One of the most important rules for macro flash photography is balance. For natural looking macro images you have to balance the ambient light and flash output. When the flash and ambient light are balanced the use of flash will not even be apparent to the viewer.
The problem is that with flash output overpowering the natural light in background it will underexpose and go dark, in some cases like the image below, it can underexpose to the point that is appears black.
Since the flower in the image above was actually surrounded by bright green foliage lit by the midday sun it is a great example of how not to shoot macro with flash. The problem is that the flash was so much stronger than the ambient light the correct exposure underexposes the background by at least 2-3 stops. In this case the exposure setting and flash output is correct for the subject but not the background. This kind of problem usually comes up when using auto flash mode combined with a small aperture like f/16.
This image above is true to life with the bright green background in natural light. The reason this image has a balanced natural looking background is because of the aperture and corresponding lower flash output. With an aperture of f/8 and flash output to match, the exposure is correct for both the subject and background lit by natural light. The use of a middle aperture allows you to use a low power output setting that balances with the natural light.
Another way to lighten the background and balance flash and natural light is by increasing the exposure time with a slower shutter speed. I don’t recommend it since a slow shutter speed will cause ghosting. Ghosting is blur caused by the ambient exposure appearing as a soft second exposure that can destroy sharpness. This is a common issue that most photographers are not even aware of.
This image below is a 100% crop of an image made at 1/60th of second to show softness caused by ghosting. A soft ambient image, or ghost image, blurs the sharper flash exposure in the same frame. Think of a image with ghosting to have a double exposure, one with flash and one ambient.
Compare the image above to the image below made below made at 1/200th.
The difference in sharpness is easy to see at 100% actual pixel view. The reason the image made at 1/200th is a lot sharper is because the faster shutter speed does not allow the second ambient ghost exposure to effect the image.
Ghosting is not always easy to spot in the field so make sure to check your image at a high magnification level to make sure your image details are sharp and ghost free. All of the Sigma macro lenses currently available are capable of resolving extremely sharp details from wide open down to f/16, even on a 36 megapixel full frame DSLR. So its important to make sure your images are not suffering from the sharpness destroying effects of ghosting.
Using the fastest shutter speed possible, 1/200th or 1/250th for example, should eliminate ghosting in a typical macro shooting situation. This is what I use as my default shutter speed when shooting macro flash. If you are shooting in bright light, a high ISO, or a large aperture this might require an even higher shutter speed in the high speed flash sync range to eliminate ghosting entirely. In any case make sure you to check closely for ghosting to make sure that you are extracting the maximum sharpness your macro lens is capable of.
If you have any questions or comments be sure to share ‘em in the comments section below.
Robert O’Toole is a Sigma Pro and has been a professional photographer for more than 20 years. As an accomplished instructor, Robert leads photography workshop tours across the US and internationally. For more info visit Robert’s web site at robertotoolephotography.com