Describe How To Use A Light Meter

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1. What about the times and places where the daylight table doesn't work? How do you find the exposure then? For the first hundred years of photography's existence, the answer was, "Guess". There were a lot of hints for making a good guess, but for the most part, experience and judgement were the only way. Today there is a much better way to find an exposure and that's with a photoelectric light meter.

2. Sometimes these devices are called "exposure meters," but that's a somewhat misleading term. The simple fact is, that no matter how sophisticated they are, they really only measure the amount of light falling on a light-sensitive element. The calculator translates this to a reading of exposure. Keep in mind that this is only suggested exposure. You must still use some judgement, but if you do you will seldom get truly bad results using a meter. For the word "judgement' you could substitute "guess," but at least with a light meter, the guessing is easier and more accurate, because you have a good place to start.

3. Before discussing how to interpret a light meter reading, let's first talk a little about how light meters work. There are two basic types, photovoltaic and photoresistant.

a. Photovoltaic meters use a selenium cell, which converts light energy directly into an electric current. (It's the same kind of cell used to power solar calculators.) The current is used to make a needle move. The brighter the light, the stronger the current, and the stronger the current, the farther the needle moves. This is an extremely straightforward way of measuring light. Its chief advantage is that it needs no batteries, because the electric current is generated by the very same light the meter is measuring. So this type of meter can be used anywhere, anytime; almost. Photovoltaic cells are not very efficient at converting light into electricity. In dim light, there just isn't sufficient current to move the needle enough to make an accurate measurement. This can be compensated for by making the selenium cell larger, but after a while it all becomes much too big to be practical. So selenium cell meters work only in relatively bright lighting situations. Even so, average room light or office light is usually enough to get a reliable reading with a good quality hand-held meter of this type.

b. Photoresistant meters, on the other hand, use a source of electricity to start with - usually a small battery. The light-sensitive element doesn't generate any electricity. Instead, it resists the electric current from the battery. As the light falling on the element becomes brighter, the resistance drops, the battery's current flows more and more easily, and the indicator needle moves farther and farther. This has several advantages. First, the cell's size is much less important. A very tiny cell works just as well as a large one. Second, this method is very, very sensitive. Extremely low light levels can be accurately measured. This has been put to good use inside many sophisticated camera, where the cell can read the tiny portion of light that is reflected off the surface of the film emulsion during exposure. In hand-held meters, indicated readings of up to 30 minutes are sometimes offered. Of course, if the battery goes dead, the meter is useless. And, just as with camera batteries, light meter batteries go dead at the worst possible times and always when they get cold. For most uses, the photoresistant type meter is preferred. But in extremely cold weather or when batteries are scarce or unavailable, the selenium cell meter still has its uses (fig 1-12).

4. Once the needle has settled down, we need a way to read exactly how far it has gone. In most cases, there is a simple scale, usually numbered, painted on the background behind the needle. You read the number the needle is pointing to and transfer it to a calculator dial to get an indicated exposure. In other meters, the needle is directly connected to the calculator dial. Turning the dial causes the needle to move to the left or right until the needle matches a mark on the background behind it. This is convenient because when the needle has been set on the index marked "nulled", the exposure indication is already set on the calculator dial, and needs only to be read. Some sophisticated and expensive light meters have done away with the calculator dial entirely and show a suggested exposure with light-emitting diode (LED) or liquid crystal displays like those found on digital watches.

Figure 1-12. Three types of light meters

5. Take a look at the calculator dial shown in Figure 1-13. Once again, this is something that looks complicated, but really isn't. First of all, there's a little window, marked "ASA". ASA is just an obsolete abbreviation for what we now call the International Standards Organization (ISO), and it is, naturally, the same film speed we talked about earlier in this chapter. Notice that only some of the film speeds are actually numbered. Others are simply indicated by tick marks (fig 1-13). This is what a complete film speed scale looks like, and what those little tick marks represent. By turning the innermost dial, you can set any ISO number you wish, and this adjusts the rest of the dial scales to match. This is the most precise scale available on the meter; each tick mark represents a 1/3-stop change in the meter settings. All the other scale markings are 1/2-stop or more apart. The outermost scale is a reproduction of the scale behind the indicator needle, and next to it is an indent mark, in this case, a large arrow. This arrow is set to the same number the meter's needle moved to when the reading was taken. The most important scale should look familiar to you by now. Remember the section on equivalent exposures and how to construct an equivalent exposure calculator? Well, that's all this is. When you set the index mark to the number the meter needle fell on when the reading was taken, you slid the f/stop scale along the shutter speed scale as you did this. So now you simply choose one of the equivalent exposure settings which match the dial and shoot your picture.

6. You probably suspect that this isn't the whole story. First of all, you need to know how to use the meter to take a light reading in the first place, and second, you need to know how to interpret what the reading really means. Let's talk about taking readings first.

a. There are two systems for taking light readings: reflected.

incident and

Figure 1-13. Calculator dial of a typical light meter

(1) Incident light readings measure the amount of light falling on the subj ect. They take no account of the subj ects or tonality. An incident reading for a black cat would be exactly the same as a reading for a white house. The only thing that matters is the amount of light falling on the subject and to some extent its direction. It's easy to recognize an incident meter, because it has a hemispherical cover of translucent plastic (or dome) over the light sensor. In some meters, the dome looks exactly like a ping pong ball that has been sliced in half and glued over the element. The dome (called a "photosphere") collects the light and diffuses it toward the inside, causing a response from the sensor.

(2) To take an incident reading, place the dome in the same relation to the light as the subject you are photographing. Hold the meter near your subject and point the meter's dome toward the camera. The meter's dome is similar to a three-dimensional subject, and you are lighting it the same way. If the sun is high, then most of the light is falling on the top of the dome and the bottom is in shadow, just as your subject is. If your subject is backlit, then the meter's dome should be backlit the same way. Once again: Hold the meter near the subject and point the dome toward the camera lens. That's all there is to it. Read where the needle is, set the calculator dial, select a shutter and lens combination, and take your picture. This is the most reliable method of getting an exposure.

(3) There are a couple of other advantages as well. For example, you don't have to hold the meter close to your subject as long as it's in the same light as your subject. Thus on a sunny day, you only need to hold the meter in the same sunlight and make the light fall on it the same way as on the subject, and you'll get a good reading (fig 1-14) .

Figure 1-14. Incident light reading technique

b. Reflected light readings are almost the opposite of incident readings. In this case, you are reading the light coming from the subject and entering your camera lens. This is the same method, in principle, that cameras with built-in meters use. To take a reflected reading, you point the light meter toward the subject. There is no dome in front of the light sensor, so the light reflected by the subject is what makes the needle move. Now for many subjects, this will give an accurate enough reading, because the great majority of photographic subjects are "average"; that is, they contain a whole range of brightnesses from very dark to very light which, if all scrambled together, would be a middle tone of gray. Look back at the description of an average subject in Learning Event 5, paragraph 4a.

c. If the subject is one that doesn't average out to a middle gray, then you've got to use judgement. To understand why, you need to know one thing about the intelligence of light meters; they haven't the slightest idea what you are taking a picture of. In fact, light meters don't have the slightest idea what a picture is. All they do is react to light, and the calculator dial is designed to generate an exposure setting that will cause the film, on average, to be 18 percent gray. If you point the meter at a black cat, it doesn't know there is a black cat present - it just tells you what camera settings will make it gray. If you want your black cat to look gray, then OK. Just remember that if you do this, then everything that is lighter in tone than the black cat will be a lighter shade of gray or, eventually, pure white. The same is true of a white house. The meter reads the bright white house in sunshine and tells you what your camera setting should be to make it look 18 percent gray. Once again, if that's what you want, fine. But usually it isn't, so you must make adjustments, and that's where the experience comes in.

d. Since you now know how the meter makes everything gray, then you must decide how much lighter or darker you want your subject to really look, and instead of following what the meter says, adjust the exposure. For example, a white house generally is about five times as bright as the gray a meter wants to make it, so you would open your camera lens (or adjust your shutter, or a combination of both) to give slightly over two stops more exposure than the meter says you should. And that black cat is really about three stops darker than the gray the meter would make it, so you should adjust your actual exposure setting to give about three stops less than what the dial tells you (fig 1-15) . You can take advantage of this in other ways, too.

Figure 1-15. Reflected light reading technique

e. Kodak and other companies make a gray card which reflects 18 percent of the light striking it, which is an average middle gray. If you are trying to take a picture of a very light or very dark scene that is not average, place the gray card in the scene and take a close-up reflected meter reading of it. Then you can use this exposure to make your picture. Since the card is midway between black and white, all the other shades will be reproduced in the print.

(1) Even if you don't have a gray card, try to find something in the scene which you know should be about the same shade as a gray card. Take a close-up reading of that one small part of the scene and use the result to set your camera.

(2) Another way to determine a light reading is to use the palm of your hand as a starting point. The average palm skin tone is about one stop lighter than a gray card, so in a pinch, you can just place your hand in the same lighting as the scene you are photographing and meter it as you would a gray card. Then set your camera to give one stop more exposure than the meter says and you're set. If you take a comparison reading of a gray card and the palm of your hand (and remember what the difference is) you can adapt the technique for your own use.

(3) To carry this a step further, white houses are about five times as bright as middle gray. In fact, almost anything painted white or whitewashed is about 2-1/3 stops brighter than middle gray. So if you meter a white painted wall and then give 2-1/3 stops more than the meter indicates, then you will be pretty close to the right exposure. Most gray cards are gray on one side and white on the other. This white side is exactly five times as bright as the gray side. You can get a very good exposure by metering the white side of the card and giving five times the exposure the meter says. You can do this by opening the lens about 2-1/3 stops, or by adjusting the ISO setting on the meter to 1/5 what you would use with a gray card. This is particularly useful when taking a meter reading in dim scenes when there isn't enough light to make a meter's needle move. By using a white card, you can effectively extend the meter's usable range by a factor of five.

f. You must take care, when metering small areas, to hold the meter close enough to the area so that you are reading only the portion of the scene you want. If you include areas that are lighter or darker than the spot you wish to read, you'll get incorrect readings. Also, be careful about shading the meter's target with your body, your hand, or the meter itself.

7. Cameras with built-in meters are highly sophisticated, but they are still basically reflected light meters, and are susceptible to the same failings hand-held meters are. Many through-the-lens meters are center weighted, which means the meter pays more attention to the brightness in the center of the scene than to its edges. This is useful in certain lighting conditions, but doesn't do much good if you are trying to meter a darker or lighter than average scene which you don't want to look middle gray. Even the most sophisticated camera meter will try to make a white house or black cat the same shade of gray, just as the hand-held meter will. But the camera meter can be manipulated the same way a hand-held meter can, and all the tips in this lesson can be used with your camera just as effectively as well. Of course, once you know how to use a calculator dial, you might find it so useful you may regret that cameras with built-in meters don't have one.

8. Ansel Adams, one of the great nature photographers, made an entire career of writing about how to make adjustments to reflected exposure readings. In the hands of an expert, reflected light readings can give absolutely beautiful exposures. But most of the time, you can get just as good an exposure by taking an incident reading and using exactly what the dial tells you. Incident readings aren't perfect, and in really unusual conditions such as very dark or very bright subjects or unusual lighting conditions you might not get a great exposure without making an adjustment or two. In general, however, the beginning photographer will find that incident readings are much more reliable than reflected ones.

9. Most important is learning not to be a slave to those ISO numbers. If you use a light meter correctly and get consistently overexposed or underexposed negatives (and the key word here is consistently), then adjust the film speed setting accordingly. If your negatives are always denser than you like them, then instead of using the manufacturer's ISO setting, use double that number and try another roll of film. Once you find an ISO setting that consistently gives you good results, then stick with it. There are all sorts of reasons you may have to do this; your camera shutter may be a little faster or slower than marked, the lens apertures may not be accurate, your metering technique may not be quite the same as that recommended by the manufacturer, the processing chemistry might be different, and so on. Don't be afraid to experiment if you don't like the results when you follow the book. Of course, if you aren't getting consistent results - some negatives overexposed, some under, others OK -then you need to examine your own techniques to find out what is wrong. It may be that you are unconsciously using a bad technique, or that your equipment is malfunctioning.

10. Care of the equipment. While this part is specifically about light meters, many of these requirements apply to all photo equipment.

a. Read the instructions. This is so simple, yet so overlooked, that it almost seems insulting to mention it. But many times a piece of equipment is damaged just because the user thought he knew what he was doing and was sadly mistaken. Most instruction books thoroughly cover the care requirements for a piece of equipment and often also offer a good troubleshooting guide to help you find out what is wrong when something just doesn't seem to work. They deserve your close attention.

b. Protect the meter from shocks. A meter which uses a needle movement is as delicate as a watch. A sharp blow will knock the mechanism off its bearings so it will not work smoothly. It's a good habit to check your meter frequently (at least before each day's use) by covering and uncovering the sensor with your hand and watching the movement of the needle. If it swings smoothly, then it's all right. But if it sticks even slightly, it could be damaged and should be turned in for repair. Also, check the zero setting. To do this, cover the sensor completely with you hand, being careful to block out all the light. The needle should come to rest on a zero mark on the background scale. If it doesn't, there is usually a small screw on the back which will adjust the needle so that it does. Otherwise, the meter will give incorrect readings.

c. Keep the meter away from extreme heat. If it is heated above 120 degrees Fahrenheit for any appreciable length of time, the meter could be severely damaged. In particular, a glove compartment on a warm sunny day can ruin camera gear.

d. Keep the meter away from moisture. Water can short out the wiring, and can also cause metal parts to corrode, affecting the smooth operation of the mechanism. If you must use the meter in wet weather, seal it inside a clear plastic bag.

e. Never point the sensor directly at the sun or any extremely bright light source; this can easily damage it. In some cases, the damage isn't permanent, and the sensor is only "blinded" for a while and will recover after a few minutes or hours. Until it does, though, it won't be reliable. In severe cases, the damage can be permanent.

f. If you don't plan to use the meter for an extended period of time (two weeks is a common recommendation), remove the battery. The battery case could rupture and acid could leak out.

11. Summary of lesson. By now you should know the theory of exposure, what f/numbers are, how shutter speeds work to control exposure, how to find equivalent exposures, how to get a usable exposure using the daylight exposure table, and how to use a light meter. To find out how much you've learned, answer the questions that follow, then check in the back of this book to see how well you did.

Lesson 1 PRACTICE EXERCISE

1. When light strikes a piece of film, what is the term used for the undeveloped image?

a. Indelible image b. Latent image c. Incomplete image d. Late image e. Inclusive image

2. The exposure formula is written E = I x T. What do the E, I, and T stand for?

a. Exposure, Index, and Time b. Exposure, Intensity, and Time c. Externity, Inherency, and Time d. Extremity, Intensity, and Tendency e. Current, Voltage, and Resistance

3. Complete this sentence: Two combinations of I and T which result in the same E are called_.

a. Actual exposure b. Acceptable exposure c. Equal exposure d. Equivalent exposure e. Equitable exposure

4 . The opening in the lens which light passes through is called the _

a. Pore b. Actuality c. Aperture d. Operculum e. Oculus

5. The device in the light path through a lens which controls the size of its opening is called the_ .

a.

Diarhomb

b.

Diatherm

c.

Diaphage

d.

Diaphragm

e.

Diastygma

6. What does the "f" stand for in an f/number?

a. Focus b. Factor number c. Focal width d. Focal point e. Focal length

7. When changing from a small f/number to a larger one, what happens to the amount of light passing through the lens?

a. The amount decreases b. The amount increases c . There is no change d. The change is minimal

8. When changing a lens' opening from f/32 to f/4, how much of an increase or decrease in brightness does this represent?

a. Increases 64 times b. Increases 5 times c. Decreases 64 times d. Decreases 5 times e. Increases 10 times

9. A four-stop change in a lens aperture changes the light by how much?

a. 4 times b. 8 times c. 16 times d. 2 0 times e. 32 times

10. (True or false) It is possible to change the aperture settings on a lens by amounts less than full stops?

11. (True or false) It is possible to change shutter speeds by less than the amounts marked on the control dial?

12 . What are the two basic types of camera shutters currently in use in most common cameras?

a. Optical and physical b. Leaf and blade c. Leaf and focal plane d. Waterhouse and focal plane e. Optical and mechanical

13. How much more light passes through a lens at 1/15 second than at 1/500? How much is this in f/stops?

a. 64 times, 5 stops b. 32 times, 5 stops c. 32 times, 6 stops d. 16 times, 4 stops e. 16 times, 5 stops

For questions 14 and 15 use an initial exposure setting of 1/30 second at f/45 .

14. What shutter speed is equivalent to the initial exposure setting if the f/stop is changed to f/11? f/4? f/64?

a.

1/500,

1/2000,

1/15

b.

1/250,

1/4000,

1/8

c.

1/500,

1/1000,

1/15

d.

1/500,

1/4000,

1/15

e.

1/500,

1/4000,

1/60

15. What f/number is equivalent to the initial exposure if the shutter speed is changed to 1/2 second? 1/500 second? 1/8 second?

For questions 16 through 20 refer to the daylight exposure table, Table 1-7. Use the shutter speed sequence for focal plane shutters.

16. Using ISO 125 film in bright sun and shooting an average subject, what is the basic exposure?

a.

1/125

second

at

f/8

b.

1/125

second

at

f/11

c.

1/125

second

at

f/16

d.

1/125

second

at

f/32

e.

1/60

second

at

f/32

17. What is the basic exposure using ISO 125 film in bright sun, backlit with a bright subject?

a.

1/125

second

at

f/8

b.

1/125

second

at

f/11

c.

1/125

second

at

f/16

d.

1/125

second

at

f/32

e.

1/60

second

at

f/32

18. Using ISO 64 film in cloudy bright sun, side lit, dark subject, and an f/number of 4, what should the shutter speed be?

a. 1/60 second b. 1/125 second c. 1/250 second d. 1/500 second e. 1/1000 second

19. Using ISO 400 film in cloudy, dull lighting, brilliant subject, and a shutter speed of 1/250 second, what should the f/number be?

20. Using ISO 200 film in bright sun, backlit, f/number of 5.6, what should the shutter speed be?

dark subject, and an a. 1/30 second b. 1/60 second c. 1/125 second d. 1/250 second e. 1/500 second

21. What is the "sunny f/16 rule"?

22. If a lens of 200 mm focal length has an opening of 36 mm diameter, what is the f/number?

23. If a lens of 50 mm focal length is set at f/11, how large, in millimeters, is the diameter of the lens opening?

a.

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Reasonable care has been taken to ensure that the information presented in this book isĀ  accurate. However, the reader should understand that the information provided does not constitute legal, medical or professional advice of any kind.

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