I just got my annual eye exam and learned for the first time that Medicare will partially cover the "medical" portion of the exam (wellness evaluation) but not the refraction. Your supplemental picks up the balance except the refraction. At my optometrist, the refraction co-pay is $30. Some/most providers will automatically do a refraction unless you specify you want a "wellness" exam.
Even though I have had annual exams (had cataracts) several times since going on Medicare , none of my previous providers had indicated they could use Medicare and I paid out-of-pocket. Glad I switched providers.