as far as
In affirmative sentences so . . . as can not properly be used except in certain restricted constructions, and where the quality referred to is to be emphasized. It occurs oftenest in sentences that, though affirmative in form, carry a negative suggestion; as, "So good a cook as Polly is hard to find," that is, "It is not easy to find so good a cook as Polly.
Few knights of the shire [in the 17th century] had libraries so good as may now perpetually be found in a servants' hall.
That is, "not many knights of the shire," etc. In a simple affirmative comparison like "Jane is as good a cook as Polly," so . . . as is not used.
In interrogative sentences, as in negative sentences, a consciousness more or less distinct of a considerable degree of the quality referred to is conveyed by so . . . as, but not by as . . . as. "Is John as old as James?" and "Is your uncle so old as my father?" convey different impressions as to what the speaker means by old. In the question where as . . . as is used there is no implication of considerable age in old.
as far as, so far as: Discriminate carefully between these terms. As far as expresses distance; so far as expresses limitation, as of one's knowledge. Therefore, "so far as I know" is preferable to "as far as I know."
as if. Compare like.