Sunday, March 22, 2009

Combining flash with ambient exposure

I've always heard that the best way to learn something is to teach it. I've been concentrating on techniques related to balancing flash and ambient exposure lately. My recent (March 5) post, "Control flash for snappier images" made its way into a newsletter article for my photo society this month. I have two classes coming up in May, where I'll include this idea. Needless to say, I've been thinking about this concept more than usual.

My friends in the Grand Lake Camera Club in Grove, Oklahoma, asked me to come speak at their meeting last week. I talked about it there, as well. But, based on some of the feedback I received, I think I still have more to learn. To be honest, as with many things I do in photography, I've taken concepts I've heard or read about and sort of "intuited" how to accomplish them. Trying to teach others, however, forces me to think through them more carefully than is my habit.

One of my friends from the Grand Lake club asked some clarifying questions by email. For instance, what flash sync speed she should use? Based on what I thought I had presented, this shouldn't have been a consideration. So, I must have missed the mark, at least a little.

In my talk, I think I described both "fill flash" and "dragging the shutter." It appears that I placed all the techniques for balancing flash with ambient light under the umbrella term, dragging the shutter. In considering my reply to her, I think I arrived at a more useful summary of the lighting approaches you can employ, using only your camera and either on-camera or external flash for more interesting photographs.

In planning your exposure—you do plan, don't you?—you have all the exposure tools your camera offers to capture the existing light. You also have a pop-up flash on most cameras, or you can attach an external flash.

For any shot, unless you're using automatic settings, you will decide on your primary source of light and set exposure for that light source. In a studio, this would be termed your key light. To simplify the discussion, flash can be thought of as either on- or off-camera. So, you will plan an exposure using ambient light or flash. In some cases, either will do. However, if you're near the extremes of bright or dark, you won't have a choice in the type of light is primary in the exposure. But you can choose to add the other type of light, and this will usually produce a more complex and, I would argue, pleasing image.

So, your options for exposing an image with typical equipment are the following:

  1. Normal to bright ambient light:

    • Ambient light only, with no flash

    • Ambient light with some flash mixed in ("fill flash")

  2. Normal to dim light (or darkness)

    • Flash-only image

    • Flash with some ambient background exposure ("dragging the shutter")

The amount of light available will determine which strategies you can use. By combining ambient with flash light, you are reducing the amount of contrast between the brightest and darkest areas of your image.

In normal to bright light, add in fill flash by forcing your flash to fire. If the ambient light is too bright for flash to have any additional impact, adjust to the lowest ISO, and you might even need to add neutral density filter(s) to lower your how much ambient light your camera sees. The flash added back in can then be balanced with the ambient light.

In normal to dim light, add in ambiently-lit background by adjusting the ISO upward till you can get the amount of exposure you want, using normal or near-normal camera exposure settings (shutter/aperture). The flash portion of the exposure will be determined by through-the-lens (TTL) flash metering. Adjust the amount of flash using flash exposure compensation, and adjust the background with aperture and shutter.

Even though the techniques seem very similar, bright or dim light will determine your starting point, when you select the primary source of light appropriate for the scene. The easy way to think of it is this:
  • In bright light, use camera exposure and add fill flash to fill in shadows.

  • In dim light, use flash exposure and drag the shutter (boost ambient exposure) to add some background.

These concepts may seem complicated at first. Once you start to employ them and see their effect in your images, you'll find that they're not so complicated, after all. They often increase the quality of the images where you use them.

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