The ratio of focal length to aperture is commonly called what?

• A. Aperture ratio
• B. Depth of field
• C. Beam ratio
• D. F-number

Answer: (D)

The ratio of focal length to the aperture is called the f-number, also known as the f-stop. It’s a dimensionless value written as f/N, where N is the aperture diameter. This ratio tells you how large the lens opening is relative to its focal length and it directly affects exposure and depth of field: smaller f-numbers mean a larger opening, more light, and a shallower depth of field; larger f-numbers mean a smaller opening, less light, and a deeper depth of field. Other terms like “aperture ratio” aren’t the standard label for this ratio, and “depth of field” describes the result rather than the ratio itself, while “beam ratio” isn’t used in this context.