The Basic Mechanics of Capturing Light
When the light of a scene enters the camera lens, it is dispersed over the surface of the camera’s image sensor, a postage-size electrical circuit containing millions of individual light receptors. Each receptor measures the strength of the light striking it in a metric called “lumens.” Each receptor on this sensor records its light value as a color pixel.
The camera’s image processor reads the color and intensity of the light striking each photoreceptor and maps each image from those initial values, producing a reasonable facsimile of the original scene. When this bitmap of pixels is viewed from a distance, the eye perceives the composite as a digital image.
Two important light issues must be addressed when capturing a photograph: light dynamic range (exposure) and color balance (temperature).
Light (Dynamic) Range
The full range of light that exists on a sunny day is virtually impossible to capture with your digital camera. Light range is defined as the dynamic difference between direct sunlight and absolute darkness.
Even though the image sensors in today’s digital cameras continue to improve, corralling all of nature’s dynamic range of light remains a futile challenge.
This statement is easier to understand when you consider the fact that camera sensors register far less light than the human eye, and not even the eye cannot tolerate unfiltered exposure to the sun. Any more than a couple of seconds of direct sunlight will absolutely damage several different parts of the human eye.
Fortunately, your body won’t allow you to stare directly into the sun for any longer. The light emitted from the sun is the strongest, most brilliant, and purest form of light in the universe. Luckily, very few scenes you will want to capture with a camera will involve shooting directly into the sun.
Most cameras have an automatic program mode that adjusts the three settings in the camera that affect exposure: f-stop, shutter speed, and ISO. The A/Av (Aperture Priority) mode allows you to set the size of the lens opening (f-stop) while the camera automatically sets the shutter speed. The S/Tv (Shutter Priority) mode lets you set the duration of the shutter opening (shutter speed) while the camera automatically adjusts the size of the lens opening.
Your camera’s ISO (International Standards Organization) setting adjusts the light sensitivity of the camera’s image sensor, allowing you to capture scenes in dim or bright light.
Your camera provides a small graph, called a histogram, that roughly indicates how well the camera is set to capture the scene. This graph displays the range of light coming through the lens and approximates the current light distribution that will be captured under the current settings. By adjusting the three settings mentioned above, you can shift and somewhat distribute this range of light so as to best record the full range of light.
My best advice about using the histogram in your camera is to adjust the big three controls (aperture, shutter speed, and ISO) to center the graph on the scale. While this won’t guarantee the most dynamic use of the tonal aspect, it will give you the latitude to adjust that tonal information in post-processing.
But remember, the histogram reads light range but doesn’t address the issue of color balance at all. Which brings us to the next point.
Setting your camera to capture the correct color of light is not as simple and straightforward as it might seem. It is certainly not as automatic as the Auto White Balance setting might suppose. You should understand what your camera means by AWB before you bet your pictures on it. I’ll explain this in some detail below.
Even if you are shooting in RAW mode, it’s a good idea to evaluate the light temperature of the scene and set your camera accordingly. RAW processing will allow you to change the color balance in post-production, but good estimating will certainly shorten the processing time.
Color of Light
Every scene’s color cast is influenced by the temperature of the light illuminating that scene. When the scene is captured outside, the sun’s position in the sky and the influence of cloud cover alters the colors of the light. Your camera offers two ways to compensate for the differences in color temperature (White Balance).
Auto White Balance
The Auto White Balance (AWB) sensor in your camera will seek any prominent white or neutral subject in the scene and will shift the entire color balance of the scene in an effort to neutralize the tint of that element. The assumption when using the AWB setting is that you (1) desire the current lighting condition to appear perfectly neutral in color, and (2) are certain that recognizable and visible elements in the scene are truly neutral in color.
Any cloud cover interfering with the sunlight will have a slight influence on the neutrality of 6500° (natural sunlight) lighting. Once again, be aware that AWB takes that slight shift in color out of the equation. Most of the time, this is a good idea. But if early morning or late afternoon (golden hour) lighting is to be recorded accurately, that AWB setting will try to neutralize those warm colors and you will, therefore, lose the mood.
Preset Color Balance Options
Your camera offers several presets to offset any known color casts caused by specific lighting situations.
Daylight sets the camera to record scenes under typical midday outdoor lighting. For outdoor photo situations under normal weather conditions, Daylight is a safe bet. The Daylight setting’s color temperature is balanced for sunny days and photos that are taken in direct sunlight. Daylight is the most neutral of the three outdoor settings and provides rich, natural-looking colors on sunny and even partly-cloudy days.
Shade shifts the colors toward orange to compensate for the bluish interference of nominal clouds. When the weather conditions are normal (sunny) but your subject is located in the shade, the scene’s color temperature actually changes slightly. If the camera’s white balance setting is Daylight, and the people are in the shade their skin tones will lack the sun’s warmth because the sun’s rays are not hitting the subject directly.
Cloudy offers a yet stronger orange shift to compensate for completely overcast (stormy) skies. When the weather conditions are overcast and clouds block the direct sunlight, the bluish color of the clouds actually removes the warmth of the sun.
On the color wheel, blue is directly opposite yellow. When these two colors influence each other, they dull each color’s intensity. When the blue-gray clouds block the sun they diminish the sun’s (yellow) warmth. For this reason, the Cloudy White Balance setting introduces more color warmth into the scene.
Flash provides a very similar color temperature lighting as Daylight and is intended to prepare the image sensor for artificial daylight or “Speedlight” type flash devices.
Tungsten shifts the colors toward the blue end of the color range to compensate for the warmer shift of incandescent lights.
Fluorescent attempts to compensate for the greenish cast of gas-charged fluorescent lights.
All of these presets attempt to correct non-neutral lighting conditions.
Custom White Balance
Among the other color balance settings, your camera offers a custom lighting (Custom White Balance, PRE on Nikon cameras) setting. With that option selected, you hold a neutral gray (18%) card in front of the lens and press the shutter button. The camera will read and lock in the color temperature of the light reflected from that card. That reading you take will now become the standard for the camera’s white balance until you select another setting.
The takeaway truth here is that you have very powerful exposure and color balance tools at your disposal. Whether you shoot with an expensive DSLR or a smartphone, take your art seriously. Invest the time to learn something new about photography every day.