In PJ circles, there's a perpetual discussion about whether we "make" or "take" images. I acknowledge this debate and must suspend it for this post. Today, we must apply the "make" paradigm in order to explain this important compositional concept.
When painters set a new canvas on an easel, they must make several choices before they begin work. They often decide a color pallet, subject and approach.
While there are exceptions, most paintings begin with a background. The artist may paint the sky, mountains or trees. Then, they apply additional layers of information leading toward the foreground elements. It would be nearly impossible to begin with blades of grass in the foreground and fill the background between them.
Photographers are presented with a new canvas each time they prepare to make an image. Like the painterly artists, it's best to build the image from the back to the front.
When arriving at a location, immediately scout clean backgrounds. We look for evenly lit, typically monochromatic areas to place subjects within. The size of the background area often determines which lens to use.
If we're using the open sky as a background and can get close to the subject, a wide-angle lens is a good choice. If background options are nothing but clutter or access is difficult, we'll often reach for a long (telephoto) lens.
In both cases, the background combined with the foreground determines the exact lens approach to the subject. Either way, we have a clean background upon which to place the remainder of the image.
Select light intensity
Light intensity is often the predominant issue when selecting appropriate backgrounds. We tend to look for backgrounds with equal or less light than the foreground elements. Backgrounds with more than three stops difference (darker) are called "low key" backgrounds. The light intensity difference allows the subject to pop off a muted background.
However, sometimes we'll opt to use a high key background to get a different feel for an image. A "high key" background is often five or more stops brighter than the main subject. When the main subject is properly exposed, most of these backgrounds appear white. As a precaution, it's best to use fill flash when shooting against most high-key backgrounds.
Whether we use darker or lighter backgrounds, the important issue is to have sufficient difference in light intensity to isolate the subject.
The light intensity on a background can also add to or negate the effects of color. For instance, a black wall with five times (five stops) more light than the foreground element appears muddy white. Consequently, it's important to also factor in background reflectivity to the light intensity to achieve desired results.
Contrast is the difference between two subjects. In simple terms, contrast is the difference between the reflectivity of black and white. In terms of color, it's also the difference between a color and its complementary color.
Surfaces with high reflectivity are often in contrast with surfaces with high light absorption. For instance, black velvet absorbs light while white ice reflects light. Although these two surfaces can be in the same (harsh) light, the contrast between the two surfaces can exceed the dynamic range.
Likewise, two subjects in the same light that reflect complementary colors from a color wheel are in contrast (red/cyan, blue/yellow, green/magenta). The complementary colors of brown, red and yellow (common skin colors) typically complement the blue to cyan color groups. This is why bluish backgrounds are often chosen as backgrounds when photographing humans.
The crux of this post has been to understand backgrounds are frequently the first compositional element chosen by PJs. Once a background is selected, PJs can move until the subject aligns with the background. Then, additional layers of information can be included via lens selection, angle and position.
While the subject of the image remains the most important factor, the background is often the difference between a successful, professional image and an amateur snapshot. When images are designed and composed from the back to the front, they're more successful.
Enough for now,