The main purpose of making the negative carrier, lens plane, and easel parallel is to insure sharp focus in all corners of the image. Thus, the most direct method of determining effective parallelism is to check for corner sharpness. Other testing methods are to some extent indirect and subject to possible errors. A clear piece of film lightly sanded with coarse and fine sandpaper makes an effective negative for checking sharpness. As long as the enlarger alignment provides sharp image corners, nothing has to be level.