In a perfect world the natural light would always be beautiful and we would never need to change it. Too bad we don't live there. You can OFTEN improve the lighting balance on your subject and background when working in close-up photography. I think that close-up photography offers more opportunity to manage the light than any type of photography that is done outside the studio. Remember the Inverse Square Law? In the examples I gave we measured light travel in feet and talked about how it falls off as it moves out from the source. In close-up photography we talk about measuring light travel in inches not feet but the same principles hold. When you are lighting things that are inches from the flash it does not take much power and you have LOTS of control within arms reach.
So what do you need to get started? Digital camera with a hot shoe flash that will work OFF the camera. Yep, that is the first necessity - get the flash off the camera. Many of the newer flash units work off camera wirelessly but an old fashion hot shoe cable will also work. I find the off camera cable to be so easy to use that I still use it even though my flash and camera will work wirelessly. Flash power is not a major issue because you are working so close to the subject. Instead of looking for a powerful flash, be sure to get one that gives you lots of ability to control the power and to turn the power down.
Lets start with the easy stuff - fill flash. How many times have you found a great subject, flower or insect, but had the light coming from behind. With back lighting you usually end up with the subject underexposed and the background correctly exposed or the subject exposed correctly but surrounded by a badly overexposed background. Fill flash to the rescue!!
Pop on a flash and place it about a foot from the subject and to the right of the camera. Turn the flash down to about -2 stops using exposure compensation on the flash unit. Set the flash mode to Auto TTL. The result is dramatic!!
Lets first think about the lighting on the subject verses the background in both images. The background lighting is the same in both images so the flash had no visible effect on the background. One reason for that is that the flash was 1 foot from the subject but 5 or 6 feet from the background. The Inverse Square Law states that every time the distance light travels doubles, light falls off by two stops. Based on that, the light that the flash put on the background would be 4 to 5 stops less than the light it put on the subject. Not really noticeable.
Why not just leave the flash on the camera? Any flash is powerful enough to light this insect from 3 feet away. I want the flash off the camera because I want more control. I want to control the direction of the light. I also want to control the lighting ratio between the subject and the background. Here is the second big thing to think about - you control exposure for the subject with the flash, you control exposure for the background with your selection of aperture and shutter speed. You need to think about the two as completely separate exposures taken at the same time.
Here is another example to illustrate the level of control you have using flash for closeup photography.
The first image was shot without flash at f22 with a shutter speed of 1/6 second. It is badly underexposed for the subject. The second image was shot at f16 with a shutter speed of 1/25 second with flash. The flash correctly exposed the subject. The aperture/shutter speed combination allowed for 1 stop less background light in the second image (aperture was +1 but shutter speed was -2). Notice the difference in the background exposure? You are making two independent exposure decisions when you use flash in closeup photography. I held the flash less than a foot from the subject. It was set to auto TTL mode and I dialed down exposure compensation on the flash to -2 1/3 stops. The flash correctly exposed the subject then turned off. The background was exposed based on the aperture/shutter speed combination.
Here is a more dramatic example of the same subject. In this example I kept the flash set to -2 1/3 stops but changed the aperture/shutter speed combination to f45 for 1/13 seconds. That is 3 stops less than the original exposure and 2 stops less than the second exposure. Look at how much control you can have over the lighting ration between subject and background!
So here is the third thing to think about. In both of the examples I have had a subject where the background was several feet from the subject. I placed the flash a foot or so from the subject, exposed the subject with the flash and the flash had very little or no effect on the background lighting. What happens when the background is closer to the subject as in 1 to 1 1/2 feet from the subject. When the background is a foot or so from the subject you have to do something that very few photographers do. You have to get the light VERY close to the subject, I am talking 3 to 5 inches! When I shoot flowers I am always on a tripod. I hand hold the light while looking through the viewfinder and put it as close to the subject as possible without getting the flash into the image.
Think about this - if the flash is 2 feet from the subject and the background is 1 foot behind the subject then the light on the background from the flash will be about 1 stop less than the light on the subject. If I move the flash to 6 inches from the subject I can lower the light from the flash on the background to -3 stops. That is lots of control! Here is an example where the background is close to the subject and is brightly lit.
I think the background is a little distracting in this image. My solution was to add flash placed very close to the flower. The first image was shot at f16 with a shutter speed of 1/15 second so I increased the shutter speed to 1/60 second. This darkened the background by two stops and made the flower stand out from the darker background.
Here are a few technical notes that I should leave you with. I am a Nikon user so most of what I talk about is based on my experience using Nikon camera and flash units. The Nikon flash is set up by default to correctly expose for the subject without trying to manage the background. If I want the system to try to balance lighting between subject and background I have to put the flash in a special "BL" mode. My experience with Canon flash units is that they are by default in a balanced lighting mode so you may need to set them up differently to make all this work properly. Check you flash instructions but you may have to put your flash in manual mode and set the power yourself. I am sure that some of the other camera systems from Pentax, Olympus, Sony, and the list goes on....may also have different requirements.
For camera setup, I usually go to manual and set both aperture and shutter speed manually. The flash is in Auto TTL mode and it handles exposure for the subject as I am selecting aperture and shutter speed combinations to get the look that I want for the background. My selection is seldom what the meter system recommends for correct exposure.
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