In the world around us light comes to in varying intensities and objects have differing degrees of reflectivity and light absorption. Black absorbs, white reflects, different surfaces have varying reflecting or absorbing qualities. An average scene has a range of tones from dark to light. A major problem for photographers is that often what our eye is able to see is more than our cameras will be able to comprehend. The human eye can simultaneously comprehend detail in the darkness of shade and the brightness of direct sunlight on a white surface. In the same scene a camera’s sensor often cannot deliver both the detail found in the shade and the high light of a white surface in direct light. By understanding light in it’s varying intensities we can control and utilize light for our own artistic purposes.
Exposure is how much light we allow to fall on our sensor when we take a picture. Correct exposure is usually a subjective evaluation. If a given image appears too dark, then we have under exposed the image. If an image is too light then we have over exposed the image. We can increase or decrease the volume of light hitting our sensor (or film) by adjusting our f stop, shutter speed or ISO. For a very technical explanation of exposure check out http://en.wikipedia.org/wiki/Exposure_(photography)
Contrast is the difference between the darkest tones in a scene or image and lightest tones. In the above images there is too much contrast. Detail is lost either in shadows if we adjust for the high lights or in the high lights if we adjust for the shadows. Contrast is highest in direct sun light at mid day and is lower on overcast days and at the beginning and end of the day.
Shooting in the shade at midday is often the best light for a portrait.
Images that lack contrast look flat, murky and drab. Having a rich tonal range can make an image pop and make all the difference in an image’s success. By learning to use light and to adjust images for the tonal range you want, you can create images that please the eye and give visual impact. The image below was adjusted for dramatic contrast, letting some areas go to pure black.
As photographers we have many options open to us for how capture and render a given scene’s contrast and tonal range. At the time of making an image, we can often employ a range of techniques to increase or decrease the tonal range and contrast of a subject we are photographing. Contrast can be decreased with reflectors, fill flash, shooting on a overcast day, shooting in the shade, shooting at the beginning or end of a day instead of the middle of the day, blocking or defusing light hitting your subject, how you position your subject in relation to your light source, and many more. Now with programs like Photoshop, we can control contrast in a number of ways with levels and curves, masks, layer blending, HDR, selectively lightening and darkening areas of an image.
Dynamic range is your camera’s ability to record details between the shadows and high lights of an image. There are many scenes where your camera will be unable to record both the detail in the brightest areas and the darkest shadow details. Usually it is best to base your exposure on capturing details in the highlights and let the shadow details go to black. Color slide film typically has a dynamic range of 5 stops, while B&W film (depending on how it’s exposed and processed) can have a range of 9 stops. Digital’s dynamic range can vary depending on the sensor, but many feel it’s similar to B&W film if you shoot in the RAW format instead of jpeg. For a much more technical explanation, try: http://www.rags-int-inc.com/PhotoTechStuff/DigitalFilm/
The Zone System was Ansel Adams’s systematic formulation for how to best control contrast and dynamic range through exposure, development and printing. It’s a complicated subject, what interests me here is his break down of a scene into ‘zones’.
Wikipedia has a detailed description. Here is a chart from their article. http://en.wikipedia.org/wiki/Zone_system
I actually feel that in the next few years – it won’t be very long – the electronic image is really going to be the medium of photography. — Ansel Adams, 1980
Here’s a good zone system link: