It could be to 'focus' on something
Eyes are used for seeing.
No, the article 'an' is never used before words starting with 'f'. The form 'a' is used. For example 'There was a frog in my garden' not 'There was an frog in my garden.' The form 'an' is used before words that start with a vowel sound. Thus, you have correctly written 'an F' in your question, because the sound of the letter-name 'F' starts with a vowel sound: 'ef'.
The US has never used an "F" mint mark. What you're seeing is almost certainly the designer's monogram. In particular, Indian Head nickels have a prominent "F" for the artist James E. Fraser.
Fantastic or Fashionable can be a compliment starting with letter F.
Finish is another word for completion starting with the letter F.
The opposite for the word plain starting with letter f is FANCY.
Parsley Feta cheese Walnuts all can be used as add-ins for a salad
The starting temperature was 11°F. Starting at 11°F, a drop of 13°F in 7 hours would result in a final temperature of -2°F.
Floccinaucinihilipilification is the longest word starting with F; it means the act of estimating something as worthless.
Frappe
Flan
Fanta!