If a handkerchief is unavailable when using a manual flash, what can be used instead?

Using one finger over the flash is an effective technique to diffuse the light when a handkerchief is not available. When a manual flash fires, it emits a strong burst of light that can create harsh shadows and overexpose the subject if not properly diffused. By placing a finger over the flash, you can significantly soften the light and reduce its intensity, mimicking the effect that would be achieved by a diffuser. This method allows you to have more control over the lighting conditions and can help in achieving a more flattering and balanced exposure.

Other options, while creative, do not effectively serve as a substitute for a handkerchief. A second camera would not aid in diffusing light, and using a piece of cardboard might not create the same soft diffusion effect; it can instead reflect light harshly. A lens cap, while it would prevent the flash from firing entirely, does not provide any diffusion and would render the flash unusable. Hence, the application of a finger over the flash is the most practical solution in this scenario.