目前分词作宾语补足语的时候可以在前面加as,譬如：
He described the picture as expressing the dream of mankind.
They considered me as having done my best.
那样过去分词作宾语补足语的时候可以不能在前面加as呢？譬如这句
James Boswell, his biographer, described the garret where Johnson worked as fitted up like a counting house with a long desk running down the middle at which the copying clerks would work standing up.
describe sb/sth as...
consider sb/sth as...
这种结构中，as为介词，作宾补的是as介词短语。as后接名词或动名词，假如动名词为系表结构或被动语态时，being一般省略，所以as可将来接形容词和过去分词。
所以这种结构并非过去分词作宾补前面加了as。as是句型中必不可少的。后接过去分词是由于省略了动名词being.
