Photography is the act of capturing patterns made by light or “drawing with light.” The description “drawing with light” is an appropriate one, since light rays entering the camera create a latent image on film by chemically changing the silver particles in the film’s emulsion. In a digital camera, the entering light rays cause electrical changes on an array of electronic sensors. Let’s look at how and why this happens.
Basic Light Theory
Light is defined as a form of electromagnetic radiation, or radiant energy, visible to the human eye. The largest producer of this energy is the sun. On a smaller scale, fire and artificial light, both fluorescent and incandescent, are examples of radiant energy.
When light is emitted from a source, it moves away or radiates in straight lines in all directions (see the sun below). The light ray moves in the form of a wave. Visible light moves at a speed of 186,000 miles per second. This is why a room appears to instantly fill with light when you turn on a light switch.
Light waves have two characteristics that can be measured. The wavelength is the distance from one crest of one wave to the crest of the next, while the frequency is a measure is a measure of the number of waves (cycles) passing a given point in one second (see graphic below). The unit of measurement for frequency is the hertz while the unit of measurement of wavelength is the nanometer.
WHY DOES THIS MATTER?
Light makes up only a tiny portion of the entire spectrum of electromagnetic radiation (frequency fiqure below). The portion of light that the human eye can detect or “see” is referred to as the visible spectrum. It consists of waves with frequencies ranging from about 400nm to 700nm. Our eyes see the different wavelengths as different colors. Structures in the eye called cones are receptive to three ranges of wavelength: 400nm – 500nm (blue), 500nm – 600nm (green) and 600nm – 700nm (red). Each type of cone is receptive to only one range of wavelengths, but the combined effect of the cones provides us with a full rainbow of colors. Although we are able to distinguish many different colors, it is usually divided into the seven traditional colors: red, orange, yellow, green, blue, indigo, and violet.
The wavelengths for some distance on either side of the visible spectrum are not normally visible to the eye, but can be recorded on film. Infrared wavelengths are longer than 700nm, so they are below red while ultraviolet wavelengths are shorter than 400nm. Film can be made sensitive to infrared radiation. Normal film is quite sensitive to ultraviolet (UV) wavelengths. Short wavelengths such as ultraviolet are more readily scattered in the atmosphere than longer wavelengths. This can create a haze (on film) which can obscure details in the distance. But by using a UV filter, the short-wavelength radiation is absorbed, eliminating the haziness.
The Color of Light
When light is composed of red, green, and blue wavelengths in approximately equal proportions, it is said to be “white light.” Such light is created by the midday sun on a clear and cloudless day. In contrast, the warm light of early morning and late afternoon is strongly red-orange in color. Different types of artificial light have different colors although the human eye can adjust to the differences and see them generally as “white” light.
The effect of the color of light on film is more obvious with color emulsions, since film can not adjust like the humane eye to see a range of colors as “white.” For this reason, color film emulsions are balanced for the type of light in which the film will be used. Film that is daylight- balanced renders colors normally when exposed with natural light or electronic flash. Tungsten-balanced film reproduces colors realistically when the light is provided by incandescent bulbs. Daylight film used under tungsten lights produces pictures with a strong red-yellow cast while tungsten film exposed under daylight conditions show an overall cold blue shade.
The effect that the color of light has on black and white film is very slight. Black and white film is panchromatic, meaning that it is sensitive to light in the blue, green, and red wavelengths. Even though panchromatic films are able to respond to light across the entire visible, they do not “see” various colors in the same way that the human eye sees them. Human vision is very sensitive to green wavelength and about equally sensitive to blue and red wavelength. Panchromatic film is slightly more sensitive to blue wavelengths and has equal sensitivity to green and red wavelengths. This difference in sensitivity often results in colors that appear quite different to our eyes being reproduced on black and white film as almost identical gray tones. This is especially true for the colors red and green. The solution is to use a contrast filter that makes the film emulsion render the colors as distinctly different. Except on those rare occasions when we include the sun for another light source in a picture, all of our photographs are made with reflected light. Light rays from the source bounce off an object and pass through a camera lens to be focused on the film plane. How the film records those reflected rays of light can be controlled by the use of filters.
When light strikes an object, one of three things will happen. The light can be absorbed, transmitted or reflected. How much of the light is absorbed, reflected or transmitted depends upon the material from which the object is made, the type of surface finish, and the object’s color. Material that is transparent or translucent reflects very little light. Most is either transmitted or absorbed. Opaque materials, those that do not transmit light, absorb or reflect light in different proportions, depending on their surface finish. A dull or rough surface absorbs a high percentage of the light rays. The reflections from such a surface are diffused (reflected in many directions). A surface that is smooth and polished will reflect a very high percentage of the light and will reflect the rays in an orderly and concentrated manner. Such reflections appear as bright spots of light and are referred to as specular reflections. Even materials that transmit most of the light that strikes them, such as glass or water, produce specular reflections from their smooth surfaces.
Objects that are white or light-colored reflect light rays readily; black or dark-colored objects absorb most light rays. Our perception of an object’s color is based upon the wavelengths of light that are reflected from that object.
white balance
Preset White Balance Settings
Auto – this is where the camera makes a best guess on a shot by shot basis. You’ll find it works in many situations but it’s worth venturing out of it for trickier lighting.
Tungsten – this mode is usually symbolized with a little bulb and is for shooting indoors, especially under tungsten (incandescent) lighting (such as bulb lighting). It generally cools down the colors in photos.
Fluorescent – this compensates for the ‘cool’ light of fluorescent light and will warm up your shots.
Daylight/Sunny – not all cameras have this setting because it sets things as fairly ‘normal’ white balance settings.
Cloudy – this setting generally warms things up a touch more than ‘daylight’ mode.
Flash – the flash of a camera can be quite a cool light so in Flash WB mode you’ll find it warms up your shots a touch.
Shade – the light in shade is generally cooler (bluer) than shooting in direct sunlight so this mode will warm things up a little.
Custom White Balance is always the best option. The method/options to create a custom setting differ from camera to camera.