SELECT count(*) FROM camera_lens WHERE id NOT IN ( SELECT camera_lens_id FROM photos )
How many camera lenses are not used in taking any photos?
[Schema (values) (types)]: | mountain_photos |  mountain : id (text) , name (number) , height (text) , prominence (number) , range (number) , country (text) | camera_lens : id (text) , brand (number) , name (text) , focal_length_mm (number) , max_aperture (number) | photos : id (text) , camera_lens_id (number) , mountain_id (text) , color (number) , name (number);