3.3 - Balancing Flash/Ambient Indoors

In last week’s post I discussed the basics of exposure with ambient light (i.e. without the use of a flash).  I wanted to give a basic overview of exposure before jumping into balancing multiple exposures.  As the Lighting 102 course explains, when shooting with ambient light the idea of exposure is fairly rigid.  There is correct exposure for a given setting.  Straying much from this correct exposure and the photo becomes either under- or over-exposed (too dark or too light). 

In contrast, flash photography has multiple exposures: an exposure for the ambient light and an exposure for each flash used.  The idea of “correct” exposure gets a little murkier because each lighting source has it's own exposure.  Is the “correct” exposure based on the overall ambient light exposure, the subject, the background, or any combination of these?  There isn’t really a simple answer as it is completely up to the photographer to decide what they are trying to create.

As a general rule of thumb for any photo is that the main subject should be fully exposed. That is to say that if the subject is a person, the person’s face should be properly exposed.  The exposure of everything else in the photo is then free to be manipulated to the desires of the photographer.

The following exercise will illustrate the balancing act between flash exposure and ambient light exposure with a simple one-flash setup.  The aperture, ISO and flash power are held constant (f/5.6, 200, ¼ respectively) and the shutter speed alone is used to balance the exposures.   Remember that aperture affects both the ambient and flash exposures; shutter speed only affects the ambient exposure.  Therefore by modifying only the shutter speed the flash exposure remains constant but the ambient exposure can be modified.

The first shot of the exercise is to the left.  I think it's a wonderful example of taking the ambient light out of the equation.  This is a base line for the exercise, and assures that no ambient light will be affecting the shot as the flash is first introduced.  This wonderful first shot is taken at 1/200 at f/5.6 and the ambient light is approximately 6 stops under-exposed.   How do I know that it is 6 stops under-exposed for ambient?    I took a second test shot in Av and the camera calculated that a correct ambient exposure would be at a shutter speed of 0.4 seconds.  Then it is just a matter of counting up the stops.

Shutter Speed: 1/200 second

Now the flash is added (through an umbrella).  The couches and wall in the background are being lit by spill light from the flash.  The shot was still taken 6 stops below the ambient exposure so the light has to be coming from the flash.  From this point forward I will slow up the shutter speed to start bringing in the ambient light.

Shutter Speed: 1/125 second

This is the same shot taken at 1/125 (2/3 of a stop slower).  It doesn’t look all that different from the shot at 1/200 and there are two reasons for this:

1.     Since I started so far below the ambient exposure 2/3 of a stop isn’t enough to begin to bring in much ambient light, and

2.     Shutter speed does not affect flash exposure.

Shutter Speed: 1/40 second

The ambient light doesn’t begin to be exposed until about 1/40^th of a second.   This is the shutter speed that begins to allow the ambient light to fill in the flashes shadowy areas.

To the right is the shot taken at 1/40^th of a second.  The difference between this shot and the one at 1/125 is very small but the ambient light is (just barely) beginning to be exposed.  Again, everything is staying consistent (aperture, ISO and flash power) but with the slower shutter speed more ambient light is allowed in.

Shutter Speed: 1/6 second

Here it is again at 1/6^th of a second.  You can start to see why there isn’t a single “proper” exposure for the ambient light.  The subject is exposed and the amount of ambient light exposure is completely up to the photographer’s discretion.

Shutter Speed: 0.4 second

Again at 0.4 seconds, which is what the camera calculated as the "correct" ambient light exposure.

Shutter Speed: 0.6 second

And again at 0.6 seconds.  I could keep cranking up the ambient exposure but I think you are starting to see the point of the exercise.  I hope that you can see that the idea of proper exposure is quite different when working with flashes.  As long as the subject, the cameras in this case, are properly exposed, the background exposure can be anything you want it to be.

If you decide to try this exercise out on your own be sure to set the subject in an area that is less exposed to the ambient light than the background.  Place it in a "shaded" area if you want to think about it that way.  Doing this prevents the ambient light from over-exposing the subject as you open up the shutter.  This concept works just the same outdoors where you have to compete with the sun.  Also please check out the Lighting 102 course by David Hobby here.

Below is a photo of the basic lighting setup that I used for this exercise.  I originally had the curtains opened during my first attempt, an instruction that I missed in the L102 course, and had to redo the exercise.  Hopefully you can learn from my mistake.

If you have been following my blog you may be confused on why this post wasn't about working outdoors.  In last week's post I explained the difficulties of working outdoors in Michigan during December.  In fact, the high daytime temperatures are struggling to get above freezing, and there is 6-inches of snow on the ground now.  This does not mean that I'm blowing off the two exercises dealing with balancing flash with the sun.  Instead I have something special planned for these two outdoor exercises that won't require me to suffer out in the cold. This "something special" is going to take a little bit of time, and with the holidays quickly approaching, there is going to be a small lag in the posts.  Look for the post on these exercises shortly after the New Year.

Until then,  thank you again for reading and I hope that you have a wonderful holiday season.

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