What material can be placed over the flash head to effectively reduce the manual flash output by about one stop?

The effectiveness of a material placed over a flash head to reduce its output depends on its ability to diffuse light without completely obstructing it. A handkerchief is commonly used in photography as a diffusion material because it softens the light while still allowing some to pass through. By placing a handkerchief over the flash, it diffuses the light, resulting in a reduction of intensity, typically around one stop. This approach helps in achieving a more flattering and soft illumination in photography, particularly for portraits or close-up shots.

In contrast, other materials like paper towels or plastic wrap may not achieve the same level of effective diffusion. Paper towels can be too thick or absorb light rather than diffusing it, while plastic wrap is quite transparent and may not significantly reduce the output at all. Cellophane can be used for similar purposes but may not provide the same softness and diffusion that a handkerchief offers. Thus, the handkerchief is preferred for effectively reducing flash output by about one stop while still achieving a good quality of light.