AMBIENT ILLUMINATIONAmbient light is probably the most common source used when photographing macro subjects. Why? Because the idea of deliberately shaping, modifying and augmenting light on this small scale is daunting to many photographers. But, it need not be intimidating. In fact, the small scale of macro photography can sometimes be an advantage to photographers who might be uncomfortable working with the larger lighting equipment necessary for full-size subjects.
When it comes to plain old ambience, though, there are some deliberate things to look for. First, a bright sunny sky is going to make strong shadows—which might mean that you're more likely to cast a heavy shadow (from your body or your camera) right onto your subject. It also makes for deeper shadows in the scene, even when you're able to keep your own shadow out of the frame. A lightly overcast sky can be a little more manageable for more even, soft illumination. It's also more forgiving if you happen to cast your own shadow on the subject, and it's not going to require quite as much deliberate positioning in order to find a pleasing angle. That said, it can also be a little too flat, and a little too cool, so you may want to improve on flat ambience with a reflector, diffuser or flag.
REFLECTORS, DIFFUSERS AND FLAGSWith flat ambient light the addition of a reflector—maybe a silver reflector for a little power, or a gold reflector that also adds a bit of warmth—can help to create an attractive ratio with light that helps to define the shape of the subject. Too-flat lighting simply can't show shape. A small handheld reflector that collapses into a pocket-sized bag is all it takes; after all, with a subject so small you don't need much.
Flags are also especially helpful for giving light some shape. Placed close to the side of a brightly-illuminated subject, a black flag will add a bit of dark edge definition that helps to add shadow—and therefore shape—to the subject. This adds to the illusion of three-dimensionality in a scene, and that's the best trick any photographer can pull off.
Diffusers are also important for macro lighting, though not when the light is already soft on an overcast day. When the light is bright and sunny, using a silk diffuser to tame the contrast and create a soft, pleasing shadow over the subject can be the difference between a contrasty snapshot and a well-crafted macro work of art. One nice thing about diffusers is that, since they're white, they also can double as a white bounce card for soft fill light. That's two modifiers in one.
All of these modifiers—flags, diffusers and reflectors—also serve another function on an outdoor macro set: they can each be used to block wind. With a delicate flower serving as subject (or even the platform on which your insect subject is positioned), the slightest breeze can make macro photography a trying chore. Position a flag or diffuser just so and the subject may stand perfectly still. For even more stability—for modifiers or the subject itself—consider a clamp like the Wimberley Plamp to affix subject or reflector to your tripod leg in order to keep it from blowing in the wind. It's like an extra hand—which can be as invaluable as it sounds.)
FLASH FOR FILL OR KEYYou can always bring your own. The ideal macro flash is designed specifically for the purpose, by wrapping around the front of the lens to provide even, direct illumination that emanates from the lens itself. And while these macro flashes are often ideal for macro photography, you don't have to have a macro-specific flash to use flash in your macro photographs. A plain old hotshoe-mountable flash will also work well—but only if you don't mount it to your camera's hotshoe. A flash positioned on the hotshoe is likely to miss a macro subject entirely (unless you bounce it from a reflector or fill card—a totally viable option—or use a device like an Orbis Ring Flash diffuser to turn a normal hot-shoe-mounted flash into a ringlight) if it's only a few inches from the lens. Instead, tether that flash to a TTL cable and hold it in your hand, or clamp it to your tripod leg, in order to position the light where it will work to show shape while illuminating the subject. That could be frontal, it could be a sidelight (great for upping the drama), or it could even be a strong backlight. A backlight like this is a great way to add depth to a scene, especially with a translucent subject such as a flower petal. Backlights can create rim light that set off the subject from the background, and it can make colorful foliage or flower petals practically glow.
No matter where that flash might be positioned, it's the power of the flash—and its ratio as compared to the ambient light—that will have a huge impact on the look of a scene. For instance, with a frontal flash on a bright and sunny day, a low-power fill flash setting will simply bring up the detail in the deepest, darkest shadows, helping to bring the contrast of a sunny day scene into a more presentable form. That's what fill flash does, by definition. But the same flash in the same position set at a higher power—a ratio that makes it the main light—will make the whole photograph look much different. There's no right way or wrong way but, in general, I find it's good policy to start with the ambient and only add as much flash as necessary to make a shot work. It's a good way to ensure the light is, if nothing else, interesting.
All of these flash options are great ways to augment ambient light. But flash isn't the only choice for lighting macro subjects. LED lights have become very popular in recent years because they're cool to the touch, very compact and some of them even allow you to change white balance from warm to cool to match daylight, indoor tungsten or any combination in between. That makes hand-holdable LED lights an ideal option for another kind of macro shot that frequently takes place indoors—food photography. If you're a foodie or a blogger, or maybe you simply like to document your culinary delights for your Facebook friends, almost all the light you need can be had in a compact LED. LitePanels offers a variety of adjustable, high-quality LED lights. Other companies—like Olympus and Joby—even offer super-compact LED spotlights attached to bendable arms absolutely perfect for macro lighting tasks. Sunpak makes a ring-style LED light, perfect for getting super-close to a small subject. Whichever LED you choose, remember that it's the placement of that light that will have a huge impact on how it affects the scene.