With the PC spotlight, focusing is achieved by moving the lamp and reflector assembly closer to or further away from the lens. However, focusing can also be achieved by moving the lens, while keeping the lamp and reflector stationary. This kind of lighting instrument is quite obviously known as a 'profile spot'.
Clearly, focus-wise, a profile spot can achieve anything a PC spot can do — you don't need Einstein's theory of relativity to see this. However, the profile spot has another trick up its sleeve: it has a 'gate' in the centre of the optical system where the beam can be modified. For instance, a profile spot will have four shutters in the gate that can shape the beam far more precisely than barn doors ever can. Also, a 'gobo' can be inserted to style the beam further.
Rumour has it that 'gobo' is a shortened version of 'goes between', and this seems a reasonable supposition. In this context, the simple gobo is a flat metal mask with a pattern cut out. So, you could project the outline of a city skyline if you wanted. Clearly, this is a simple light/no-light projection — if you want a detailed image, you will need a projector. But that's not the function of the profile spot. It is simple and effective in what it does, and versatile too. The gobo doesn't have to be of a recognizable image. It can be a pattern that is used slightly out of focus to give the beam a texture that it would not otherwise have.
Although the beam of a profile spot can be shuttered and masked, it is always worthwhile selecting the right kind of instrument in the first place, so that the beam width needs minimal trimming. Since the beam is shuttered and masked by placing objects partially in its way, a significant amount of heat is generated. The carbon-neutral lighting instrument has yet to be invented, but even disregarding waste of energy, there is enough heat being generated around the stage anyway, and the hazard it creates can be reduced simply by being less wasteful. Some profile spots have an iris, similar in function to the iris of the eye.
Simple profile spots provide a beam angle that can be adjusted through focusing, and through shuttering in the gate. However, a much wider range of beam adjustment can be obtained with a 'variable-beam profile spot'. This has an additional lens element that effectively results in a 'zoom lens' for lighting. The advantage of this is that one variable-beam profile spot has the same range of capability as several conventional profile spots that have different beam angles, through the use of different lenses. Of course, the additional complexity comes at additional expense. But the rig is now more versatile and easier to set up. The variable-beam profile spot also offers a wide range of beam qualities, as well as angles.