Keep the eyes visible behind glasses
The white rectangle in a lens is not necessarily an exposure problem. Trace its route back to the camera.
A lens returns the image of a source
A bright rectangle may be a reflected softbox or window. Lowering output dims the face as well as the reflection. First inspect the source–lens–camera geometry. A reflection follows the surface normal, but real lenses add curvature, front and rear surfaces, and coatings. Locate the reflection obscuring the eye, then verify a small source adjustment in an actual test frame.
Three checks for a glasses portrait
Inspect the photograph and tick what you observe. These are personal viewing notes, not automated glare detection or image edits.
Move the source a little before asking the person to move
A small shift upward or sideways may send the reflected source away from an eye. Check both lenses and the brow shadows after each adjustment. Raising the source indefinitely can darken the eyes; an uncomfortable head angle can make the final portrait feel strained.
A polarizer or empty frame is not a universal answer
Polarization may reduce some reflections but cannot remove them at every lens and angle; light loss and coating color also matter. Record real eyes with a considered setup. Enlarge both lenses and take another frame after a slight gaze change to make sure the result holds.