Focal Length

In technical terms, the focal point is the geometric place where parallel light rays converge after passing through a lens. In photography, the focal point stays at a small distance from the camera sensors called focal length.

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A longer focal length means that light rays coming from objects far away will converge way before reaching the sensors, so they will disperse and the object will appear more blurry. Conversely, light rays coming from objects very near the camera will tend to converge further behind the sensors and will appear more blurry as well. In other words, a longer focal length causes a shallower Depth of Field.