The image of a galaxy can be characterized in an entirely objective and non-controversial way by (1) its total integrated magnitude (usually the Holmberg (1958) magnitude based on the total luminosity inside isophotes of 26.5 and 26.0 mag arcsec−2 in the photographic and photo visual regions, respectively), (2) its integrated colors, U–B, B–V, V–R etc., (3) its isophotal radius r, in which r=(a×b)0.5, and where a and b are the semi-major and semi-minor axes to some specific isophote, (4) its effective radius re, defined as the radius within which half of the total galaxy light is emitted in projection, and (5) for many (but not all) galaxies the disk and bulge scale-lengths. More detailed characterizations, such as those provided by the Hubble, de Vaucouleurs and DDO type, are much more difficult (or perhaps impossible) to carry out in an entirely objective fashion.

Inspection of the images of the SRC Southern Sky Survey on IIIaJ emulsion (van den Bergh 1989b) show that it is just barely possible to recognize grand design spiral galaxies at redshifts of up to about 1×104 km s−1. The images of such objects have diameters of ∼0.5 mm (corresponding to 34') and contain ∼1×103 picture elements. Experience shows that the images of galaxies with redshifts of 1000–2000 km s−1, which typically contain 1×104−1×105 picture elements, can be classified with confidence on plates obtained with the SRC Schmidt telescope.