This is not 'f', although the letter is somewhat similar, but rather 'long s'. It is derived from an antique Roman cursive letter that resembled letter 's' with the upper part similar to a straight line pointing upwards and to the right. This form of the letter, only slightly simplified has been used in the half-uncial script using ascenders and descenders that dominated writing across continental Europe in 4th-8th century, where Roman elongated 's' took a form resembling letter 'f'. This was transferred to the Carolingian minuscule adopted in 9th century and merging the popular insular maiuscule with Merovingian minuscule. The round 's' made its way into the alphabet however, usually as the ending letter, quite possibly because of the existence of two variants of the Greek letter sigma that were used in initial or medial position (σ) or at the end of the word (ς). This form of script was considered perfect for the time and thus there was little pressure to change it, so the documents written throughout Europe in the Middle Ages were quite quite similar visually, even if they used different languages. It fell into disuse in the end of 18th century (for example, in France it was phased out as the relic of 'ancien régime'). It can still be found in some books written until the middle of the second decade of 19th century though and in Germany it remained in cursive until mid-1970's (this is one of the reasons it can be still found in heavily stylised inscriptions there). It should not be, however, mistaken for German 'Eszett' or 'scharfes s' (ß) that is derived from the ligature between 'long s' and 'z'.