CAMERA FLASH: EXPOSURE

CAMERA FLASH: EXPOSURE

Using a camera flash can both broaden the scope and enhance the appearance of your photographic subjects. However, flash is also one of the most confusing and misused of all photographic tools. In fact, the best flash photo is often the one where you cannot even tell a flash was used. This tutorial aims to overcome all the technical terminology in order to focus on the real essence of flash photography: how to control your light and subsequently achieve the desired exposure.
The first part of the camera flash tutorial focused on the qualitative aspects of using a camera's flash to influence a subject's appearance; this second part focuses on what camera settings to use in order to achieve the desired flash exposure.
FLASH EXPOSURE OVERVIEW
Using a flash is fundamentally different from taking a normal camera exposure because your subject is being lit by two light sources: your flash, which you have some control over, and the ambient light, which is likely beyond your control. In this part of the tutorial we'll focus on the other two consequences of this fact, as they pertain to flash exposure:

Illustration shown roughly to scale for a 1/200th second exposure with a 4:1 flash ratio.

Flash shown for first curtain sync. A pre-flash is not emitted with much older flash units.

1. A flash photograph actually consists of two separate exposures: one for ambient light and the other for flash. Each of these occurs in the split second between when you hold the shutter button and when the shutter opens. Newer SLR cameras also fire a pre-flash in order to estimate how bright the actual flash needs to be.
2. A flash pulse is usually very brief compared to the exposure time, which means that the amount of flash captured by your camera is independent of your shutter speed. On the other hand, aperture and ISO speed still affect flash and ambient light equally.

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