not so ... as ...
From Longman Dictionary of Contemporary Englishnot so ... as ...not so ... as ...NOT formal used in comparisons to say that something or someone has less of a particular quality than another person or thing The bed was not so comfortable as his own.GrammarIn comparisons without ‘not’, you say as ... as: Your writing is as bad as mine. ✗Don’t say: Your writing is so bad as mine. → so