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Flash ghosting comes about through the interplay of ambient light and flash lighting, causing shadows behind objects where there isn't actually a solid shadow plane for the shadow to fall on at all.

If you're mixing ambient light and flash, also known as 'dragging the shutter', and have mysterious unexplained hairline shadows beside your subject, you've got flash ghosting.

Flash ghosting on the hairs of a moth at 1:1. Flash ghosting is a problem in extreme macro, and comes about through the interplay of ambient light and flash lighting.

The ugly flash ghosting shadow behind the hairs can be avoided by using the rear sync flash mode on your setup. This means you'll need to move away from using PC-Sync, as it can't do rear sync.

Mixing Flash and Continuous Lighting

When you mix flash and continuous light, the photographer freezes the subject with the flash, but if a subject is otherwise silhouetted against a brighter background, the remaining exposure will capture strange shadows if there is any sort of movement at all. This happens in photos of people at sunsets, where the photographer has used their camera flash, and also with handheld shots at low magnification, when mixing flash and continuous light to take in ambient light. This is called flash ghosting, but goes under a variety of labels including classic ghosting.

Flash Ghosting Effect

Flash ghosting seems to be especially visible if your ambient light is at about the same level than your flash

Ghosting is an effect caused by the movement of the subject during the time that the shutter is open but the flash is not on. The flash may perfectly freeze the subject, but if there is enough light on the subject, ie if the ambient light is bright enough, subject movement either before or after the flash pulse, whilst the shutter is open, will result in either the blurred image of moving subject or a shadow caused by the dodging of the background by the subject or a combination of both. Flash ghosting is especially visible if your ambient light is at about the same level than your flash and there are basically four ways to avoid or minimise this sort of flash problem.

Prevent Flash Ghosting

First, you can avoid the problem by underexposing the background by 2 or 3 stops. This is effective but results in images where the background is black or very dark and we don't always want a dark background. It makes your images look as if they're all shot at night. Second, you can use flash as the primary means of lighting the background rather than using ambient light. This is also effective, but can only be done if the background is also shaded and, unless the background is very close, might require a separate flash remotely triggered to light the background up.

Thirdly, another method is to use the high speed sync flash function of your flash and use a fast shutter speed. This works, but may result in insect blur if you use it in field macro. The avoidance of blur was of course the reason that you wanted to use flash in the first place. Lastly, use rear curtain sync. In my case the blurring was caused by the shutter opening and having a flash pop at the start of an exposure allowed the mirror wobble to affect the image. On rear sync flash mode, with enough time to settle down, it doesn't.

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