'f-stop' originates from English, specifically from the letter 'f' (ultimately from Latin 'focus') used to denote focal ratio, combined with 'stop' from Old English 'stoppian' where 'stoppian' meant 'to stop or block'.

'f-stop' changed from the related technical term 'f-number' (used in scientific and optical texts in the 19th century) and from the mechanical 'stop' used on early cameras to limit light; over time 'f-number' and 'f-stop' became interchangeable in photographic usage.

Initially it referred to a mechanical 'stop' that blocked or limited light; over time the term came to denote the numeric ratio ('f-number') that describes aperture size.