user: Question: What is the maximum aperture of the lens(es) with a focal length of 20mm?
Columns: ['Brand', 'Product Name', 'Focal Length', '35mm EFL and equivalent aperture', 'Max. aperture']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Olympus', 'Olympus M.Zuiko Digital ED 12mm f/2.0', '12mm', '24mm f /4', 'f /2.0']
SQL:

assistant: SELECT MAX("Max. aperture") FROM "Table" WHERE "Focal Length" = '20mm';