Not necessarily. A good letter-writer will highlight all your wonderful attributes that made you a stellar medicine or surgery student and say something wonderful like "dk23 will make an excellent physician." That being said, you should provide them with your CV and personal statement so that they can reinforce these aspects of your application in their letter. If you know that you want to do ophtho, tell them, but the bulk of the letter should focus on the qualities that are making you an outstanding physician. Your letter from ophtho will detail all of your qualities that will make you an excellent ophthalmologist. Good luck!