In working with Mr. Obama — someone he said he had always wanted to
portray — Mr. McCurdy employed his usual process, in which he carefully lights a photograph (taking about 100 in a
session) that he then works from to create his painting.
这里是说每拍一张照片前要调好光，总共拍一百张吗？ a在这里是表示"每一”的意思吗？
