Why use exposure compensation

why use exposure compensation


EV , Footcandles and LUX (For The Truly Dedicated)

Imagine an exposure computer so advanced that it uses your eyes as a sensor. The processing unit is as powerful as your brain. The computer is accurate over a light range from reflected starlight through the light produced in a hydrogen fusion reaction. This computer weighs nothing and operates without batteries. It comes with instructions to allow you to implant the capabilities of the computer directly into your own memory so you can accurately judge a correct exposure by simply looking at the type of light that the subject is in. You are using this computer right now!

Everything I've said above is true. However, I'm sure you've noticed that the only thing before your eyes is a World Wide Web page. An entire photographic industry has been built by convincing consumers that the subject of photographic exposure is so arcane, difficult and impossible to understand, that it is best left up to technology. This, of course, allows companies to sell billions of dollars worth of equipment to figure it all out for you.

The fact is that the concept of photographic exposure is extremely simple. It can be mastered by anyone who can multiply or divide by two. Everything you need to know is contained on these few Web pages.

Knowledge of photographic exposure is essential to controlling the creative side of the photographic process. This knowledge increases the chance that the photograph that comes back from the lab is the one you envisioned when you pushed the shutter release. Your camera or light meter can not make creative decisions for you. If you want to control the creative side of photography, you need to understand the interrelationship of Exposure Value, film speed, aperture and shutter speed. You must have this understanding even though you own the most sophisticated equipment available. I have watched photographers (amateur and professional) struggle with this subject for a number of years, confused by the misinformation that abounds in the photographic press. I decided to publish this document to clarify, simplify and demystify the issue.

But first, a word about copyright. This document is copyrighted. In order to keep the cost down, it has been published in a form that would be easy to copy. Copyright law strictly forbids copying this document. You may print a copy or two of the Ultimate Exposure Computer to keep in your camera bag for personal use. In fact, I would encourage it. All I ask is that you send me an e-mail (fred at fredparker dot com) to let me know that you've downloaded this document. If you get a chance, drop me another e-mail after you've worked with it and let me know if you have any suggestions. However, please do NOT make copies for your friends, students or any other person. Originals are inexpensive and easy to obtain. If you have a related site (commercial or otherwise) please hyperlink to this site for access to this document. If you download all or part of this document and post it directly to your site, it will be assumed that you have decided to purchase redistribution rights, which START at $3500. Thanks!


Light meters can be less accurate than you might imagine. Their response to various colors of light may not match that of your film. Their response in low light and high light conditions may not match response at the mid-range of light. But the worst characteristic, for practical purposes, is that all reflected light meters make one basic (erroneous) assumption. They are calibrated to render an exposure that will make the subject look like a middle tone in the resulting photograph.

How do you compensate for the fact that your meter is lying? You have to lie to your meter. If you are photographing a light toned subject, such as snow or sand in bright light, you must convince your meter that it is looking at something even lighter than your subject, so that the meter's attempt to make it a middle tone will result in the right tone. The same is true for dark objects. You must tell your meter that the object is darker than it really is so that the meter's compensation will register the correct tone in the photograph.

You can tell this white (or black) lie to your meter in a couple of ways. You can meter a middle-toned area in the same light as your subject and manually set your camera accordingly. You can lie to the camera outright by telling it that it is using film of an ISO rating different from what is actually in the camera. For instance, if you are using ISO 100 film and are photographing a light subject, you would tell the meter that you have ISO 50 film. If you are photographing a dark object, tell your meter that you have ISO 200 film in the camera. This method works well for cameras in automatic exposure modes. Be careful to tell your camera the truth if you move to a middle toned subject.

Another way to get the right exposure from a lying meter is to politely ignore it. Switch from automatic to manual exposure (a good idea anyway) and simply move to the next larger aperture or the next slower shutter speed (for a light object) or to the next smaller aperture or faster shutter speed (for a dark object).

So it goes. Your meter deceives you, so you deceive it. But what kind of relationship is this? The Ultimate Exposure Computer does not make erroneous assumptions, because you tell it what type of light your subject is in. As long as you tell it the truth, it will not lie to you. Definitely a better relationship.

There is a chance that the preceding section may have confused you. If that is true, don't worry about it. The next section defines Exposure Value, film speed, shutter speed and aperture (f/stop) and describes the interrelationship among these factors. After you've read the next section, revisit this section again, because the concept is important.

Exposure Value. FILM SPEED, SHUTTER SPEED AND f/stops -- What Are They?

Each of these four factors is represented by a series of numbers. Your camera may have only a portion of each series indicated.

Exposure Value:

In most light meters, photons of light that are reflected from your subject put pressure on a photo-sensitive receptor in your light meter and are converted into electricity (this is why some meters do not require batteries). The greater the number of photons, the greater the electricity that is produced. Internationally accepted standards specify exactly how much light pressure equals a certain EV number. This value is measured by the following number series (for ISO 100 film):

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23

Easy! However, even though this appears to be a linear progression, each number represents twice as much light as the numbers increase. This provides an easily memorized scale to clarify the concept of "absolute value of the amount of light falling on a subject". This term is related to illuminance, footcandles and lumens. But it doesn't matter. If you are using a light meter this number is irrelevant, and you can ignore it. You may never see these numbers, because the meter uses them internally to calculate a set of combinations of shutter speeds and apertures. However, knowledge of Exposure Value can significantly enhance your ability as a photographer. The Ultimate Exposure Computer gives you the knowledge to take control of your photographs. See the section "Throw Away Your Light Meter!" for details.

In order to be useful, this Exposure Value number must be converted into an f/stop and shutter speed combination that will provide the proper exposure for the film speed you have chosen. The relationship between these four elements is represented in the Ultimate Exposure Computer.

Film Speed:

Film speed is a number that indicates the sensitivity of film to light. Film sensitivity is measured by a set of standards established by the American Standards Association (ASA) or the International Standards Organization (ISO). For all practical purposes, the ASA and ISO numbers are interchangeable. In general, films with a higher sensitivity (larger number) have coarser grain and do not register detail as well as films with lower sensitivity (lower number). The number series for film speed is:

25, 50, 100, 200, 400, 800, 1600, 3200

Pretty straight forward! Moving to the right, each number is twice the preceding number, and represents twice the sensitivity to light as the preceding number. There may be some intermediate steps (such as 64 or 125) on your dial. Set the light meter or camera for the same number that is on the film. Your camera may do this automatically.

This leaves only two things to adjust to achieve the correct exposure while making a photograph; shutter speed and aperture (f-stops). Shutter speed and aperture are very important to the creative photographic process.

Shutter Speed:

Shutter speed indicates how long the camera shutter remains open to let light onto the film. The number series for shutter speed is:

15, 8, 4, 2, 1, 2, 4, 8, 15, 30, 60, 125, 250, 500, 1000, 2000, 4000, 8000

This looks more complicated, but it's actually straightforward. These numbers are whole seconds or fractions of seconds. They aren't expressed on your shutter speed dial as fractions to save space, so they should read as below:

15, 8, 4, 2, 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000, 1/8000

Again, each number moving to the right is half the value of the preceding number, and represents half as much light as the preceding number.

There is an important rule regarding shutter speeds:

If your shutter speed is slower than the reciprocal of the focal length of your lens, you must use a tripod.

For example, if you are using a 200 mm lens, your shutter speed must exceed 1/200 second (I.E. 1/250 or faster). If not, you should use a tripod. If your subject is moving, double this shutter speed. If you are moving (such as in a boat or plane) triple the speed. If you are doing macro work (.25 magnification or greater), always use a tripod. If you're using a format larger than 35mm, use a tripod. To be safe, weld your camera to the tripod. If you are a serious nature photographer you will always use a tripod, anyway. A tripod will allow you to make photographs with slower film speeds. Slower film speeds equate to better detail and sharpness in your photographs. More important, using a tripod makes you slow down and allows you to examine your composition more carefully. Modern electronic cameras may lack certain features of the older, mechanical cameras (such as a button that allows you to see the depth of field that will be present in your photograph), but every camera has a tripod socket. You'll make better photographs if you use it.

Aperture (f-stops):

Aperture refers to the size of the opening inside the lens that the light must go through to reach the film. Aperture is measured in f/stops as indicated in the series below:

1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45

This looks tougher, but the solution is the same as for the shutter speeds. These are actually fractions. They should read as follows:

1/1, 1/1.4, 1/2, 1/2.8, 1/4, 1/5.6, 1/8, 1/11, 1/16, 1/22, 1/32, 1/45

Like the shutter speed series, each progression represents half as much light (moving to the right) as the preceding number. But why do the numbers look so odd? I recommend that you ignore it, because it doesn't matter anyway. Just accept that each progression represents half as much light as the preceding number.

If you are curious as to how we got to such a seemingly illogical progression of numbers, read the following three paragraphs. If you aren't curious, you can safely skip them.

The numbers represent the ratio of the focal length of the lens to the diameter of the lens diaphragm opening. That's why it's called an f(ocal)/number. The designation "f/2" means that the diameter of the aperture is 1/2 the focal length of the lens. The designation f/32 means that the diameter of the aperture is 1/32 the focal length of the lens. f/2 on a 100mm lens means that the diameter of the diaphragm opening is 100/2, or 50mm. Unfortunately, the amount of light reaching the film is dependent on the SURFACE AREA of the opening NOT the DIAMETER. As you no doubt learned in high school, the method of calculating the surface area of a circle is Pi times the radius, squared (Pi is approximately 3.14; the radius is half the diameter, squared means that the number is multiplied by itself). Therefore, in our example, the surface area of the opening would be 3.14X25X25, or approximately 2000 sq. mm.

Now, let's look at the next f/stop, which is f/2.8. 100/2.8=35.7mm. The surface area would be 3.14X17.85X17.85. If you multiply it out, you will see that the surface area is now approximately 1000 sq. mm, or HALF the surface area of f/2.Therefore, each succeeding smaller aperture lets in half as much light as the previous f/stop.

The reason we use the ratios instead of the actual surface area of the diaphragm opening is that the actual surface area would be quite different between lenses of different focal lengths. Photographers would have to memorize a series of numbers for each focal length lens they owned. Think about what you'd have to do with a zoom lens! Ratios allow us to use the same number series for all of our lenses. f/2 on a 100mm lens lets in exactly the same amount of light as f/2 on a 500mm lens. Trust me, f/numbers are the best way to do this. I would still recommend that you simply accept that each successive f/stop represents a doubling (or halving) of light. However, since you've read this far, I'll give you a hint about how to memorize the f/number series. The good news is that you only have to remember two numbers; 1 and 1.4. Double each one alternately and you will have two series:

1, 2, 4, 8, 16, 32 and 1.4, 2.8, 5.6, 11, 22

So, combining the two sequences in numerical order gives the full sequence:

What you need to remember is that, within each factor, as you move from one number to the next, the amount of light is either doubled or halved. Some cameras may include numbers between the numbers mentioned above. Those numbers represent half or third stops. For the purpose of what you are learning here, ignore the in-between numbers.


Well. maybe not. But you could if you wanted to. The purpose of this section is to show you how Exposure Value, Film Speed, Aperture and Shutter Speed are interrelated, and why you don't need a light meter to know the proper exposure.


Automatic cameras are designed to give an average photographer a better chance of getting properly exposed pictures while shooting average subjects in average light, without a tripod to steady the image. These cameras work well for family pictures and many urban subjects. Automatic focus and exposure (especially TTL flash) are extremely useful for some specialized types of photography, such as sports or photojournalism.

Nature photography is a different matter. Many of your subjects will not have "average" tonality. Most of your subjects will not be located in "average" light. Many times your subject will not be in the center of the frame (where an automatic camera takes its light readings). However, the most important reasons for putting your camera on manual while photographing natural subjects are:

1) There are times when you will want to stop motion, such as a flower waving in the breeze. Other times you may want to exaggerate the effect of motion, such as when photographing flowing water. Decisions about these matters address APPARENT SUBJECT MOTION. This is controlled by the shutter speed. Faster shutter speeds stop action better than slower speeds (1/1000 second will stop most motion, while 1/8 second will blur even slow moving subjects).

2) Sometimes you will want everything in focus, such as a foreground flower AND a distant mountain. Sometimes you will want the flower to be in focus while keeping a

busy background softly out of focus. When you make this type of decision, you are considering DEPTH OF FIELD (the distance in front and behind of the subject that is acceptably in focus). The aperture (f/stop) controls the depth of the image that is acceptably in focus (f/32 keeps more of the subject in focus than f/2).

You must make these aesthetic choices. The camera has no way of knowing what you want the photograph to look like. Remember that the camera's automation is designed to make technical decisions based on average subjects in average light. It is not designed to make aesthetic choices regarding natural subjects (which are rarely "average").

Because of these factors, the same camera that will give you dozens of perfect photographs of your children opening birthday presents will churn out dozens of badly exposed nature photographs that look nothing like what you experienced in the field. The only way for you to get good results in nature photography is for YOU to make the choices.

Get your camera. Purchase two sets of batteries and install one set in the camera. Do this even if you changed batteries last month. Keep the spare set with the camera, always. Keep a copy of the Ultimate Exposure Computer with the camera and the batteries.

Find your instruction manual. Now comes the hardest part of the entire process (unless you've lost the manual). You must find out how to set your camera body to its "manual" setting. I can't help you here. Good luck and congratulations! You've just taken the first step toward becoming a serious nature photographer!


CHART A: Take a look at Exposure Value Chart A. The left column of the chart contains the Exposure Value (EV) numbers. The right column of the chart contains some lighting situations that equate to EV numbers. The chart ranges from EV -6 to EV 23. This represents a range of light from below reflected starlight to the brightest light in the Solar System.

Notice that most daylit subjects fall within a narrow range from EV 11 to EV 15. It is a good idea to memorize the characteristics of these daylight Exposure Value factors. Pick a favorite lighting situation, note the EV and turn the Ultimate Exposure Computer over to Chart B.

CHART B: Exposure Factor Relationship Chart B shows how Exposure Value, film speed, aperture and shutter speed are related.

On the left side of this chart, along the top row, are the ISO/ASA film speed numbers. Pick the film speed you use most often. Starting with that film speed, move down the column until you reach the EV number that you chose from Chart A. Now move to the right along the row until you cross the double line. To the right of the double line is a row of shutter speeds. As you will recall, a fast shutter speed stops motion, while a slow shutter speed enhances the effect of motion. Choose an appropriate shutter speed for your subject. Now, move straight up the column to the top row to find the aperture (f/stop) for use with the shutter speed. Enter your chosen shutter speed on your camera and set your lens to your chosen f/stop. It's that easy!

Suppose you are in the field photographing with ISO 100 film, and the battery in your camera just failed. You can't take any light readings, and you only have one mechanical shutter speed (1/125 second). This situation is extremely unfortunate, since you just stumbled onto an extremely rare species of flowering plant that blooms for one hour every hundred years. It's in full bloom. The sky has started to cloud over and things look bleak. Have no fear! Your Ultimate Exposure Computer will save the day! You already know from Chart A that subjects under heavy cloud cover are at EV 12.

Look on the left-hand side of Chart B. Locate ISO 100 film along the top row. Look down the column until you find EV 12. Look along the row to the right of EV 12 (across the double line) until you find the shutter speed you are looking for (your mechanical 1/125 second). Move straight up the column and find that the aperture of your lens should be set to f/5.6. Easy! No light meter required!

But wait! Suppose the f/stop you landed on isn't on your lens. Or perhaps the f/stop you landed on offers too much or too little depth of field (the distance in front and behind of the subject that is acceptably in focus). In that case, move to an appropriate f/stop along the top row to achieve your desired result. Then move down the column to the row represented by your original choice of film speed and EV number. At that intersection you will find the appropriate shutter speed for the desired aperture.

Perhaps this exercise indicates a very slow shutter speed that cannot be hand held. That means one of two things. You must put the camera on a tripod (where it should have been to begin with), or you need to determine the proper film speed to use for your photographic situation.

The Ultimate Exposure Computer is useful for several photographic tasks, such as planning a shooting session. For instance, you may know that you are going to be shooting scenics of a field of flowers with mountains behind. It will be a sunny day (EV 15), but the wind will be gusting to twenty miles per hour. You need a small aperture to get the greatest depth of field (the distance in front and behind of the subject that is acceptably in focus). but you will also need a fast shutter speed to stop the motion of the flowers in the wind. What ISO film speed will you need to do the trick?

Go to the upper right of the Table B. Find f/22 aperture for maximum depth of field. Go down the column until you reach a shutter speed fast enough to handle the flowers in the wind (1/125 second). Move to the left along the row (across the double line) until you find the correct Exposure Value (15). From there, move up the column and find that ISO 200 speed film will solve your problem.

The Ultimate Exposure Computer is designed for use in existing light. If you need to use flash, or you are taking bellows shots, by all means use a camera with a through-the-lens meter. When using flash, use the manufacturer's dedicated flash unit that allows through the lens control of the flash. Those situations are excellent reasons to use the meter in your camera. Some of the lighting situations described in Chart A occur outdoors during daylight. In these cases it is assumed that the sun is at an elevation greater than forty-five degrees. If the sun's elevation is less than forty-five degrees, you can use the next lower Exposure Value, although you'll probably like the results of using the chart without modification. For some films, exposures involving shutter speeds in excess of several seconds may require additional exposure because the film's sensitivity decreases with continued exposure to light for long periods (this is called "reciprocity failure"). Light meters do not correct for this phenomenon, because it varies according to the type of film. Consult the manufacturers' specifications for details. There are some tricky exposures where you can improperly expose the film whether you are using a camera meter or the Ultimate Exposure Computer. Many of these situations are addressed in "What to do in Tricky Light Situations" in Appendix A.


The Ultimate Exposure Computer is easy to use and extremely accurate. But there is an easier way. Why not memorize the whole thing? It's a lot easier than you may think. This section will show you how.

There is a rule called the "Sunny f/16 Rule". It says:

On a bright, sunny day, the correct exposure for any subject is f/16 at the shutter speed nearest to the reciprocal of the film speed.

For example, if you are using ASA/ISO 100 film, the correct exposure would be 1/100 second (or rather the closest available shutter speed - 1/125 second) at f/16. The next three paragraphs address this example.

We know that a bright, sunny day equals EV 15. The "Sunny f/16" rule gives us an anchor point to use in mentally calculating the entire contents of Chart B, since it contains all four elements of exposure: Exposure Value (EV 15), film speed (ISO 100), aperture (f/16) and shutter speed (1/125).

From that anchor point, it is easy to calculate any other set of correct exposures. We know that each step up or down in one variable represents a doubling or halving of the amount of light required to make a correct exposure. For example, an overcast day would halve the light falling on the subject (EV 14). If the light reaching the film is cut in half, ONE other variable needs to be changed to increase (double) the amount of light. In this case the shutter speed could be reduced to 1/60 second OR the aperture could be increased to f/11 OR the film speed could be increased to ISO 200. Any ONE of these corrections would provide the correct amount of light. Make the correction that best suits your photographic purpose (decreased depth of field -- the distance in front and behind of the subject that is acceptably in focus -- or enhanced apparent subject motion).

In the same sense, a change in any of the variables can be offset by a reciprocal change in any other variable. For example, you may need a faster shutter speed to stop some action. If you choose 1/500 second, the light will have been reduced by two steps (cut in half from 1/125 to 1/250 and cut it in half again from 1/250 to 1/500). You could compensate for this by opening up the aperture two steps (from f/16 to f/11 will double the light and f/11 to f/8 will double it again). You could make the same correction by increasing the film speed by two steps (from ISO 100 to ISO 200 doubles the light and from ISO 200 to ISO 400 doubles it again). Or, you could increase the aperture to f/11 AND increase the film speed to ISO 200. Again, make the adjustment that best suits your purpose.

Now, let me say that the "sunny f/16" rule is one of the most misunderstood rules in photography. I have heard very competent professional photographers say that the rule is based on reflected light. Nonsense! The rule is based on the light falling on the subject (incident light) not the light reflected from the subject. The rule gives precisely the same measurement that you would get from an incident meter or a reflected meter using a perfect gray card. If your subject is very light or dark, you will need to adjust exposure to bring it within the five stop range of transparency film. But you would also have to make an adjustment from a reflected reading in the same circumstance. The difference is that an incident light measurement is a consistent anchor from which to adjust. A reflected measurement is much more subjective and prone to error. I have also heard that the "sunny f/16" rule doesn't work for backlit or sidelit subjects. This is also false. Just open up a half stop, and make a series of exposures in half stop increments one and a half stop above and below the correct exposure. This is called "bracketing" and it is an important concept. You should always bracket in difficult situations. Anyone who says differently is misleading you. See the section on "Difficult Exposures" for guidelines.

The "sunny f/16" anchor point, combined with your knowledge that each change of one step in a factor doubles or halves the exposure, makes it easy to select a correct exposure for any photographic situation you may be confronted with.

Practice this technique with the Ultimate Exposure Computer in hand. Look at (or imagine) a photographic situation and try to mentally figure out the correct exposure using the "Sunny f/16" anchor point and any adjustments you feel are necessary. For the purpose of this exercise, it is best to concentrate on EV 11 through 16 (where most of your outdoor photography will occur). As you perfect your accuracy in this range, you can move on to more exotic lighting situations. Write down the factors (Exposure Value, film speed, aperture and shutter speed) that you believe will make the best exposure. Consult the Ultimate Exposure Computer to assess the accuracy of the calculations you made mentally.

With a little effort you will soon be achieving close to 100% accuracy, without the use of a light meter or any other exposure aid. The more you practice with the Ultimate Exposure Computer. the sooner you will be able to simply look at a lighting situation and immediately know the correct exposure for the job.

Once you have mastered this technique, you can use it to judge the accuracy of the light meter in your camera (or anyone else's). You will be able to amuse and edify your photographic friends by accurately stating the correct exposure in any situation before your friends can take a reading with their meter!

When you reach that point, the real Ultimate Exposure Computer will be


I get e-mail from all over. Photographers use this document on every continent. Even Antarctica. People taking photographs far to the north or south of my sunny San Diego, California (USA) location (32d N) have asked me to make charts adapted to extreme (from my perspective) latitudes. Here's why: If you live anywhere between the equator and about 50 degrees north or south, the charts and tables in the Ultimate Exposure Computer should work fine for you (remember to bracket). Just move on and don't read the next few paragraphs. If you live further north or south, it should work for you in summer. But things aren't so simple if you live toward the poles. Light from the sun is diminished by traveling farther through the atmosphere or blocked by the curvature of Earth. Look at it this way: near a solstice, natural light at mid-day can range continuously from "Sunny f/16" at the equator to near dark at one of the poles. So you have the variables of latitude and season and time of day to consider. There are other variables such as altitude and snow cover (partially counteracting the effect of latitude and season).

"So," you say, "You've taken me this far and it turns out this won't work for me?" I say, the Ultimate Exposure Computer will NOT let you down. "Well then," you say, "Are you going to give me some complicated, hashed-up chart that tries to fit in the five variables you talked about?" Well. Actually I DID try to do this. What a mess! I'm striving for simplicity here, not trying to model the light falling on every square inch of Earth! Fortunately, the solution is simple, because I know something about you that you've never admitted to me. You DIDN'T throw away your light meter, did you. Hah! I knew it! Good! Read on!

You can easily calibrate the Ultimate Exposure Computer to fit your exact location. Go outside and take a meter reading (preferably incident or reflected from a standard gray card). Use whatever light is available, but it is best if it's full sun on a bright day (Sunny f/16). Take a few readings and average them. How many stops difference between your reading and the "Exposure Value Chart"?

Print out two copies of the "Exposure Value Chart" (below). Using a sharp implement (your choice) cut one chart apart, separating the "EV" section from the "Type of Lighting Situation" section. If, for example, your metered reading was two stops below "Sunny f/16" you would slip the "EV" section down two rows. Tape it in place. You now have a chart customized to your latitude, season, time of day, altitude and any other variable you can think of (remember to bracket). You'll need to repeat this exercise as the seasons and conditions change. This is only necessary for sunlit subjects. Other subjects in the chart will work using the copy you didn't deface. There are limitations. Here's just one example: If you're at McMurdo Station on June 21, you'll need to find another way to calculate exposure. Drop me an e-mail and tell me how you make photographs. For most of you intrepid poleward photographers. enjoy!

Source: www.fredparker.com

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