Translation guide
The focal ratio (f-number) of a lens or telescope, expressing the ratio of focal length to aperture diameter. In Japanese, this is primarily a technical term used in optics, astronomy, and photography.
The ratio of the focal length of a lens or telescope to its aperture diameter, often written as f/N.
Standard term for 'f-number' or 'focal ratio' in photography and optics. Pronounced 'efu-chi'.
このレンズのF値は2.8です。
The focal ratio of this lens is f/2.8.
Literally 'focal ratio', used in astronomy and technical optics. Less common in everyday photography.
F値 is the everyday term in photography and is always written with the Latin letter F. 焦点比 is more formal and used in scientific literature, especially astronomy. Both refer to the same concept (f-number).
Do not translate 'focal ratio' word-for-word as 焦点比率 (しょうてんひりつ). This is not a standard term and may cause confusion.
The focal ratio of this telescope is f/10.
Literally 'aperture ratio', sometimes used synonymously with focal ratio in technical contexts, but can also refer to the ratio of aperture to focal length (the inverse). Use with caution.
May be ambiguous; '焦点比' or 'F値' are preferred for clarity.
口径比が小さいと、暗い天体の観測に適しています。
A small focal ratio is suitable for observing faint celestial objects.