Flash with Macro Photography
Most flash units are larger then the actual object with macro photography. This would be comparable to using a 10 foot umbrella to reflect the light from the flash while you take a picture of a 6 foot tall person.
As you move closer the flash can easily overpower the exposure. Most flash units aren’t designed for Macro Photography. The flash can't turn off fast enough. You will need to experiment with using a smaller aperture by a few stops to compensate for this, or a macro flash unit. You could use a faster shutter speed but usually we are battling Depth of Field with Macro Photography and so a smaller aperture will put more of the subject in focus. This is why some photographers use macro flash set ups, a smaller aperture can be used in low levels of light, and any movement can be frozen. Watch out for the ring flashes. They can show rings of light in the eyes of the insects which isn't natural. There have been other problems of improper or unnatural looking exposures with ring flashes.
The closer you get with the flash, the more light there is, but the light gets exponentially brighter as you get closer. The closer I am, the more critical the flash exposure becomes.
The ideal angle for a flash is a 30 degree angle from the lens. So if I am 12" away I should have the flash 4" high. Move the flash closer down to the end of the lens as you get closer with the flash. Also, watch the flash doesn't get behind the lens and shade your subject! You will need a camera bracket, or make your own custom bracket. Sometimes a flash off to the side can give a neat effect instead of directly overhead.
There are a lot of specialized and expensive macro flash units available. Sometimes the brackets that are also needed for the flash units are almost as expensive as the flash unit. There are less expensive alternatives to this, such as using a diffuser to cut the amount of light your non-macro flash is putting off, bouncing the flash off of a white cardboard… Be creative!
Try just holding a white cloth single or double layer over the pop up flash to reduce the amount of light and diffuse the harsh glare of the flash. Another technique is to block the light from hitting your object directly from the flash, and hold an aluminum foil piece or reflector to catch the light and reflect it down at the angle you want.
A Gary Fong flash attachment can be helpful because it gives off a lot of diffused lighting, and its height helps get the light from the flash over the top of the lens to your subject. Also a Gary Fong decreases the light output from the flash which is useful up close.