MANUAL COMPENSATION
This metering method is designed for cameras without a built-in meter, or situations where the built-in meter fails, such as with dead batteries. Manual compensation is also necessary in circumstances where a hand-held meter is used. To use these methods with a programmed or automatic camera, the manual or metered-manual exposure method must be used. If the camera is left in any automatic mode, it will over-ride these changes.
F-16 RULE -- when purchased, most films include general instructions for the use and handling of the film. Some include a miniature guide to proper exposure with adjustable cameras, but few photographers pay any attention to this guide assuming that their expensive in-camera meter is much more accurate. Unfortunately, this is a mistake, for this mini-guide is based on the F-16 rule, a highly accurate technique which is usable under a variety of conditions.
FIGURE 10-1
F-16 Exposure guide
Condition(9am-3pm) Aperture Shutter Speed
Sun on snow f22 1/ISO
Sunny f16 1/ISO
Hazy with shadows f11 1/ISO
Cloudy without shadows f8.0 1/ISO
Shade on sunny or hazy day f5.6 1/ISO
Heavy overcast f5.6 1/ISO
Shade on cloudy day f4.0 1/ISO
The F-16 rule is only appropriate from about 9am until 3pm because the intensity of the sunlight tends to diminish outside of the times. In the early morning or late afternoon there can be a 2 to 3 F-stop difference from these recommended settings, even when the sunlight is direct. The human eye can quickly adapt to this light loss and may not notice it even when underexposure of the film will be severe. Only a meter can accurately measure these changes.
Despite its limitations, the F-16 rule is useful and important. First, it's very easy to use; there are no buttons to push and no calculations to make, just memorize the table or make a copy for your camera bag. Be careful though when defining the light that is falling on your subject. For example a front lit subject on a sunny day requires f-16, but a backlit portrait on the same day calls for f 5.6, because although the day is sunny the face is in the shade. Secondly, the f-16 rule is essential when the camera batteries die and no replacements are available. Even though a meter reading is impossible without the batteries (except for selenium meters) a picture can still be taken outdoors using the f-16 rule as a guide. On some cameras, however, the shutter is also dependent on the battery and even though the exposure settings can be made using the f-16 rule, an exposure is impossible because the shutter may need electricity to operate; some cameras have mechanical shutters which will operate without a battery.
EXERCISE 10-1
meter calibration
The f-16 rule can be used as a very easy check of your meter's accuracy; all that is needed is a grey card and a sunny day around mid-day.
1.On a sunny day set up the grey card with the sun illuminating the grey side.
2.Set the ISO of the camera or meter to be tested at 125.
3.Set the camera on any automatic or semi-automatic mode which gives an exposure readout in f-stops or shutter speeds.
4.Set the aperture at f-16 and/or the shutter speed at 1/125 making sure to maintain an exposure readout.
5.The meter should indicate that f-16 and 1/125 provide a correct exposure when the meter is pointed at the grey card.
6.If the meter doesn't indicate a correct exposure, make sure you are using the meter and the grey card correctly, and then bring it in for repair or adjustment.
Camera meters, with use, tend to stray from what is a correct reading. Don't be surprised if your meter is off by more than one f-stop.
The f-16 rule does not mean, of course, that for all pictures the shutter speed must be the reciprocal of the ISO (1/ISO) or that only the f-stop listed in the guide can be used. The f-stops and shutter speeds listed in the guide just make it easier to remember. The shutter speed and f- stop can be varied according to the reciprocity rule which is often written as the formula:
E = I * T
where E = total light hitting the film
I = intensity of light created by the f-stop
T = DURATION OF LIGHT CREATED BY THE SHUTTER SPEED
This formula states that an exposure (E) is determined by two variables; first, the intensity (I) of the light hitting the film (determined by the f-stop) and secondly, the time (T) that the film is exposed to the light (determined by the shutter speed). Using Figure 10-2 it can be seen that any recommended exposure can be varied, regardless of whether the exposure is determined through the f-16 rule or with an exposure meter. If a shutter speed is shortened, the amount of light hitting the film is decreased; if the aperture is then proportionately changed to increase the light hitting the film, there is no change in the total exposure. For example, the f-16 rule on a cloudy day recommends f-8 and 1/60 for any ISO 64 film (1/60 is the closest shutter speed to 1/64). The film will receive the same total amount of light if the shutter speed is increased to 1/125 and the f- stop is opened-up to f-5.6. Some other equally correct combinations are, for example, 1/125 and f-4.0, 1/1000 and f- 2.0, or 1/15 and f-16. When making these reciprocity rule changes remember that the shutter speed varies by a factor of 2 while the equivalent change in the f-stop is the square root of 2 -- 1.414.
FIGURE 10-2
Sample Reciprocity Rule Variations
f-stop I----I----I----I----I----I----I----I----I----I----I (1.4) 1.0 1.4 2.0 2.8 4.0 5.6 8.0 11. 16. 22. 32
shutter I----I----I----I----I----I----I----I----I----I----I (2.0) 1000 500 250 125 60 30 15 8 4 2 1
less light <-------------------------> more light
Hand-held Meters -- Admittedly, there are a wide variety of interesting photographic conditions that the f-16 rule doesn't cover, and the exact difference between a hazy day and a cloudy day is subtle and difficult to differentiate. There are also many situations that are too dark for a grey card to produce a reading with a standard camera meter. For these and other reasons many photographers turn to a more accurate and sensitive hand-held meter. There are two general types of hand-held meters.
Reflectance Meters -- One type of hand-held meter is the reflectance meter, sometimes called a luminance meter. These meters operate in the same general way as the built-in camera meter in that they read reflected light. Typically, reflectance meters have angles of view from 25 to 50 degrees depending on the manufacturer and the design. Within that angle of view they average all the tones of the scene and recommend shutter speeds and apertures according to the set ISO. The only advantage this type of meter can have over the in-camera meter is a wider sensitivity range. Depending on the meter, you may be able to read in lower light conditions than with a built-in camera meter.
Care is recommended when using a hand-held reflectance meter because the angle of view of the meter can be dramatically different from the angle of view of the lens being used on the camera. For example, when using a hand-held reflectance meter with an angle of view of 40 degrees to take a picture using a camera with a 200mm lens (angle of view = 12 degrees) the meter and the lens are seeing very different parts of the scene and will more than likely recommend different exposure settings. It's necessary to know the angle of view of the meter and make sure it's reading only what you want it to read by changing your distance from the subject.
A specialized type of hand-held reflectance meter --the spot meter -- solves this problem of conflicting coverage. Spot meters have very restricted angles of view, typically from 10 to 1 degree, which is equivalent to having from a 250mm to a 2000mm lens on a 35mm camera. There are even zoom spot meters which allow the photographer to change the angle of view as with a zoom lens. These spot meters allow the photographer to pinpoint small areas in a scene for metering without having to move closer to the subject. This is very useful when dealing with unapproachable subjects that are under different lighting conditions than the camera. Fortunately these meters are very small and lightweight despite their long focal lengths. Unfortunately, these meters are not inexpensive, and for less money a photographer can purchase a 500mm lens and use it with a TTL camera meter to selectively read small parts of the scene as is done with the spot meter. The 500mm lens, despite its greater size and weight, has the added advantage of being able to take pictures, as well as meter readings!
Whether using an averaging or a spot hand-held meter, it still maintains the limitations of any reflectance meter --they assume that whatever they are pointed at reflects 18% of the incident light. Despite its added cost, a light or dark subject will confuse the meter just as it fools the built-in camera meter. Consequently, whenever using these types of hand-held meters make sure you are using a grey card as described in Chapter 9 or meter off an object in the scene which reflects 18% of the incident light.
INCIDENT METERS -- The second type of hand-held meter is the incident or illuminance meter. These meters measure the amount of incident light that is falling on the subject as opposed to the reflectance meter which measures the amount of incident light being reflected off of the subject. As a result, these meters are not pointed at the subject as with the reflectance meter, but rather are pointed away from the subject in order to capture and measure the incident light. Unlike the reflectance meter reading which varies depending on the reflectance value of the subject, the incident meter produces the same reading regardless of the subject's reflectance value. In the example scene, the black and white faces both produce the same reading as the grey card when an incident meter is used, but different readings when a reflectance meter is employed. The incident meter is subject to the same basic limitations as the grey card. The meter must be held in the same light as the subject, but not necessarily immediately in front of the subject. Distant subjects can be metered as long as the lighting is the same at the subject position and the meter position. This is very helpful with many unapproachable subjects. If the subject is in a different lighting condition however, it becomes necessary to move the incident meter closer until the lighting is the same. If the subject is unapproachable and a substitute reading cannot be achieved, such as with a spot-lit stage show, it becomes necessary to switch to a spot meter or a TTL camera meter with a telephoto lens to get an accurate reading.
An incident meter with its white, flat diffuser, which transmits 18% of the incident light to the photocell, simulates the grey card but without having the reflection problems that plague the grey card user. In use, the flat diffuser is carefully placed on the most important plane in the scene, just as with the grey card, and a reading is taken. As with a grey card, the flat diffuser does not have to be placed immediately in front of the subject but must be in the same light as the subject if used in daylight. With artificial light sources it is essential to place the incident meter as close to the subject as possible.
Incident meter companies supply flat diffusers as an accessory for their meters, and while many photographers find them more appropriate for general use, the incident meter normally comes equipped with a white, hemispherical diffuser preferred by portrait photographers. This dome is roughly shaped like a head or any other rounded object and gives good overall results with subjects of this shape. In use, the meter is not placed on the most important plane but rather, the dome is pointed toward the camera and the reading is taken. The dome does not simply average the light illuminating the subject from all sides but rather produces a bias toward the brighter side of the subject, normally the most important part of the scene for which to expose in portraiture. The domed incident meter will give the same results as a grey card or an incident meter with a flat diffuser held between the light source and the camera position, the most important plane in portrait work.
Results comparing the domed diffuser and the flat diffuser are seen above. The two meters give the same results when pointed at the light source and, in this case, at the shadow areas. When pointed at the camera position, the two meters recommend radically different exposures. The domed meter is still effected by the light source and is biased toward this end; while f-8 is the exact midpoint between the highlight and shadow readings of f-16 and f-4, the domed meter is biased by the light and recommends f-11. This is the same reading that a grey card or flat diffuser will recommend if positioned on the lighter cheek -- the most important plane. When pointed at the camera position, the flat diffuser is not effected by the light source and it recommends f-4, the same reading that a grey card will produce if positioned on the forehead. Clearly the domed diffuser is easier to use in this type of situation with a rounded object due to its bias toward the brighter side.
A flat subject under the same illumination is better exposed if a flat diffuser is used on an incident meter. The subject in this case is flat, just like the flat diffuser, and the results are better than those obtained with the domed diffuser. Once again a grey card reading would produce the same results as the flat diffuser in this case. Clearly, it is essential whenever using a flat diffuser or a grey card to identify which plane in the scene is the most important for which to expose. If the subject is more rounded than flat it is easier to use a domed incident meter because positioning is not quite so particular.
There are manufacturers who supply plastic, transluscent caps for cameras which transform the cameras built-in TTL reflectance meter into an incident meter. They are frequently advertised in photography magazines and are substantially less expensive than a separate incident meter. They screw into the filter threads or snap on to the front of the lens and come with instructions for use. As with the hand-held incident meter, the camera with the dome attached is used in the metered-manual mode and carefully placed in the same lighting as the subject. The aperture and shutter speed are dialed in according to the meter, then the cap is removed and the picture taken.
These transforming caps vary in price, size, shape and durability; some have a flat shape while others are more rounded, and the selection should be made depending on the type of work you do. A cheaper, less durable alternative to these transforming caps is to slip a styrofoam cup over the end of the lens and use the TTL camera meter. Fortunately, styrofoam cups come in a variety of sizes so that nearly any lens can be accomodated. Feel free to cut off any excess around the rim of the cup for a better fit. If the sides of the cup are completely cut away, the cup bottom will operate like a flat diffuser on an incident meter. This will probably make aperture and focusing adjustments easier too. Make sure that the cup doesn't have any designs or colors on it as these can unintentionally cut out some of the incident light. In addition, be careful to keep your fingers away from the front and sides of the cup as this can also cut out some of the light. The only significant advantage a separate hand-held meter has over the coffee cup approach is that the coffee cup uses the camera meter and is dependeent on its sensitivity range while a hand-held meter will probably provide the photographer with a wider sensitivity range.
Like the grey card, the coffee cup can be used to determine the accuracy of your BLT built-in camera meter. On a sunny day the meter should match the f-16 rule when pointed toward the sun; if it doesn't, it's time to have your meter adjusted. Whether you are using an incident meter or a reflectance meter, don't assume that you can get accurate exposures without work and thought. Make sure you're using the meter correctly, make sure you're metering the light you want to expose for, and make sure the grey card or incident meter is positioned on the most important plane. In addition, make sure the meter is correctly calibrated; to be sure, make sure it matches the f-16 rule on a sunny day. Many meters won't, even when new; with age and use, wide discrepancies are not uncommon. Finally, another important consideration when purchasing a meter is what accessories are available. A list of meter manufacturers is available in Appendix A. A list of meter accessories and buying tips is found in Appendix C. A comparison of the various metering methods is found in Appendix G.
HAND-HELD METERS
Admittedly, there are a wide variety of interesting photographic conditions that the f-16 rule doesn't cover; there are also many situations that are too dark for a grey card to produce a reading with a standard camera meter. For these and other reasons many photographers turn to a more sensitive hand-held meter. There are two general types of hand-held meters.
Reflectance Meters
One type of hand-held meter is the reflectance meter, sometimes called a luminance meter. These meters operate in the same general way as the built-in camera meter in that they read reflected light. Typically reflectance meters have angles of view from 25 to 50 degrees depending on the manufacturer and the design. Within that angle of view they average all the tones of the scene and recommend shutter speeds and apertures according to the set ISO. The only advantage this type of meter can have over the in-camera meter is a wider sensitivity range. Depending on the meter, you may be able to read in lower light conditions than with a built- in camera meter.
Care is recommended when using a hand-held reflectance meter because the angle of view of the meter can be dramatically different than the angle of view of the lens being used on the camera. For example, when using a hand- held reflectance meter with an angle of view of 40 degrees to take a picture using a camera with a 200mm lens (angle of view = 12 degrees) the meter and the lens are seeing very different parts of the scene and will more than likely recommend different exposure settings. It's necessary to know the angle of view of the meter and make sure it's reading only what you want it to read by changing your distance from the subject.
A specialized type of hand-held reflectance meter -- the spot meter -- solves this problem of conflicting coverage. Spot meters have very restricted angles of view, typically from 10 to 1 degree, which is equivalent to having from a 250mm to a 2000mm lens on a 35mm camera. There are even zoom spot meters which allow the photographer to change the angle of view as with a zoom lens. These spot meters allow the photographer to pinpoint small areas in a scene for metering without having to move closer to the subject. This is very useful when dealing with unapproachable subjects that are under different lighting conditions than the camera. Fortunately these meters are very small and lightweight despite their long focal lengths. Unfortunately, these meters are not inexpensive, and for less money a photographer can purchase a 500mm lens and use it with a BTL camera meter to selectively read small parts of the scene as is done with the spot meter. The 500mm lens, despite its greater size and weight, has the added advantage of being able to take pictures, as well as meter readings!
Whether using an averaging or a spot hand-held meter, it still maintains the limitations of any reflectance meter -- they assume that whatever they are pointed at reflects 18% of the incident light. Despite its added cost, a light or dark subject will confuse the meter just as it fools the built in camera meter. Consequently, whenever using these types of hand-held meters make sure you are using a grey card as described in Chapter 9 or meter off an object in the scene which reflects 18% of the incident light.
The second type of hand-held meter is the incident or illuminance meter. These meters measure the amount of incident light that is falling on the subject as opposed to the reflectance meter which measures the amount of incident light being reflected off of the subject. As a result, these meters are not pointed at the subject as with the reflectance meter, but rather are pointed away from the subject in order to capture and measure the incident light. Unlike the reflectance meter reading which varies depending on the reflectance value of the subject, the incident meter produces the same reading regardless of the subjects reflectance value. In the example scene, the black and white faces both produce the same reading as the grey card where an incident meter is used, but different readings when a reflectance meter is employed.
The incident meter is subject to the same basic limitations as the grey card. The meter must be held in the same light as the subject, but not necessarily immediately in front of the subject. Distant subjects can be metered as long as the lighting is the same at the subject position and the meter position. This is very helpful with many unapproachable subjects. If the subject is in a different lighting condition however, it becomes necessary to move the incident meter closer until the lighting is the same. If the subject is unapproachable and a substitute reading cannot be achieved, such as with a stage show, it becomes necessary to switch to a spot meter or a BTL camera meter with a telephoto lens to get an accurate reading
The incident meter normally comes equipped with a white, hemispherical diffuser preferred by portrait photographers because it is roughly shaped like a head or any other rounded object and gives good overall results with subjects of this shape. In use, the dome is pointed toward the camera and the reading is taken. The dome does not simply average the light illuminating the subject from all sides but rather produces a bias toward the brighter side of the subject, normally the most important part of the scene for which to expose. The domed incident meter will give the same results as a grey card held between the light source and the camera position, the most important plane in portrait work.
To expose for other planes on the subject or to measure incident light for non-rounded subjects, a flat diffuser is more appropriate than the domed diffuser. Many incident meter companies supply flat diffusers as an accessory for their meters, and many photographers find them more appropriate for general use than the domed diffuser. Simply stated the flat diffuser simulates the grey card but without having the reflection problems that plague the grey card user. In use, the flat diffuser is placed on the most important plane in the scene, just as with the grey card, and a reading is taken. As with a grey card, the flat diffuser does not have to be placed immediately in front of the subject but must be in the same light as the subject
Results comparing the domed diffuser and the flat diffuser are seen above. The two meters give the same results when pointed at the light source and, in this case, at the shadow areas. When pointed at the camera position, the two meters recommend radically different exposures. The domed meter is still effected by the light source and is biased toward this end; while f-8 is the exact midpoint between the highlight and shadow readings of f-16 and f-4, the domed meter is biased by the light and recommends f-11. This is the same reading that a grey card or flat diffuser will recommend if positioned on the lighter cheek -- the most important plane. When pointed at the camera position, the flat diffuser is not effected by the light source and it recommends f-4, the same reading that a grey card will produce if positioned on the forehead. Clearly the domed diffuser is easier to use in this type of situation with a rounded object due to its bias toward the brighter side.
A flat subject under the same illumination is better exposed if a flat diffuser is used on an incident meter. The subject in this case is flat, just like the flat diffuser, and the results are better than those obtained with the domed diffuser. Once again a grey card reading would produce the same results as the flat diffuser in this case. Clearly, it is essential whenever using a flat diffuser or a grey card to identify which plane in the scene is the most important for which to expose. If the subject is more rounded than flat it is easier to use a domed incident meter because positioning is not quite so particular.
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