|Ansel Adams, like most of the early piioneers in photography, recognized that exposure was the most critical part of image making. Using light we paint an image onto a light sensitive material. For Adams that material was film but for the modern photographer that is now an electronic device. The following discussion is based on FILM but the concepts remain the same for digital. Now, however, rather than adjusting our film processing time to compress or expand the contrast range of our image we turn to software and adjust the LEVELS. |
This is a sample lesson from the Intermediate Digital Photography Course offered by CLASS.SCPHOTO.COM
THE ZONE SYSTEM - Film Photography
The "Zone System" was created by Ansel Adams to break the exposure process down into parts that can be examined and controlled. The first step is to divide the shades of grey from white to black up into 10 steps or zones. Here we find pure white with no texture at all is a zone 9 and pure black with no visible texture is a zone 0. Neutral grey or half way between black and white is a zone 5. Light meters are designed to work at zone 5. The meter averages all the light values and places the result as a neutral reading that will produce average shades of tone when the film is processed normally. But what if our subject is mostly white as in the snow or what if it is a night time concert with black the dominant shade -- then the meter sets exposure to get neutral grey which is under exposed for white and over exposed for black.
The zone system uses a spot meter that can measure the light in a small area of our photo. The photographer "pre-visualizes" the VALUE desired for the area measured and adjusts the exposure appropriately. If the area measured needs to be almost black it is made into a zone 2 or 3 which is 2 or 3 f-stops less light than what the meter says. The zone system also records the contrast of the scene or its brightness range. If the scene has a range or 3 to 5 f stops it is normal contrast and has normal film processing. If the scene is 6 to 10 f stops the film must be slightly under developed to reduce the contrast of the bright scene. For an overcast or soft lighting scene with less then 3 stops range the film must be over developed to increase the contrast. The goal is to match the contrast of the film to the contrast of the scene. If we over expose we push detail off into the highlight area and it becomes white and invisible. If we under expose we drop out detail in the shadow and turn it black. Ideal prints have an exposure based on the shadow with the meter reading adjusted to the proper zone and film processed to match the brightness range or contrast of the scene
ZONE 1 = This is the threshold or the area where film begins to see an image. There is no visible texture here and nothing will print on paper.
ZONE 2 = First suggestion of texture in the shadow area of a subject (darkest part of black hair).
ZONE 3 = In most prints this is where the shadow detail becomes visible. In normal film this is just barely visible with normal development and exposure. Dark clothes, black hair are zone 3 examples. This will be 2 stops less than the zone 5 exposure.
ZONE 4 = This is average dark leaves on trees, blue jeans, brown hair or a dark stone or a shadow on a landscape (grass). A meter reading of this shadow in a scene will be 1.5 to 2 stops more light than is needed. Exposure on the camera would be set to 1 to 1.5 f stops less exposure (example, if f4 is normal then expose at f5.6).
ZONE 5 = This is the middle gray between black and white. This would be dark tan skin, average grass, or the north sky on a clear day. This is the normal value to be used in exposure metering. A neutral gray test card is this value and this is what the light meter is designed to reproduce on film. This is the one shade that is the same tone on the film and in real life.
ZONE 6 = Average Caucasian skin, light hair, normal concrete, shadows on snow or sand. This is the high end of the mid tones. A spot reading increased 1 stop for this zone.
ZONE 7 = Bright - Very light skin or snow in full sun, white concrete. This spot meter exposure is increased by 2 stops for this zone.
ZONE 8 = Very white - Whites with faint texture - this is direct noon sun on a clear day on a white reflective surface, something that hurts the eyes to look at as snow or sand or white clothes. Exposure is increased by 3 stops.
ZONE 9 = Glaring white surfaces that will show up without a texture on print. This is a reflection on metal of bright light....called a highlight.This should only be a small area on a print...a reflection. All other values above this are light sources such as the sun or a flood light. This represents the highlights on a print and have no texture. Exposure should not be set for this area on a subject.
Zone system exposure is made after previsualizing the tonal values of the final print. Select the area of the subject you want to show the minimum amount of detail in the shadows (such as black hair) and meter that. Set the camera to two stops less than what the meter says.
When pointing the meter at something that you wish to be zone 1 and show a minimum amount of texture or shadow the meter says 125 at f 11 for the area you want to turn minimal, so set the camera to f22 or to 250 at 16 or 500 at 11 to turn it black rather than neutral gray. This is calculeted to shift the value the light meter created which is ZONE 5 into ZONE 2 which is 3 STOPS LESS light. Changing the shutter from 125 to 250 is one stop and changing the aperture from 11 to 16 to 22 is two more stops for a total of three stops.
NORMALY WE USE ISO 100 FOR BEST RESULTS.
BASIC SETTINGS and their equivalents:
SHUTTER: 15 - 30 - 60 - 125 - 250 - 500 - 1000
APERTURE: 22 - 16 - 11 - 8 - 5.6 - 4
H & D CURVE - Film Exposure Response
This is a technical name for a graph that records the result of exposure on film or paper. The vertical line on the graph represents the density of the negative and the horizontal line represents the brightness of the light. Notice that the curve illustrated above starts with a line that is not at ZERO on the density scale when the light is at zero. This represents the darkness of the film base material. Then we notice that as light is added for a small portion there is NO change in density from this zero point. This area is called the shadow area or TOE of the curve. Eventually the light begins to cause a change in negative darkness or density. This starts as a slow change until it begins to take off on a straight line portion where a change in brightness results in an equal and uniform change in density on the negative. This portion is called the STRAIGHT LINE portion and represents the part of the exposure process that produces a GOOD photo. This is where we want the exposure of our image to fall. As light is added it comes to a maximum point the film can handle and the density of the film gets as black as it can get. The straight line portion levels off . This portion is called the SHOULDER. Ideal exposure of the film in the camera will produce results where the film will record the darkest part of the subject on the very tip of the toe and not into the area where no image is recorded while still getting the whitest part of the image to appear on the tip of the straight line where the shoulder starts. In short, the exposure needs to be on the straight line portion. Overexposure results in loss of detail in the hightlights as the exposure shifts to the right on the graph pushing detail over into the shoulder.
Histogram - Digital Response Graph
The digital world makes the need to taylor fit the film development to the number of zones of light that are available in our subject. Right the viewing screen of just about every digital camera the modern photographer can view the Histogram to see how the image has been exposed.
This is a normal exposure and the histogram that can be viewed for it on your camere. This view comes from Photoshop but the graph looks the same on the camera minus the labels and gray scale. The histogram is basically a graph that counts all of the pixels or picture elements in the 3 or 4 million that make up the photo and plots them on a graph that goes from black on the left to white on the right. In the center is neutral gray or ZONE 5. After taking a photo most DSLR's will let you view the histogram and the key you are looking for is that none of the data is lost by dropping off one side or the other. In this graph some of the highlight data is clipped but there was little that could be done for this because it was a very bright day with high contrast lighting. Ansel Adams would have under developed this film to lower the contrast and extend the graph to the right a little and that is one big advantage of film over digital. .
PHOTOSHOP ADJUSTING THE ZONE
In Photoshop, or editing software, we can adjust the photo and accomplish some of the same tricks Adams was able to do in film processing. Here is how this graph works:
SHADOWS are on the left side of the graph and in this photo the shadows dominate the composition. The key to reading this graph comes in making sure that the whole "mountain" or group of shadow pixels is visible and not clipped or cut off. For the shadow on this one we are OK. Under the graph is a triangle that sets the gray scale level for Zone II which is the minimum visible level in the dark side.
MIDDLE TONES are in the middle and the great thing about digital photography is that this can be adjusted easily. Under the histogram are three triangles that can be slid from left to right to adjust the location for black - gray - white. In this photo the shadow might want to be adjusted a little to the right to see what it does to the final shadow detail. By sliding the middle one to the left a little on this photo we can change the LEVEL that is selected to be gray or ZONE V. What will that do? It will lighten up the shadow side of the photo and darken, slightly, the highlight side of the photo. The great thing is that if you don't like it you just press the RESET button and start all over. With film if we made an error in film processing it was lost forever. The location of ZONE 5 is the most critical for digital and film. With film the photographer used a spot meter that would let them meter the value of specific parts of the photo to expose them and turn them into neutral gray ZONE V.
HIGHLIGHTS are found on the right side of the graph and they represent the white side of our pixel levels or ZONE 9. They should not be clipped either but in this case there was no way to capture both highlight and shadow because the brightness range was wider than the ability of the camera to see it. This is the kind of light that hurts our eyes and is when we reach for our sun glasses. When either the shadow or highlight side of the exposure is clipped the data there is lost forever so the important thing to do in the field is review the histogram of important shots and make sure one side or the other is not clipped. If they are then under expose a quarter f-stop or so to shift the graph to the left or over expose a quarter f-stop to shift the graph to the right.