Depth of field

 The depth of field is the distance between the closest and furthest object. In simple words, it is the point of focus where objects remain sharp and in focus. The depth of field can be calculated through the focal length and the distance of the subject. Focal length is measuring the distance in millimeters between the optical center of the lens and the camera sensor.

Depth of field can be used to blur out the distracting details of the object and to pay attention to the main subject. The more distance depth of filed is said to be wide/deep and closeup is said to be narrow/shallow. 

There are two types of lenses prime/telephoto, the prime is the one with the fixed focal length and is used by high-end production companies whereas telephoto has a variable focal length is used for close-range photography and is commonly used to show faraway objects.