user: Question: What is the brand of the Sigma 30mm f2.8 DN, which has a maximum aperture of f /2.8 and a focal length of 30mm?
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 "Brand" FROM "Table" WHERE "Max. aperture" = 'f /2.8' AND "Focal Length" = '30mm' AND "Product Name" = 'Sigma 30mm f2.8 DN';