Flash photography can be somewhat challenging at times. However, understanding the basic concepts of flash photography opens up a whole new world of creative possibilities. If flash photography seems intimidating to you than this little series of blogs will hopefully help you to use your flash creatively and with more fun.
Two light sources
With the rare exeption of taking pictures in a pitch dark room flash photography always involves the use of two separate light sources: flash and ambient light. Whether the ambient light is daylight or the artificial lighting in a room is of no real importance (it does have an influence on the color rendition of the scene, though. But that’s a topic for a future blog). What really is important is the fact, that you have to deal with the characteristics of the two different light sources. However, as a rule you have no influence on the ambient light. Therefore, the look of the image can be mainly controlled by the proper use of the flash light.
The flash is a point light source
The built-in flash of a camera or even an external flash unit are what is usually called a point light source. The light intensity or brightness of such light sources follows the inverse square law. If that sounds a bit technical than that’s because it is, well, a bit technical. But don’t worry, the following illustration will clarify this.
If the brightness at a distance of 1 m from the light source is exactly 1 (don’t worry about the units of measurement here) than it is just 1/4 at 2 m and 1/9 at a distance of 3 m from the light source. Simply put, the farther from the light source the dimmer the illumination. This is very different from sunlight, which is very uniform thanks to the very large distance between the sun and the earth. You’ll see shortly why this difference in the characteristics of flash light and sunlight (and other forms of ambient light) is so important.
Matching ambient light and flash
Assuming that the ambient light does not change over time (that’s obviously not always the case, but assuming changing ambient light would not help to understand the general principles), different power settings of the flash result in quite different pictures.
The following pictures were taken in manual exposure mode using f8 for all 5 exposures. The shutter speed, however, varied between 1/125 sec. and 1/8 sec. The flash was in fully automatic (E-TTL or i-TTL) mode. The power output of the flash is then based on the cameras exposure metering.
Note that the exposure of the little white dog (yes, I know who this guy is …) is almost identical in all five pictures. The background however changes from black with no visible detail to its actual (blotchy) green. If you are not familiar with flash photography this may seem strange. Here is the explanation.
All pictures were taken in low light conditions with the sun just below the horizon. At the fast shutter speed of 1/125 sec comparatively little light enters the lens. Hence the power output of the flash needs to be quite high to correcly expose the stuffed animal. Since the background is several meters away from the camera it is practically not illuminated by the flash at all (due to the inverse square law, see above) and therefore appears black. At a shutter speed of 1/8 sec, however, the ambient light is enough for a correkt exposure of the background at f8 and so the flash, with a rather low power setting, just fills in the subject in the foreground.
You may have noticed, that the last two pictures are slightly blurred. This is due to the fact, that all images were taken handheld.
Part 2 of this series we’ll cover some rather technical details of flash photography such as flash sync speed and high speed synchronisation.
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