It could be to 'focus' on something
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'.
Eyes are used for seeing.
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.
One word for being late starting with the letter F is forgot.
Frappe
Flan