A number of different choices are possible on WFPC2 in order to approximate the Johnson-Cousins system often used in ground based observing. These choices differ in throughput, wavelength fidelity, color transformability, and cosmetics. The HST science program as a whole benefits if a standard set can be agreed upon by the community for broad band photometry. This will allow theoretical isochrones and other models to be published in the standard system, and allow ready comparison of the results from different observers. Furthermore, although all filters will be calibrated photometrically and with flat fields, a core set must be chosen for monitoring the instrument both photometrically and in imaging performance. There was a substantial consensus between the accepted Cycle 4 GO programs and the WF/PC-1 and WFPC2 science teams that F336W, F439W, F555W, F675W, and F814W should be the preferred set to approximate the Johnson Cousins U, B, V, R, I passbands. These filters form the basis for the WFPC2 broad band photometric system. As will be seen from the figures in Section 8.8 (page 172), the preferred set is accurately transformable with the exception of the U bandpass.
On the other hand, there are situations where concerns such as maximum throughput must override the above arguments. For example, filters F300W, F450W, F606W, and F814W were chosen for the Hubble Deep Field (HDF), due to their wider bandpasses.