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

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'