The frame difference is the difference between which measurements?

• A. The difference between C and D
• B. The product of A and B
• C. The difference between A and B (horizontal & vertical measurements)
• D. The sum of A and B

Answer: (C)

Frame difference measures how much the frame’s width differs from its height by comparing the horizontal and vertical measurements. In common frame notation, the horizontal measurement (width across the lenses) and the vertical measurement (lens height) are the two values used. The frame difference is the absolute difference between these two, |horizontal − vertical|. For example, if the frame width is 50 mm and the lens height is 40 mm, the frame difference is 10 mm, indicating a more rectangular shape. If they’re close, the frame is near-square. This concept focuses on the relationship between width and height, not on any product or sum of measurements, and not on other paired measurements.