CREATE TABLE table_79371 ("Brand" TEXT, "Product Name" TEXT, "Focal Length" TEXT, "35mm EFL and equivalent aperture" TEXT, "Max. aperture" TEXT)

What is the maximum aperture of the lens(es) with a focal length of 20mm?
SELECT "Max. aperture" FROM table_79371 WHERE "Focal Length" = '20mm'
SELECT "Max. aperture" FROM "table_79371" WHERE "Focal Length" = '20mm'