit lens, you can get faster shutter speeds to freeze the action, and keep your ISO low. It's great for concerts, indoor events, or any poorly lit situations.
5. Tiny and light, perfect for traveling. Keep that weight in your bag down.
6. EF mount. This means that it will work on a full frame camera, or crop sensor (APS-C). That being said, keep in mind that while it's 50mm on a full frame, if used on a crop sensor camera, the focal length will be 80mm on Canon cameras, as you'll have to apply the 1.6 crop magnification - check the image below for a visual representation:
7. Affordable. When you start looking at lenses other than your kit lenses you'll realize that photography can be very expensive. For the price/quality ratio, there's nothing better than the 50mm.
8. Forces you to think more about your composition. We can get lazy with zoom lenses. With prime lenses, the zoom are your feet.
9. If you're only shooting stills, the version II of the lenses is fine, if you're going for video, consider the newer version (STM) for much, much quieter focus. The newer version of the lens also has a metal mount, as opposed to plastic.
Want to spend more or less? If you want to spend even less, then you have the Yongnuo EF YN 50mm f/1.8. I haven't tested it, so I can't tell you the difference in image or build quality.
If you want better quality at the same focal length, then get the wallet ready - there are plenty of options out there: