The kit lens is a lens that comes with your camera, and is usually of lesser optical quality and durability. A Canon L lens is a branding marker for the highest-end lenses that canon makes. The major differences between a kit lens and an “L” lens lies in the build quality and optical performance.
An easy way to tell what kind of lens you are using is to look for the red stripe. All L-series lenses feature a red stripe that wraps around the top portion of the lens, effectively categorizing these lenses as “luxury.” You may also notice that many of the more expensive lenses for Canon are a white/cream color. But not all L series lenses have this coloration.
Most L-series lenses feature rubber seals which keep out fine particles and dust. Many are weather sealed to keep out moisture and water if getting a bit wet is unavoidable while shooting in damp or rainy environments. Kit lenses are not sealed and can collect fine dust particles within the lens. They also cannot withstand heavy moisture or rainy conditions, unless properly protected by cumbersome, and often faulty, aftermarket rain proofing materials.
L series lenses are known for their outstanding performance because they decrease distortion and chromatic aberration that is apparent in many kit lenses. They also really stand out when it comes to rendering accurate colors. Kit lenses are often made with an inferior glass quality which results chromatic aberrations (you’ve seen it, that unsightly purple border around any area of your image where there is a dramatic change in contrast)! If you don’t want to spend time removing that distracting purple fringe in post processing, an L-series lens, with its superior glass, can dramatically cut down in the occurrence of chromatic aberration.
Unlike L-series lenses, many kit lenses often have a rotating front element that moves as the camera focuses the lens. This can make using a filter such as a polarizing filter difficult to use. L-series lenses have mechanical elements that rotate within the body of the lens and allow the outside of the lens to remain stationary while focusing and also allowing easier use of filters.
L series lenses also boast real time focusing, which means you can keep your auto focus system running while manually fine tuning your focus. Kit lenses simply do not offer this feature and you must either focus completely manually, which is difficult if your subject is moving or at all wiggly. The only other option with a kit lens would be to put all of your faith in the cameras autofocus system and hope (close your eyes and cross your fingers!) that the autofocus nailed those eyes tack sharp. Modern DSLR cameras often do a great job with focusing, especially if you are using a single manually placed AF point, but if you are using the focus and recompose method, real time focusing gives you the best of both worlds by allowing the camera to autofocus and then without switching to manual mode, you can use your hands to more precisely adjust the focus ring if necessary.
Lastly and most importantly, unlike kit lenses, most L-series lenses utilize a ring type USM (ultra sonic motor). This motor is quick and efficient and allows for a very fast and easy autofocus transition. Kit lenses often cause the camera to hunt for an appropriate focus point if conditions are less than ideal.
L-series lenses have obviously beneficial qualities and definitely live up to their label of “luxury.” That being said, a luxury is not always a necessity, and if you don’t have the pocket change to spare for one of these fancy lenses, don’t despair. Many great photographers capture beautiful images with non L-series lenses too. The simple difference being, that if a standard kit lens or general non L-series lens was used, some simple post processing of color and contrast correction may be all that is needed to perfect your image.