The proper position of the flash is directly over the lens. This will ensure that any shadows from the flash will occur behind the head and body and not to one side. Of course, when shooting square format like Hasselblad, the flash can be fixed in that one position. For shooting with a rectangular format like Bronica ETR or Canon DSLR, the flash must be mounted on a swinging bar so that the flash can be positioned over the lens in either a horizontal or vertical shot.
I have determined through many tests that the ideal distance of the flash head above the lens is twelve inches. There are many advantages to using this distance. First, the unavoidable reflections on the forehead, nose, cheeks and chin are conducive to making the subject look good. The forehead reflection is positioned higher on the forehead, almost into the hair line and much diminished in strength. This leaves the flat portion of the forehead reflection free and retaining the natural color of the skin.
The shape of the nose is determined by the fall off of light along the sides and the position of the reflection on the bridge of the nose. When the flash is positioned closer to the lens, the bridge or indentation receives no highlight but rather the bony lower part of the nose is emphasized. The tip of the nose also benefits from the twelve inch distance of the flash head by appearing smaller and less intense.
Cheek reflections are considered acceptable when they are centered on the upper portion of the cheek. With a lower positioned flash head, the reflection highlight the unattractive line of muscle from the cheek to the nose. The twelve inch flash position also enhances the cheek bones. Chin reflections lower down on the point of the chin are unattractive and make the chin look wet. Alternatively, the twelve inch flash position just places a small crescent shaped highlight under the lip. An added benefit also occurs in the form of a more defined chin line and the placement of some double chins in shadow.
Those terrible eye glass reflections are greatly minimized with the twelve inch flash. The flash highlight now appears near the top of the eye glass, completely avoiding the area of the iris and pupil. The eyes are the most important feature of the face and ugly flash reflections can wipe out the eyes completely. An added benefit is a slight darkening at the bottom of the picture, enhancing the composition with a natural fade out.
Remember that since most natural light comes from a position over the horizon the most natural flash lighting will do the same for the face. Don’t let convenience prevent you from capturing your flash subjects in the most attractive light.