I'm trying to get a grasp of the basics of a Tolman SB test. Please correct me if I'm wrong: To be able to do a Tolman SB test you first have to observe a couple of identical objects at different redshifts. "Identical" here means objects of the same spectral type and luminosity class. Then you measure their apparent areas and their apparent magnitudes, and from these two parameters you get their surface brightnesses*. Now, if the universe is expanding, the surface brightness of an object at, say, z = 0,4 should differ by a factor of 1,4^4 to an identical one at z~0 (that is, the ratio of SB (z~0) to SB (z = 0,4) should equal approx. 1,4^4). But all of the above is of course highly simplified. For example, the observed SB isn't the "real" SB, correct? To find the "real" SB you have to correct for, among other things, interstellar extinction and interstellar reddening. *I'm thinking of the relation S=m+2,5logA here, where A is the apparent area.