The longer the extension tube the larger the image on the film plane and the longer the exposure, the longer the focal length the larger the image, so combine a longer extention tube and a longer focal length the larger the image on the film plane, you will have to take into account exposure factors when metering as the 501 does not have built in TTL metering, unless you have a metered finder, also take into account the closer the macro work the shorter the depth of field, which means you will have to stop down more to attain better depth of field, which again results in longer exposure times, which in turn can induce resiprocity failure in your film.

Dave