What's Wrong With Flash?

The problem with most flash illumination is that the lighting typically looks flat and unnatural. The flash's light overwhelms the natural lighting, appearing as if the flash is the only light source. If a flash is shoe-mounted (on the camera), then the light is harsh and produces strong shadows. Your background goes black, and your bug looks lit by bad stage lighting. It's simply not the natural look that you had imagined (although this look can be effective for some shots).

So, the main issues with macro photography with flash tend to be the following:

• Backgrounds go black, causing an unnatural appearance

• Lighting appears flat and staged
  

This picture was shot in open shade in the middle of a summer day. Really.

Why does the background go black?

There are several reasons:

• The flash is several times brighter than the ambient (existing) light

• Digital image files (as opposed to images captured to film) tend to be less forgiving than film, "blocking up" all the shadow detail. Some shadow detail can be retrieved, but the shadowy area often gets "noisy" (grainy, with compromised image detail).

• The flash can't expose for foreground and background equally. (see Inverse Square Law, below)

So, with thinking cap on, you ask, "Why can't I just drop the shutter speed down to burn in the background?" Sure, you can do that, but you'll risk ghosting of your primary subject if she buzzes about, and may overexpose your subject due to the additive effect of flash and available light. Additionally, you may encounter motion blur from breezes or camera motion. But it's a valid technique, and you'll see a sample of that in the coming pages.

Inverse Square Law
As referenced here regarding flash illumination, the inverse square law dictates that at twice the distance, one quarter the amount of light will fall. So, if your subject is 8 inches away, and there is a lovely flower behind the subject at 16 inches away, the flower will be two stops underexposed (receiving one-quarter the amount of light) relative to the subject.
If the flower were 32 inches away, then only 1/16th the amount of light would strike (a 4-stop underexposure), providing virtually no illumination to the flower. So, the lesson here: Wherever possible, keep the background close. Move your position slightly to place some foliage behind your subject. It can make a huge difference. Or, even better, set up another flash to illuminate the background.

Next: Improving Flash Light

Copyright 2006 Eric Delmar