What is the difference between focal length and focal distance of a lense?
Focal distance refers to the distance from the front of the lens to the beam crossing point (see figure). To define the focal length, the lens is replaced by a line, AB. The positon of the vertical line is such that the two parallel beams are deflected, as shown in the figure, so that the two beams cross. The distance measured from this line AB to the focal point is referred to as the focal length of the lens. For the case of a thin lens (thickness of the lens/focal length is small), the value of focal length and focal distance are almost the same. For the case of a thick lens, the location of the line AB will be noticeably different from the front of the lens. Hence, the focal length and focal distance will differ.