Custom Function 12
Ready, SET, Go
Most contemporary 35mm cameras feature what are called Custom Functions. For those just entering the fray, or if you've been under a photographic rock for the past 15 years, Custom Functions are a response to modality in camera design.
Back in the good old days (before about 1985) cameras had mechanical dials and knobs. You turned them and they changed some function of the camera ‹ always a single function like shutter speed or aperture, because they were mechanically coupled.
Today cameras have a plethora of buttons, wheels and knobs, and almost every one of them is an electronic control. In other words, what it does at any time depends on what the designer (or photographer) wants it to do. That's modality, and that's what Custom Functions allow you to do ‹ change the function or "mode" of a particular switch. (Aren't you glad you asked?)
The D30 allows, though Custom Function 12, for you to turn the rear SET button into a control to quickly change either the ISO speed or the Image Quality level. Both of these are normally settable via the rear CRT panel. But by setting the one that you prefer with Function 12 you can change them on the fly without accessing the CRT.
While some might want to use this to change the Quality Level, for me the magic of this control is being able to set the ISO speed almost instantly. The D30 is remarkable in its ability to shot very high quality images at speeds up to ISO 400. My tests, as well as those of others, have shown that there is only a slight image degradation when shooting at 400 Vs. 100. For anyone coming from many years of shooting colour film, having an ISO 400 capability that's this good is close to astonishing. Having the ability to instantly switch to it between shots, without having to change camera bodies or reload film, is close to magical.