Earlier in my career, I used to shoot a lot of physician portraits. And it seemed that I was constantly dealing with reflections in eyeglasses. In this helpful video produced by Adorama TV, there are some excellent tips for coping with the situation during portrait sessions.
But like most things in photography, you usually have to give up one thing to get another. For subjects with very curved eyeglasses lenses, I often find that raising the off-camera flash does get rid of the reflection, but then creates a lighting effect that I'm not thrilled about for portraiture. The next step usually requires additional fill cards and reflectors.
In other words, there are no pat answers when battling unwanted reflections. But this video does provide a good starting point.
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