Macro photography presents a unique set of challenges to the photographer. When overcome though, it's possible to create intriguing images of things that may be overlooked or unseen with the naked eye. The difficulty lies in the close subject-to-sensor distance, which magnifies the subject and results in very shallow depth of field.
To get as much of the subject in focus as possible, it's necessary to stop down the aperture of the lens. For fast shutter speeds to avoid blur caused by subject or camera movement, you need a lot of light. Opening up the aperture will let more light in and diffuse the background, making the subject stand out.
Flash allows a smaller aperture to be used, giving greater depth of field. The built-in flash on some cameras is best avoided as the light is harsh and doesn't allow enough control over the power of the flash or its direction...
Flash can be used as fill light to bring out the details, colours and textures of your subject, especially in the shadows. The trick to achieving a natural look is to balance the ambient light with the flash, and if done well, its use should be almost undetectable...
Consider the background
Having flash as the only light source can be especially useful when shooting handheld. The short duration of the flash freezes subject movement, allowing a smaller aperture to be used with the optimum ISO setting for image quality. The downside is that the flash will correctly expose the subject, but the background will be underexposed and may look dark and artificial.
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