It is one thing to know the various classification systems, but quite another to apply these to real galaxies. As an art, galaxy classification has many stringent requirements as well as limitations. For instance, a system is usually defined by a set of standards or prototypes as they appear on a selected type of image material. Thus, reproducibility and consistency will depend on different observers using similar material to classify other objects. For Hubble's system, this image material included prime focus and other bluelight plates taken with 1.5-5m class telescopes. In general, telescope focal length and ratio, image resolution, and the depth of exposure, in addition to the characteristics of a given galaxy itself, all play a role in determining what information is available for classification.
However, there are important problems, such as high inclination, and the fact that large-scale plates or digital images, ideal for classification, are available for less than 15% of the approximately 25,000 galaxies having a standard isophotal diameter 1'. This means that for most galaxies, morphology is judged from small-scale sky survey plates, prints, and films. The pitfall of the sky survey images, in addition to the much smaller image scale than is typical of Cass or prime focus large reflector images, is frequent overexposure, making it difficult to see the crucial inner regions where high surface brightness bars, rings, lenses, etc., may be present. Distinguishing E and SO galaxies on such image material, for example, can be difficult 48. On the other hand, the pitfall of large reflector plates in the past has often been underexposure, causing very low surface brightness disks, rings, arms, etc. to be completely missed. (In fields with significant galactic extinction, these same details can still be lost even on deep SRC-J plates.) Thus, it is important when using published morphological types to know where the types came from and their limitations.
The standard Hubble classification system can also simply be difficult to apply to certain kinds of galaxies. Some of the worst offenders can be early-type ringed, barred galaxies, where the bulge can be relatively small in spite of the fact that a spiral pattern is tightly wrapped and not highly resolved 44, 15. Particularly interesting and important in my view also are what Vorontsov-Velyaminov 67 calls "double or triple stage spirals", which refers to those spirals which exhibit distinct sets of spiral structure on different scales. The use of one pattern could lead to a different type from the other. A good example is NGC 3504 in the Hubble atlas, where a well-resolved oval and bar dust lanes lead to a classification of Sb in the RSA, but a fainter, larger, and smooth outer pseudoring with almost zero pitch angle would clearly warrant a type of Sa. De Vaucouleurs 20 compromises on a type of Sab for this one. The classification of galaxies with hierarchical spiral structure is an interesting problem hardly discussed in the literature, but this kind of structure plays a selective role in the application of Hubble's system 44.
Perhaps the most serious problem with galaxy classification is that it is still largely a subjective visual exercise. The human eye is very good at pattern recognition, and is capable of integrating the information in an image quite quickly. However, a morphological type is not a measured quantity even if it is coded on a numerical scale. De Vaucouleurs 23 emphasized that in spite of the subjective nature of classification, such classification is a time-honored tradition in astronomy that has been largely successful. This has been recently demonstrated very definitively by the extensive analysis of published type (T) and luminosity (L) classifications that provided the coded types given in the Third Reference Catalogue of Bright Galaxies 26 (=RC3). From this analysis, which is based mainly on sky survey (PSS or SRC-J) classifications, the average uncertainty in both of these parameters is 0.9 step, where one step represents a difference such as Sb to Sbc, Sa to Sab, etc. Classifications based on large-scale reflector plates, or for large face-on galaxies based on sky survey images, axe generally considerably better than this.