AF Lens
AF lens allows us to enjoy an easier method of photography experience which is not complicated and also focus on capturing the moment than trying to focus. AF lens compromises of both passive and active categories. However most cameras use a cheap passive AF lens whilst the expensive cameras will have an active AF lens or sometimes professional cameras will have both.
AF has its limitations when trying to capture more fuzzy artistic expression photos which you will not be able to do so with an AF lens. Moreover the AF implementation can fail in certain scenarios. In most cases the camera will move back and forth the AF lens and will eventually not be able to focus.
With a passive AF lens you will be able to measure the distance between the camera and the object that you wish to capture in the picture. If you know the distance between the object and AF lens, you will be able to set it to focus on the position. The AF system will shoot a beam of invisible light which is generally infrared, at the object which is at the center of the picture and then measures the distance towards that object. Based on the distance calculated the focus is then set.
Most expensive and professional cameras will have both AF passive and active lens. These cameras will be able to identify the right lens according to the scenario or sometimes will check between the lenses at the same time. The photographer can decide between the systems that he/she wishes to use manually. In cases of capturing blue skies the camera can try to use the active system and then measure the distance. The camera can set the focus and then skip the passive focus.
In some cases the AF lens can fail due to not properly focusing and when the picture is of fuzzy nature and sometimes the picture might be in focus but the camera adjusts itself. When capturing low light pictures, the passive AF lens needs to focus on the picture to actually work and this may not be the case in low light cameras. Sometimes these will use a series of flash to adjust in these situations but in the end will fail to capture the perfect shot.
AF lens will fail with objects that tend to absorb the infrared beam that it uses. These will cause the system to measure the distance incorrectly.