The equation of transfer is a two-faced equation, easy to solve at times, and difficult to solve at other times, depending on the values assumed by its coefficients. It is rather tractable when applied to the deep layers of atmospheres and for frequencies from their continuum spectrum, but this solvability is in stark contrast to the difficulty in solving it close to the surface of atmospheres or within spectral lines. It follows that the photons most rich in information that reach our telescopes are also the most difficult to interpret. The purpose of this note is to stress this point and to illustrate it with some examples taken from the solar atmosphere.