Table: lens_specs
Columns: brand
product_name
focal_length
35mm_efl_and_equivalent_aperture
max_aperture

Question: What is the maximum aperture of the lens(es) with a focal length of 20mm?
SQL: SELECT "max_aperture" FROM lens_specs WHERE "focal_length" = '20mm'

Question: What is the 35mm EFL and the equivalent aperture of the lens(es) with a maximum aperture of f /2.5?
SQL: SELECT "35mm_efl_and_equivalent_aperture" FROM lens_specs WHERE "max_aperture" = 'f /2.5'

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?
SQL: SELECT "brand" FROM lens_specs WHERE "max_aperture" = 'f /2.8' AND "focal_length" = '30mm' AND "product_name" = 'sigma 30mm f2.8 dn'