focal length

...Strictly speaking, it is defined for paraxial light. When a bundle of rays parallel to the optical axis intersects at a point F' on the axis after passing through an optical system, F' is called the image-side focus, while the point F where parallel light is incident from the image space and converges on the object side is called the object-side focus. Focal length refers to the distance between the focus and the principal point, and is usually positive when the direction of the principal point coincides with the direction of the light rays as seen from the focus. There is the front (object-side) focal length, which is the distance between the object-side focus and the object-side principal point, and the back (image-side) focal length, which is the distance between the image-side focus and the image-side principal point. If the refractive index of the object space and the image space are the same, the two will coincide, and in this case the front focal length is simply called the focal length. ...

*Some terminology that mentions "focal length" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…厳密には近軸光に対して定義されるもので，光軸に平行に入射した光線束が光学系を通過後，軸上の1点F′で交わったとき，F′を像側焦点，一方，像空間から平行光を入射させたとき物体側で収束する点Fを物体側焦点という。　焦点距離focal lengthとは焦点と主点の距離をいい，ふつう焦点から見て主点の方向が光線の方向と一致するときを正とする。物体側焦点と物体側主点との距離である前側(物体側)焦点距離と，像側焦点と像側主点との距離である後側(像側)焦点距離があるが，物空間と像空間の屈折率が等しければ両者は一致し，このときは前側焦点距離を単に焦点距離という。…

※「focal length」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報