如果你照完一张相片后不往前进一张，底片就是会曝光。 If you don't wind on after taking a picture, your negative will be double-exposed.
安全曝光的时限价值或情况的范围，尤指曝光范围，在此期限内摄影用的底片能产生可用的影象 A range of values or conditions, especially the range of exposures over which a photographic film yields usable images.
