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THE EXPOSURE METER
Exposure is the most valuable link in the chain of events that lead to a good photograph, the more important link being the initial "seeing" of the subject or scene to be photographed. Exposure links what the photographer sees with what the viewer will see in the final outcome. Therefore the ability to control the chain of events leading to a pleasing photograph is the reason for a good technical background in exposure. A photographer often times can relay on past experience as a guide in making the necessary camera settings, but this is not always possible due to different conditions or the lack of experience.
Another way to judge exposure, providing it is a bright sunlight situation, is the "rule of thumb" for exposure that works as follows: Convert the ISO (film speed number) to the nearest shutter speed and use with f16. For example Kodak Plus X or Ilford FP4 (ISO125) would be exposed at 1/125 of a second at f16. Remember this is for bright sun conditions only. Any other lighting conditions would vary from this exposure.
There are other ways to evaluate the intensity of the light rather than to rely on experience and/or simple (often ineffective) rules. Printed charts graphs and dials can be helpful in determining proper exposures. The simplest of all exposure guides is the instruction sheet packed with the film, or printed on the paper backing of the film. See the sample below.
In the final analysis, however, there is no better way to determine exposure settings than by the use of the photoelectric meter, sometimes simply referred to as a light or exposure meter.
Before using a meter what exactly does exposure effect? The effects of exposure can be divided into two major categories: The contrast of image and image density.
Contrast of Image--includes the difference in each tone and separation of each tone throughout the negative. In other words, the density of the tones creates the image and the contrast in the tones gives the image its density. Good separation is needed between lines and tones so the photograph will not be flat or fall apart.
Image Density--refers to stability of tones composing the images. The density must be even throughout each tone if that tone is to come out evenly in printing the photograph.
The basic effects of incorrect exposure (for black and white film) start with loss of definition. This occurs when the density of the image is either weak, resulting in a flat print with little separation in lines and tones, or strong, resulting in too much contrast which causes areas to "block up" or "bleach out" details.
Change of contrast is another major ill-effect of improper exposure. The loss of detail and lack of overall balance is normally the result of an over or underexposed negative. Basically color negative-type films suffer the same ill-effects of incorrect exposure as black and white films. Some additional complications occur because color balance is greatly effected by exposure. These shifts in color are especially hard to correct in printing because printing papers are not made in contrast grades, thus contrast changes caused by exposure are more serious in color than in black and white. Reversal type color films have considerably less latitude than negative films, thus plus or minus 1/2 stop differences off correct exposure are the maximum that will be tolerated under conventional lighting situation and subject properties.
Considerations Prior to Exposure
Even before using the light meter to find the correct exposure certain considerations must be observed, which are the following:
Special Exposure Situations
Selenium Cells
Not all meters work, or are used, in exactly the same manner, but they all have certain similarities. Once the photographer has learned to use one meter, then it is a simple matter to use any meter. The basic principles are the same.
All photo-electric meters have a power source consisting of a photo-electric cell which generates a small charge of electrical energy when directed toward light. This cell may either be a selenium cell or a cadmium sulphite cell (generally called a CdS cell). The fundamental difference between those two is that the selenium cell meters have an extremely delicate wire for transmitting the energy to the pointer, and the size of the cell is comparatively large. On the other hand cadmium sulphite light meters (CdS) have a small battery which works in conjunction with the light cell to offer greater sensitivity, ruggedness and compactness.
THE CDS METERS
The most sophisticated of general photographic exposure meters is the Cadmium Sulfide (CdS) type. Rather than having a selenium cell which generates its own current, the CdS cell must have an electric current from a small battery for it to work.
In effect, when light strikes the CdS cell it acts just the opposite of the selenium cell in that it acts as a proportional variable resistor and its resistance decreases as the intensity of the light increases. As this resistance increases or decreases it controls the flow of electricity from the battery through the circuit proportionately and causes a needle to deflect. The amount of deflection is read out on a scale as a particular value of light intensity which can then be computed in terms of exposure settings on appropriate dials on the meter.
The CdS meter circuitry is even more unique in that it is generally incorporated with an optical system to restrict the angle of acceptance to a much narrower cone than the standard incident light meters. In some instances there are models of CdS meters having highly developed optical systems to limit the reading angle to one or two degrees of arc. This makes for an amazing instrument which literally can read a tiny spot (area) of the subject. This permits precision brightness range method readings to be made even at some distance from the subject.
Because the CdS system includes a power supply (battery), its range of sensitivity is greatly increased--so much so, in fact, that it will give accurate exposure readings ranging from milliseconds to many minutes. The sensitivity is so great that readings in moonlight can be made.
The "heart" of most of the automatic exposure systems built into cameras is the CdS circuit. Exposure meters vary in their ability to be sensitive to light, and in respect to what they "see". This is known as meter's "angle of acceptance" or simply the angle of view or how much area it covers in its measurement. Small angles of acceptance enables one to measure very small areas for more accurate readings.
THE MOCK-UP EXPOSURE METER
Before using any exposure meter it is recommended to practice setting the dials with the mock-up model of the Weston meter. By practice using the mock-up meter it will save time and eliminate mistakes.
1. Located on the top is a scale that represents the light scales which show units of light intensity. The numbers range from 0 through 1600. The larger numbers indicate brighter or more intense light, and the small numbers low levels of light. The pointer is located beneath the scale represents the pointer of a real meter which moves according to the intensity of light it receives.
2. The outside circular dial has numbers that correspond to the light scale above.( range from .1 to 1600)
3. Examine the middle dial. It is the shutter speed dial. It has black numbers around the outside ranging from 2 through 1200 which represent fractions of seconds for shutter speeds. For example, the number 100 means 1/100 second. The white numbers on a black background indicate full seconds of time.
4. There are also the printed words "EXPOSURE INDEX". This represents the ASA, ISO (Film Speed Dial) which is used to indicate on the dial the sensitivity of film.
5. Finally, note the top small dial. This contains two items of significance: (A) the arrow shaped "pointer" called the normal arrow and (B) a scale of numbers ranging from 1 through 32. This is the f/stop (aperture) scale. (C) the letters "U", "A"and"C", and "O"which represent a system for under and over exposure.
How To Use The Mock-Up Meter
1. Set the Exposure Index Window over the film speed to be used. In this instance select 400 ASA.
2. Imagine that the meter is pointed toward the light to be measured and the pointer at the top is registering 200 units of light on the top light scale.
3. Locate the shutter speed and f/stop scales on the middle and top dials. You will see for instance that "300" is opposite the number "16". This means that with this film under this light condition, an exposure of 1/300 second with an aperture setting of f/16 could be used, other f/stops and shutter speeds that are opposite one another on the dial could also be used. For example, "150" is opposite "22" which means a setting of 1/150 second and f/22 could be used as an exposure setting. Any combination of shutter speeds and f/stops that appear opposite one another can be used.
HOW TO USE AN EXPOSURE METER
Take the exposure meter and carefully hold it, be very careful because if it is dropped it will not be accurate( if it is dropped report it to the instructor). Open the protective case and locate the following parts:
1. Photo-electric cell
2. Light intensity scale
3. Pointer
4. Pointer lock
5. Baffle
6. ISO dial
7. Normal arrow
8. Light scales
9. f-stop scale
10. Shutter speed scale
After examining the parts, activate the light intensity pointer lock, and aim the photo-electric cell at various objects of different tones and brightness. Keep the photo-electric scale about six inches away from the area being measured. Note how the pointer varies in its readings. You are now measuring reflected light-that is light reflected from the subject toward the camera. This is the most common way to measure light. Try readings with the baffle in the closed position and also in the down position. Note the light scale in each baffle position.
For some situations it is better to measure the light falling on the subject. This is known as an incident light reading. In order to measure light by this means, attach the incident light reading cone over the baffle and point the cone covered photo-electric cell toward the source of light rather than toward the subject.
Procedure for making a reading
Make practice readings using the following steps:
Not all films react to light in the same manner; some are faster (more light sensitive) than others. Neither are all lighting situations the same; light varies in intensity (strength) from one situation to another. Because of these light and film factors there can be no single camera setting on adjustable cameras that can be used for taking all pictures. Exposure meters are instruments used to measure the light and to indicate the correct camera settings to use. The information supplied by the exposure meter, when applied to the camera settings, will enable you to produce properly exposed film. When the film is correctly developed it will produce negative of sufficient tone and detail to be easily printable.
Latitude Readings
("Latitude" is a term which refers to the amount of under and over exposure that can be given to film and still produce an acceptable image detail.)
NOTE Latitude readings: (Exposure Latitude is a term which refers to the amount of under and over exposure that can be given to film and still produce an acceptable image detail.
Some exposure meters have indicator for under and over exposure - a "U" and "O" mark on the dial for black and white film readings which simply means that any area of the subject which gives a reading between the "U" and "O" marks will give sufficient exposure on the film to show detail. If the reading of a shadow or highlight does not fall in between "U" and "O", then it will be either over or under exposed so as to render no detail. The "A" a nd "C" positions is the latitude scale for color film. For proper flesh tone reading it is recommended that you use the "C" position as a marker rather than the arrow.
CONCLUSION
The exposure meter reads for middle gray whether the subject is black or white or light or dark. Most exposure errors fall within the plus or minus one f/stop range, which falls within the latitude of most black and white or color negative films on the market. The world is not composed of middle-toned scenes. A half stop of underexposure can turn a light green spring foliage to a muddy green mass. The camera may deliver exposures that are acceptable(within specifications of the camera manufacturer) but are a stop off. This is the best argument for a meter that is not in the camera. When choosing a meter many decisions must be made. The first is for what purpose is it going to be used and what is the cost. Other decisions include: reflected or incident or system, selenium or cds, ease of use versus more variety in readings. Regardless of your choice the principles presented in this unit can be used with any meter whether it is in or out of the camera.
Kinds of meters
Basically there are two kinds of light meters reflected and incident. The reflected light type measures light that is reflected from the subject. A special kind of reflected meter is the spot meter. The spot meter is like a telephoto lens placed on a meter. It measures small parts of a scene. The second type is the incident meter which measures the light as it falls on the subject. Incident light is measured by pointing the meter toward the source of light. This type of reading is done by use of a special opalescent glass adapter or by a meter designed for this purpose. Most light meters are designed for reflected light and some can be adapted for use with both types.
Methods for taking light meter readings
Once these methods are learned and the correct operation of the light meter is achieved then accurate meter readings can be taken in any light. Always use the one most suited to the picture taking situation. Remember that the meter reads for gray.
Camera Position Method
This is the simplest and fastest method for measuring exposure by reflected light. The meter is held at the position of the camera and pointed directly toward the scene. Nearly all outdoor exposures of landscapes, seascapes, mountain views, and buildings can be accurately measured in this manner. Light from the sky is more actinic (violet and ultraviolet) than light reflected from the ground. Care must be exercised to direct the meter to avoid the light from the sky so it does not affect the meter reading.
A simple test will illustrate how the actinic rays from the sky can erroneously affect a meter reading. Direct the meter toward a scene, aiming the meter above the horizon line, then slowly tilt the meter down until a sharp deflection of the needle toward the lower end of the scale is noticed. At the point of deflection, the sensitive cell of the meter will no longer be picking up the actinic rays of the sky.
A good rule to follow when photographing scenes that include sky is: hold the meter at the camera position and point it toward the scene, tilt meter downward halfway between the ground and the horizon line. This will place the angle of the meter to measure the reflected light from the scene without allowing the intense light of the sky to influence exposure.
Close-up Method
This method is used to measure the reflected light from a single important object in the picture. For example when photographing a person, the most important area in the picture would be the person's face and a method for measuring the light from this area should be used. The meter is held close to the brightest side of the face, about eight inches away, and the reading taken off the reflected light. When taking the reading, care must be used so that a shadow from the meter or hand of the photographer is not cast on the face to influence the reading.
A close-up reading will produce a normal negative for the subject measured and other areas of the picture will reproduce according to their relative brightness.
The close-up method can be applied in different ways to give the photographer the greatest control of exposure. For example, if separation of detail in the shadow areas of a scene is needed, the close-up method is used to measure the shadow areas only. Exposing for shadows will produce a negative with full shadow detail while the other tones will produce with decent contrast providing the brightness range of the subject is not excessive. This same procedure can be followed when the highlight areas of a scene are the most important to the quality of the photograph.
Brightness Range Method
The brightness range is the difference between the brightest highlights and the darkest shadows of a scene. A knowledge of brightness range is very helpful in determining exposure, especially when the desired results are to make a photograph that reproduces all objects of the scene in the same brightness relationship as in the original scene.
With this method, a close-up reading is taken of the shadows and highlights and a light reading midway between the two extremes is selected. The photographers judgment should "lean" toward the shadows or the highlights depending on the importance of each of these areas in the picture.
Substitution Method
The photographer may desire to take a close-up reading of a particular area of a scene, but the scene may be inaccessible. In the event that it is not convenient to take a close-up reading, the substitution method is used. This method consists of substituting an object, close at hand, that has the same reflective power of the area we desire to measure in the scene. The meter is then used to take a close-up reading of the substituted material.
This material can be a piece of paper, clothing, a hat or any item that is judged to reflect the same amount of light as the area in the picture. When taking portraits, or pictures of people the palm of the hand can be effectively substituted for the skin tone of the face.
Some photographers carry several sheets of paper in their camera case for this purpose. These sheets range in tone from white to black with three or four shades of gray in between. With a little experience, considerable skill can be developed in matching tones.
A neutral gray card having a standard reflectance value of 18% can be used for accurate and effective exposure meter readings for subjects of average reflectance. These 18% gray cards can be purchased in camera shops.
The card is held in the same position as the subject and a reflected light meter reading is made from its surface. Be careful you do not cast a shadow of the meter on the card in the area that the meter cell is reading. The reading from the gray (18%) card will be the same as one taken with an incident light meter since the latter meter is calibrated on this basis.
The substitute card method is particularly good with standard reflected light meters of the selenium cell type having a fairly wide angle of acceptance for the cell. This is especially true when the background is considerably darker or lighter than the subject and the meter reading angle is such that the background influences the overall measurement.
Care should be taken to read only the light reflected from the paper or material, and the reading should be made without casting a shadow from the meter or hands.
MEASURING INCIDENT LIGHT
When a ray of light strikes a surface and is diverted, it is called a reflected ray. When measuring incident light, measure the source of illumination directly before it is reflected from the scene. The various methods for measuring reflected light have been discussed. In discussing incident light measurements, it is somewhat simpler because the incident light method measures the source of illumination--the light falling on the scene.
Using the incident light meter is very simple. The meter is held near the subject and pointed towards the camera position. It is not necessary to tilt the meter or compensate in any other manner because the sensitive cell of the meter is measuring only the light falling on the subject. The important rule to remember when measuring incident light is: The meter should receive the same light as the subject. It is therefore important to place the meter as close to the subject as possible--especially when using the meter in the studio where several lights may be in operation. Outdoors, the incident light meter may be held at the position of the camera because the intensity of the sunlight falling on the subject will be the same at the position of the camera.
Because the incident light meter reads the light falling on the subject, it will give an identical reading for subjects that are white or black, under the same conditions. As with the gray card reading method, if the subject is lighter or darker than average, the photographer must exercise judgment and correct the exposure setting by a factor of 1/2 to one stop.
Before learning how to take a reading REMEMBER THIS: The exposure meter will give an exposure that will render the area measured as a middle shade of gray on the negative and print. Therefore, always measure the light from an area that you desire to be a middle shade of gray. Or to state it another way, measure the area of the subject that you wish to be reproduced in the greatest detail. Finally, remember too, that this is a very delicate instrument that should be handled very carefully. Always carry it with the strap around your neck.
SPOT METERS
The spot meter reads a very small area. All spot meters are reflected light meters and are rated by the angle, or degree of arc of the acceptance of light. The smaller the angle of acceptance the smaller the "spot" that it will read. The greatest advantage of the spot meter is accuracy of reading from the area that the meter is directed toward. Spot meters can reach areas that are not accessible.
A reflected light meter (left) is aimed at the subject. Here it measures the light bouncing off the man’s clothing. The incident meter (right) is faced away from the subject and towards the camera to measure the general light falling on the man from the direction of the camera.
The opening of a reflected-light meter (left) admits light from a limited angle (usually 30 to 50 degrees) and therefore takes in a small area of the subject when used at a close range. The incident meter’s coverage, or angle of acceptance (right), is 180 degrees, to enable it to measure all light coming toward the subject from the general direction of the camera.
SYSTEM METERS
The best meter is the meter that can make a variety of meter readings in a variety of situations. The system meter can do this, it can make reflected light readings and by moving a cone it can be used to make incident light readings. Other attachments available are a spot meter and flash metering system. These meters will cost more but offer more flexibility and accuracy.
THE ZONE SYSTEM
Of all the available systems for determining correct exposure and development, the zone system is the most often used and misunderstood. It is based on the same orderly mathematical progression already employed by most photographers. The instrument that provides this is the camera by using the shutter and aperture. An example of this would be: f/8 allows twice as much light to enter the camera as f/11 and half as much as f/5.6. For shutter speed 1/60 allows half as much light as 1/30 and twice as much light as 1/125.
When taking a picture the area of contrast found in the picture may be expanded or contracted by exposure and development. To understand the zone system visualize that each zone is either black, a shade of gray or white. Any picture can have its contrast adjusted just like the contrast on a television set can be adjusted for more or less contrast. However there are limits. The zone system can be used to add or subtract contrast in any picture. The individual that is serious about photography should learn and utilize a system similar to the zone system.
Processing Compensation
A general rule concerning exposure control that is well worth remembering and applying is: Expose for the shadows and develop for the highlights. In use, exposing for important shadow details will ensure that they will register adequately on negative. However realize that the highlights will be overexposed.
To control this situation, development time is shortened in order that the highlights do not become extremely dense while the shadow detail is brought up to the printing image level. The amount to reduce development time should be based on the photographer's working knowledge of the film, development and camera. In general, development time reduction can start with 10% of the total processing time for each stop of exposure increase (over the normal reading). For instance, if the exposure meter reading indicated that it needed three stops more exposure to hold shadow details and normal development time with our particular developer is 10 minutes, then reduce development time to 7 1/4 minutes to compensate for the three stops of overexposure in the highlights.
The original development time of 10 minutes is reduced by 10% which gives a time of 9 minutes (or one stop compensation). Nine minutes is 540 seconds; 10% of 540 is 54 seconds subtracted from the 9 minutes leaves us with 8 minutes 6 seconds, (486 seconds). 10% of 486 is 48.6 seconds. Subtracting this from 486 we get 7 minutes 17.4 seconds which can be rounded off to 7 1/4 minutes for the three stop compensation.
Remember this is only a general starting point. Each photographer must modify it according to personal experimentation so as to arrive at the proper compensation to produce full scale negatives in which details will be visible in both shadow and highlights in your prints. Once you have arrived at a satisfactory adjustment figure stick with it and your negatives should maintain a desirable consistency in printing density range which will reproduce in your prints.