Flash Exposure – Adjusting for Black Background
The first few exposures I did with handheld flash with macro were a load of fun. I seemed to have the best of two worlds. I could work with a small aperture and also a faster shutter speed. All of the sudden the macro subject was exposed and could be frozen at 1/200 and F16.
I soon started to wonder why the background was dark or black with these flash enhanced images. What happened to the pleasing out of focus bokeh?
Fortunately the solution was to try to balance the background ambient or available light with the flash. The idea is to set an exposure for background and then use flash to expose the subject. It is an iterative two step process, well three steps if you count the Rinse and Repeat Step 3.
Why not one exposure? Shouldn’t the flash expose the whole image? The flash has limited strength or range. When I use a fast shutter speed and ridiculously small aperture setting the flash is even more constrained in its range. The concept is similar to being in a large stadium, sitting miles from the playing field and hoping your flash will illuminate the action on the field. Of course it can’t and is only effective for a few meters near the camera. With macro, the flash works in centimeters not meters.
The photos of the bee may help to demonstrate. The bee was sleeping on the bottom side of a low lying flower. Since the bee and flower were relatively stable I experimented with a longer exposure time in an attempt to catch some of the background color. The length of the exposure is all that changed between exposures, the f/stop remained constant. The dark back ground had a shorter exposure time (1/200 sec.) and the brighter background was longer (2.5 sec.). I thought of the photograph as two exposures – one for the background exposed by natural light and the other for the bee exposed with flash.
The manual process employed was as follows:
1. Determine Background Exposure - I set the exposure for the background and locked that into the camera. With macro work, I tend to shoot in manual mode. So locking in was a matter of setting the shutter speed to the proper value for an aperture of f16. I now ensured that the background was exposed to my tastes. With that part of the photo covered I concentrated on the bee.
2. Expose the Subject by Moving the Flash Closer or Farther Away - I used manual flash to expose the bee. Manual flash, sounds intimidating and it is not. Manual exposure means moving the flash closer and farther away from the bee and changing the angle of the flash.
The flash strob was being moved into many positions until I was satisfied. Because exposure was set for the background in Step 1. I did not have to worry about that part of the photo exposure changing – just the bee. Sort of a “set it” and “forget it” approach to the background bokeh.
3. Rinse and Repeat - After a few trial shots (chimping) I had enough for my tastes. My camera’s lcd monitor allowed me to verify the exposures and let me know when I was done with Step 1. and Step 2.
Below are two additional reference and links to popular flash tutorials. While not dedicated macro articles – the basic concepts may help to reinforce some ideas. These articles have helped me adapt flash for my macro and non macro images.
http://strobist.blogspot.com/2006/03...flash-and.html
http://strobist.blogspot.com/2007/09/lighting-102-balancing-light-twilight.html
a) If the bee was too dark, or under exposed, then I moved the flash in
b) Too washed out, or over exposed, then I moved the flash father away
c) If the light was too harsh on the photo then I changed the angle and position of the flash
