Understanding exposure

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As newer, better, faster digitalcameras enter the market, many ofus who were hesitant at first arenow taking the plunge into theworld of digital imaging. Alongwith the promise of moreresolution and better colorreproduction comes the myth thatphotography no longer needs to bea manual process. Today, manynewcomers to photography cannotresist but to set their top-of-the-line digital SLR to the Programmode and postpone learning aboutany of the other modes or basicsettings available to them.

In reality, even with the most advanced camera systems and software, photography has notchanged much since the day that the very first photographic image was recorded. To this daywe are still painting with light, albeit using a much more advanced arsenal of tools.

In this lesson, we will take you all the way back to the basics. We will explore thefundamental, manual process that makes photography what it is, "painting with light." By theend of this lesson, you'll be able to take full control over your camera settings includingshutter speed, aperture, and ISO. These basic camera settings fall under the umbrella ofexposure. Understanding exposure is the first and most important step to unleashing your fullphotographic and artistic potential.

(Most images can be clicked for an enlarged view.)

Topics Covered:

How Cameras WorkUnderstanding the Digital SLRThe ApertureExploring Depth of FieldThe Shutter SpeedThe ISO SettingThe Light MeterTying It All Together

Equipment Used:

18/04/2009 Understanding Exposure

webphotoschool.com/…/index.html 1/18

Camera/Media

Olympus E-510

From the very first box cameras to be mass-produced in the 19th century all the way to themost current digital cameras, the mechanics of exposure have remained relativelyunchanged. The most notable difference today is that the camera is now able to do most ofthe "thinking" and arrive at various exposure settings automatically. Nevertheless, the camerais not right 100% of the time. More importantly, the camera does not know, nor does itcare, about your personal vision for any given photograph. Therefore, in order to gaincontrol over one's photographic vision, one must understand how a camera works and howto manually control the mechanical process of exposure.

How Cameras WorkFirst, let's explore the internal workings of a camera. Simply stated, the camera is reallynothing more than a lightproof box with a hole on one side and a light sensitive material onthe other side.

Figure 1

The very first camera actually existed longbefore there was such a thing as film or lightsensitive material. It was called the CameraObscura (translated from Latin as "darkchamber"). The mechanics of a CameraObscura are simple (figure 1).

It is a light-tight box with a tiny pinhole onone side. Light enters through this tiny pinhole and an upside down, laterally reversedimage of whatever is outside or in front of thepin hole is projected onto the interior wall ofthe dark box. It is really quite magical and itreally does work. However, without any filmor digital sensor, it is practically useless tophotographers.

The next logical step to the Camera Obscura is a basic, homemade pinhole camera. Anyonewho has taken a beginning photography course in the pre-digital age was probably taughthow to make one. A pinhole camera is a light-tight box with a hole on one side and a pieceof photographic paper on the inside. As with the Camera Obscura, light enters through thepinhole and an image is projected onto the light-sensitive paper. By controlling the amount oflight entering through the hole and reaching the paper, one is able to control the exposure ofthe recorded image.

If this concept is still a little unclear, don't worry. For now, just remember that a cameraneed not have any dials or LCDs in order to be a camera. Rather, a camera is simply a darkbox.



Understanding the Digital SLRCamera Obscuras and pinholes aside, you might still be asking yourself how all of thisrelates to your brand new Canon 20D, Nikon D200, or Olympus EVOLT E-3. Well, thetruth of the matter is that every one of these modern digital cameras are build around thesame design concept as the basic dark box example shown above.

The Digital SLR is also a dark box with a hole on one side and a sensor on the other side.SLR is an acronym for Single Lens Reflex. Single Lens Reflex means that the lens that youare looking through when you look into the viewfinder is the same lens that is taking thepicture. There are other types of cameras where this is not the case, but for the purpose ofthis lesson, we will direct our attention to the SLR as this is the most common type ofcamera being used today.

The diagram on the right (figure 2) illustratesthe inner workings of an SLR (Single LensReflex) camera. In a nutshell, here's how itworks. Light enters through a lens instead of apinhole. The lens is made up of numerousglass elements, which work together to focusthe light coming in. Having a lens instead of apinhole makes it possible to let in a muchlarger amount of light while keeping theprojected image in focus. After the light passesthrough the lens it encounters a mirror, whichsits at a perfect 45-degree angle to the lens.The mirror reflects the light upward into aspecially shaped prism. The prism bounces thelight around its various surfaces turning theimage right side up.

Figure 2

When you look inside the viewfinder of an SLR, you are seeing the image reflected inside ofthe prism. Keep in mind that this entire mirror and prism mechanism is only there so that youcan see the image right side up in the viewfinder prior to taking the picture.

As soon as you press the shutter release, the mirror pops up and a shutter curtain opens,exposing the digital sensor or film. At this point, the light simply travels in a straight paththrough the lens and if you've focused the lens correctly, a sharp image is projected onto thelight sensitive material (CCD or film). This is where the concept and technique of controllingexposure comes into play.

One of the primary skills in photography is properly determining how much light reaches thesensor, how long the sensor is exposed to the light, and finally how sensitive the sensor is tobegin with. This is what we mean when we compare photography to "painting with light." Asyou read on, we will revisit each of these points in detail.

The 45-degree mirror described above is an essential part of every SLR, film or digital.Here are two illustrations that show the mirror inside of an Olympus EVOLT E-510 (figures3 & 4). For demonstration purposes, the lens has been removed. When you remove a lensfrom an SLR body, you will see where the mirror is positioned directly behind the lens(figure 3). When the shutter release is pressed, the mirror instantly moves up, out of the way,



exposing the digital sensor (figure 4).

Figure 3 Figure 4

Now that you've got a grasp on some of the basic parts that make a digital SLR or anyother camera work, we can begin a more in-depth discussion of exposure and its variouselements. As mentioned above, exposure is comprised of three major factors, all of whichwork together to control how light or dark the recorded image will be.

The three factors that effect exposure are:

1. The amount of light coming in through the lens. This is controlled by the aperturesetting.

2. The length of time that digital sensor is allowed to be exposed to the light coming inthrough the lens. This is controlled by the shutter speed setting.

3. The sensitivity level of the digital sensor or film. This is controlled by the ISO setting.In the case of traditional film, each individual roll of film has an ISO rating.

The ApertureInside every lens there is an adjustable opening called the aperture. The aperture controlshow much light is allowed to enter through the lens. The larger the aperture opening, themore light can enter through to the digital sensor. The smaller the opening, the less light canenter.

The relative size of the aperture is measured in f-stops. The range of f-stop settings availableon the camera depends on each lens. With older 35mm film cameras, the apertureadjustment setting was usually located on the lens, and not the camera. Each lens wouldhave an aperture ring right behind the focus ring. With most of today's digital SLRs,however, the aperture adjustment setting is located on the camera body. But regardless ofhow the aperture is controlled on whatever camera you are using, the range of f-stopsettings have essentially remained the same.



Most lenses have the following standard range of f-stop settings available:

[f/2.8, f/4, f/5.6, f/8, f/11, and f/16]

For the purpose of this tutorial, we will not delve into the complex equation from whichthese f-stop numbers are derived. Much more vital to one's understanding of exposure arethe following aperture related facts:

The smaller the f-stop number, the larger the aperture opening inside the lens, and themore light is allowed to enter.The larger the f-stop number, the smaller the opening, the less light can enter.The list of apertures given above is the standard list expressed in full stop increments.A "full stop" increment can be expressed as either twice or half the amount of light asthe f-stop before it. In other words, an aperture setting of f/5.6 will allow twice asmuch light in through the lens as f/8 or half as much light in as f/4. This will make moresense later, when we discuss how the shutter speed interacts with the aperture.Many current Digital SLRs will allow the aperture to be set in 1/3-stop increments.This allows for f-stops like f/9 and f/10, which allow more precise exposure control,but this can be confusing for the absolute novice.Some lenses will allow for additional stops at the widest and smallest aperturesettings. For example, some lenses will allow apertures as wide as f/2, f/1.8, or evenf/1.4. These lenses are considered fast lenses because they let more light in, whichmakes them useful for low light situations. We will explore this concept later in thelesson.

The following five images (figure 5) are illustrations of 5 full-stop increments in aperture,ranging from f/11 at the smallest lens opening to f/2.8 at the largest lens opening. Thewindow towards the bottom of each image shows a picture of an actual aperture inside of alens, shot through the back of a 35mm film camera. For each shot, the shutter speed and theISO setting were kept constant in order to demonstrate the effect of the aperture on theexposure of this shot.

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Figure 5



As you can see, a larger lens opening (represented by a smaller f-stop number) lets in morelight, which causes the image to be lighter. A smaller aperture lets in less light, which causesthe image to be darker. In this example, the third aperture setting (f/8) resulted in the mostaccurate overall exposure for this particular lighting situation, shutter speed, and ISO setting.At this point, it is important to remember that the shutter speed and ISO settings were keptunaltered throughout. As we will soon demonstrate, each of these other exposure settingsplay an equal role in determining exactly how much light is recorded.

Exploring Depth of FieldEvery exposure setting (aperture, shutter speed, and ISO) has its own unique side effect,which can be used by the photographer to achieve specific creative goals. The main sideeffect of aperture is depth of field.

Depth of field is a photographic term that relates to how many things remain in focus alongan imaginary line drawn from the lens into infinity. The wider the aperture or the bigger thelens opening, the less will be in focus. The smaller the lens opening, the more will remain infocus across a longer distance range.

For example, imagine that you are photographing a portrait and the model is in front of adistant mountain landscape. With an aperture setting of f/2.8, you either have the model orthe mountain in focus, but not both at the same time. However, with an aperture setting off/16 (or the smallest lens opening), both the model and the background will be in focus.

Figure 6

Here's another example (figure 6). This imageof a branch set against a distant backgroundof foliage is a composite of two shots taken atdifferent aperture extremes. The left portionof this comparison shows the result of usingf/2.8 as the aperture setting. This wideaperture setting caused the foreground tosharp, while the background was renderedcompletely out of focus. This is known as a"shallow" or "limited" depth of field.

On the right, an aperture setting of f/22 wasused. There the background is almost assharp as the foreground. This is referred to as"long" or "great" depth of field.

Another important thing to notice in the example above is the shutter speed setting, which isdramatically different for the f/2.8 and the f/16 portions of the comparison. This will makemore sense when we discuss the shutter speed later on in this lesson.

For now, just keep in mind that by closing down the aperture from f/2.8 to f/16, yousignificantly reduce the amount of light coming in through the lens. In order to keep theoverall exposure the same for both images, you have to compensate by using a longershutter speed. In other words, you have to let the light in for a longer duration in order tohave the same amount of light recorded on the digital sensor.



The image on the right (figure 7) is a classicexample of great depth of field being used tokeep the foreground and backgroundcompletely sharp. In this case, an aperturesetting of f/16 was used to maintain sharpfocus from the closest plants to the farthestpine trees.

Figure 7

Figure 8

This next image (figure 8) is an example ofshallow depth of field. Here the photographerused an aperture of f/2.8, while carefullyfocusing on the single flower in theforeground. This resulted in a completelyblurry background, which helps to isolate thesubject. If a smaller aperture had been used,the flower might have been lost amidst a verybusy background. Using shallow depth offield is a great way to keep the backgroundelements simple.

Before we move on, let’s review the key points that you need to understand about aperture.

The aperture is an adjustable opening inside of the lens, which controls or limits theamount of light entering through the lens and reaching the film of digital sensor.The aperture setting on the camera consists of a series of f-stops.The larger the f-stop number, the smaller the opening in the lens.The smaller the opening in the lens, the less light can come in and the darker the imagewill be.Depth of field is a side effect of aperture. The smaller the lens opening is, the greaterthe depth of field.

The Shutter SpeedWhile the aperture setting is responsible for limiting the amount of light entering through thelens, the shutter speed is responsible for limiting the length of time that the film or digitalsensor will be exposed to that light. Shutter speed is the second factor that helps to controlthe total quantity of recordable light. In essence, the shutter speed is a direct counter-balance to the aperture.

Just like apertures, shutter speeds are measured in full stop increments. Here is a list of



standard shutter speeds (measured in seconds or fractions of a second).

[BULB, 1 sec, ½, ¼, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, and 1/1000]

It is worth noting that most modern cameras will allow you to set the shutter speed in thirdstop increments. For the sake of keeping things simple, we will only concentrate on full stopsfor this lesson.

Many cameras will also allow shutter speeds as fast as 1/4000th of a second and/or as slowas 30 seconds. Most if not all SLR cameras offer an alternative shutter speed setting calledBULB. When the camera is set to BULB, the shutter will stay open as long as the shutterrelease is pressed. This makes it possible for much longer, manually-timed shutter speedsthat can be in excess of several minutes or even hours in length.

As you can see from the list of standard, full-stop shutter speeds listed above, eachconsecutive shutter speed (from left to right) is half the time of the preceding speed. 1/250thof a second is half the time or half the amount of light reaching the sensor as 1/125th of asecond. We will explore this concept in more detail later.

In its basic form, the mechanism that makes the shutter speed work consists of two opaquecurtains which move in front of the sensor exposing it to light. When the shutter release ispressed, the mirror pops up and out of the way. Then, the first curtain moves to the right,revealing the sensor and exposing it. To end the exposure, the second curtain follows thefirst and covers the sensor. Then the mirror can come back down and into position. Whenshutter speeds are faster than 1/15th of a second or so, the second curtain will actually beginmoving before the first curtain has reached the end. This causes a small window or slit tomove laterally across the film. The shorter the exposure time, the narrower this window willbe.

Keep in mind that this description is based on a much older style of shutter found in older35mm film cameras. Modern cameras feature more advanced versions of essentially thesame process.

The series of images shown below (figure 9) illustrate how the shutter speed affects theexposure. For each of these examples, the aperture and the ISO settings have been keptconstant. The window below each image illustrates how the curtain shutter mechanismmoves across at different speeds.

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Figure 9



From this series of illustrations, you should be able to understand that as the shutter speedgets shorter, less light is allowed to reach the sensor. Less light results in a darker image.

In addition to controlling the length of the exposure and the total amount of light reaching thesensor, the shutter speed is also responsible for either freezing movement or blurringmovement. Just as depth of field is a side effect of aperture, motion blur is a side effect ofshutter speed. This is where your creative decisions come into play.

When the shutter speed is slow (i.e. 1 full second), the shutter is open for a relatively longtime. During this long exposure, anything that is moving (subject and/or camera) will causemotion blur in the image. If the exposure is really short (i.e. 1/1000th of a second) whateveris moving will inevitably become frozen. It is almost like taking a thin slice out of time.

In some situations you might prefer to freeze the moving subject. Other times, you mightwant to use a slow shutter speed to add motion blur to your shot as an aesthetic element.Perhaps the most important point to remember is that the shutter speed must be fast enoughto freeze any camera shake that can occur by the movement of your hands and body whileholding the camera. If a fast enough shutter speed is not possible, then the camera must bekept still using a tripod, monopod, or other camera support.

Rule of Thumb: To prevent motion blur due to camera shake when hand holding thecamera, make sure that the shutter speed is greater than the focal length of the lens. Forexample, if you are using a 60mm lens, make sure that the shutter speed is 1/60th of asecond or faster to prevent camera shake. If your lens is a 25mm wide-angle lens, then youcan use a shutter speed as slow as 1/30th, or if you have really steady hands, maybe even1/15th of a second.

Figure 10

Here is an example of intentional motion blur(figure 10). The amount of available light inthis situation was extremely low, whichrequired a very slow shutter speed of about15 seconds. The camera was mounted to atripod in order to prevent any motion blur dueto camera shake. During the 15-secondexposure, a large truck with bright headlightsdrove past the camera. The truck wasrendered as several long streaks of light,while the other parts of the image -- the roadand landscape in the background -- remainedabsolutely sharp.

This next shot of a dog frozen in mid-stride isan example of how a fast shutter speed can beused to freeze action. Here, the photographerused a shutter speed of 1/2000th of a second,which resulted in every element, including thetiny droplets of water splashing around thedog, to be recorded as absolutely still and



tack sharp.

Figure 11

Keep in mind that in order to use a super fast shutter speed like 1/2000th of a second, theremust be enough available light to record a proper exposure in such a short amount of time.In this example, the bright midday sun offered plenty of light to warrant the use of a fastshutter speed. Whereas in the previous example taken at night, there was not nearly enoughlight to use such a fast shutter speed.

The ISO Setting The third factor that directly affects exposure is the ISO setting. ISO is an acronym for theInternational Standards Organization. In relation to the photographic process, ISO is ameasurement of film or digital sensor sensitivity. In other words, the ISO rating tells us howsensitive a film or a digital sensor is to light. Therefore, the ISO rating determines how muchlight is needed for an accurate exposure.

In the days of film, every roll of film came with its own ISO (or ASA) rating, expressed infull stop increments as follows.

[25, 50, 100, 200, 400, 800, 1600, and 3200]

The higher the ISO, the more sensitive the film is to light. An ISO 200 film is twice assensitive (or 1 stop more sensitive) to light as an ISO 100 film and requires 1 stop less lightto achieve the same exposure.

The same principle applies to digital photography, with one major difference. The sensitivityof a digital sensor is adjustable. This means that the same range of ISO ratings can be dialedin as a setting right in the camera. No longer is it necessary to match the appropriate film toany given lighting situation. Now we have the entire range of film speeds built in to thecamera and accessible on demand.

The side effect of ISO is grain or digital noise. The higher the ISO setting, the more noise ispresent in the image. Digital noise lowers the overall clarity of the image. This can be a reallytough compromise, especially in situations where both a fast shutter speed and high imagequality are important.

The illustration below (figure 12) compares two extremely different ISO settings. Here, wecan see the effect of these settings on this composite shot of several, fast-moving billiardballs.



Figure 12

In this scene, the light level was relatively low,as it would be in a pool hall type of setting.ISO 100 is a relatively low ISO setting,which makes the sensor less sensitive to light.This means that more light or a slower shutterspeed is needed for an accurate exposure.The result in this case, however, is motionblur.

When the camera is set to ISO 1600, lesslight is needed for the same exposure, whichallows the use of a faster shutter speed. Thishelps to freeze the billiard balls that are inmotion. However, notice that the higher ISOsetting here also caused more noise in theimage, which significantly obscures the finedetails in the subject and lowers the clarity ofthe image. (Click on the image for anenlarged view to see this effect more clearly.)

Before we move on to see how all three exposure factors, aperture, shutter speed, and ISO,are used in relation to one another, let’s recap some of the most important informationwe’ve covered so far. Here is a list of points that you should be clear on before proceedingto the next portion of this tutorial.

Exposure is a process for controlling the amount of light that is allowed to record onthe digital sensor.The main objective of making an exposure is to record just the right amount of light sothat the image is neither too dark (underexposed), nor too light (overexposed).The aperture setting controls the amount of light coming in through the lens.The shutter speed setting controls how long the sensor is exposed to the light comingin through the lens.The ISO setting controls how sensitive the digital sensor is to light.

With all of this in mind, one might ask: "How is it possible to know which shutter speed andaperture settings to use for any given situation? Surely, you can’t just pick random exposuresettings out of a hat! There must be some method for arriving at an accurate exposurewithout having to do a ton of trial and error!"

The Light Meter Fortunately there is such a method. In the next and final portion of this lesson we will tie allof this information together by exploring the most essential tool - aside from the camera -that every photographer must know how to use: the light meter.

A light meter is simply that - a device that measures the amount of light in a scene. Lightmeters come in several varieties and usually have several modes of operation. In general,there are two main categories of light meters: incident and reflective. Incident light metersmeasure light that is falling onto the subject, while reflective meters measure light that isreflecting from the surface of a subject. Within these two categories, there are many differenttypes of light meters. Some are separate devices and some are built into the camera. Somemeasure ambient or available light and some measure flash or studio strobe lights



measure ambient or available light and some measure flash or studio strobe lights.

In this lesson, we will concentrate on the type of light meter that is built into the camera.Every modern SLR has a built in light meter, which is an essential tool for determiningproper exposure.

How does a light meter work? What exactly does it do? Essentially, a light meter measures the strength of light and determines an appropriate shutterspeed and aperture combination based on the ISO setting selected. It is then up to you, thephotographer, to decide whether to use that specific aperture and shutter speed combinationsuggested by the meter, or to adjust these settings to tailor to your own creative vision.

All light meters are calibrated to read the light as middle gray. Whether the subject is allwhite, all black, or a mix of different tones, the light meter will average everything togetherand offer up an exposure that will render the metered area(s) as a middle gray tone. Thismight sound confusing at first, but we will clarify this point with some examples below.

Most modern digital SLR cameras offer several metering modes. The most commonmetering modes are described and illustrated in the diagram below (figure 13).

Figure 13

The best way to explain how the light meter reading translates into an actual exposure is withthe following experiment. After reading through the next section, we recommend setting up ai il i t f lf t if hi th lt



similar experiment for yourself to see if you can achieve the same results.

As previously mentioned, the light meter is calibrated to read everything it sees as a middlegray tone. To demonstrate this, we positioned our model, Mary, in an ordinary scene withflat, overcast lighting. We asked Mary to hold three 8x10 cards: a white card, a black card,and a Kodak 18% gray card.

Note: The Kodak 18% gray card is an inexpensive light metering tool, which can bepurchased at virtually any camera shop. It is called the 18% gray card because it reflects18% of the light falling on it. In terms of light reflectance this is considered a perfect, middlegray. Therefore, this gray card can be used as a metering guide in conjunction with a lightmeter to arrive at accurate exposures in any lighting condition. We strongly recommendgetting your hands on one, as it will definitely come in handy in more situations than youmight realize.

To use the built-in meter in your camera, press the shutter release down halfway. This willactivate the meter and you will see the current shutter speed and aperture settings displayedinside the viewfinder. Next to the shutter speed and aperture is an exposure indicator, whichusually has markings ranging from -3 to 0 to +3. Adjust the aperture or shutter speed untilthe indicator reads 0. This is the exposure that the light meter deems accurate for thatparticular amount of light.

When metering, remember that the ISO setting will also affect what the light meter will deemaccurate because the ISO setting determines how much light the sensor actually needs.Remember to avoid shutter speeds that are slower than 1/30th of a second (depending onthe lens, as was discussed earlier) in order to prevent motion blur.

For the following examples, we used the Center-Weighed metering mode in order to limitthe metering area to just the center of the viewfinder. This mode excludes any light valuesaround the edges of the frame. We set the camera to ISO 200 and kept the aperture at itswidest setting of f/2.8.

First, we asked Mary to hold an 8x10" inch white card. To take the meter reading, thephotographer stepped in close, almost filling the frame with the white card. Then heproceeded to meter the card (figure 14). The light meter suggested 1/500th of a second asthe correct shutter speed.

Remember that the light meter is designed to read everything as middle gray. The result,then, is underexposure. 1/500th of a second at f/2.8 with ISO 200 rendered the white cardas a nice middle gray tone or about two stops darker than it actually was in real life. And ofcourse, this caused all of the other tones in the image to be two stops darker as well (figure15).



Figure 14 Figure 15

Next, we repeated the same process using a black card (figure 16). This time, the metersuggested an exposure of 1/30th of a second (same aperture, same ISO setting).

The result, as you might guess, is overexposure. The meter performed its function flawlessly,turning the tone of the black card into a middle gray tone, which in turn recorded all of theother tones in the image two stops lighter than they should be (figure 17).

Figure 16 Figure 17

Then, we conducted the experiment once more, this time using a middle gray card. The lightmeter suggested 1/125th of a second as the shutter speed (figure 18). This resulted in anaccurately exposed image, with all tones rendered as close as possible to how they areperceived in real life (figure 19).



Figure 18 Figure 19

With this concept of metering in mind, the next step is learning how to visually perceivemiddle gray in whatever subject matter you are photographing, and then take your lightmeter readings from that. For example, green grass is generally a good middle gray. Let’ssay you are photographing a big white house surrounded by a lush green lawn. If you takethe meter reading from the house, you will render the house as a middle gray and cause theimage to be underexposed. If you meter off the green grass, then you will be more likely toachieve an accurate exposure.

This is primarily why an Auto exposure mode cannot always guarantee optimal exposures.Of course, modern digital cameras feature highly advanced metering algorithms, which canaverage the house and the grass together and arrive at a fairly accurate exposure.Nevertheless, if the scene is predominantly dark or light, the camera will not intuit yourintention is to keep it that way and will try to compensate by making the image unnecessarilylighter or darker.

Tying it All Together Now that we’ve covered the basic exposure mechanisms of a camera, which work togetherto record light onto a digital sensor or a piece of film, let’s consider how we can apply thesetechniques to achieve a desired creative outcome in our images. As you already know now,the shutter speed, aperture, and ISO settings work together to control the exposure. Andthe light meter can measure a given lighting condition and offer up a combination of shutterspeed and aperture settings to use with a given ISO setting.

There is no such thing as a single "correct" exposure in any given situation. In fact, there arealways many different possible combinations of settings, which can produce the sameexposure. However, as you are already aware, each of the settings (aperture, shutter speedand ISO) has their unique set of limitations and side effects. The combination of settings youend up selecting is ideally driven by the look you want to achieve for your photograph.

For example, let’s say you metered a scene made up of predominantly green foliage. Withthe ISO set to 200, the light meter suggested an exposure of 1/500th of a second at f/5.6,which turned out to be an accurate exposure.

Based on this reading, we automatically know that the following list of aperture and shutterspeed combinations will produce an identical exposure with an ISO setting of 200:

1/30 @ f/22



1/30 @ f/221/60 @ f/161/125 @ f/111/250 @ f/81/500 @ f/5.61/1000 @ f/41/2000 @ f/2.8

Remember that each change in shutter speed or aperture represents either half or twice theamount of light. This is called a full stop increment. So if we change the shutter speed from1/500 to 1/2000, then we are letting in two stops less light. To compensate for this, wewould need to open the aperture two stops from f/5.6 to f/2.8.

If we were to change the ISO setting from 200 to 100, then we would be making the digitalsensor one stop less sensitive. This would mean that we would need one stop more light toproduce the same exposure. Therefore 1/500 @ f/5.6 with ISO 200 is equal to 1/250 @f/5.6 with ISO 100.

At this point, the preferred combination becomes creative decision. If your priority is tofreeze action, then 1/2000 @ f/2.8 is your best bet. If you want to maximize depth of field,then go with 1/30th @ f/22.

Here are a couple more examples of how exposure decisions are made with purely creativegoals in mind.

The image below (figure 20) was taken about 1 hour after sunset. The light was rapidlyescaping. In fact, when this image was taken, there was barely any light left in the sky at all.Believe it or not, this is often the best time to photograph. The last trace of light after sunsetcan be extremely unique and surreal.

Figure 20

Because of the dim lighting conditions, anextra long shutter speed had to be used inorder to record the tiny bit of light that wasilluminating this landscape. The camera wasmounted to a tripod to prevent motion blur.The ISO setting was set to 100 to ensuremaximum image quality. The aperture was setto f/16 to ensure that the cliff wall in theforeground and the cliffs in the distantbackground would both be in focus. Aftertaking a few light meter readings fromdifferent surfaces in the composition, thephotographer was able to calculate a shutterspeed of 4 minutes, which required the use ofthe BULB setting and a stopwatch (figure20).

In this case, the photographer could have altered the ISO setting and the aperture setting togain as many as 10 stops of extra light. However, this would have compromised his creativevision for this image. The severe motion blur, caused by the long exposure, turned thecrashing waves into a soft mist, while the tripod ensured that everything else in the shot that



was motionless would be rendered sharp.

Here is a different example, which shows howto handle low light situations without using atripod. This dog portrait was also shot aftersunset with a limited amount of available light.To pull this off, the photographer used a highISO setting of 1600. At ISO 1600, the lightmeter suggested an exposure of 1/60th @f/2.8, just fast enough to hand hold the cameraand prevent any motion blur. Of course, theside effect of using such a high ISO setting isdigital noise and a reduction in image clarity.However, for this image, the compromise wasworth it and the extra noise, in our opinion,adds a kind of impressionistic feeling thatseems to work well for this photograph.

Figure 21

These examples make it clear that photography is really a world of compromises. Thecamera does not see things the same way that the human eye perceives. Sometimes, thecamera can see things that we can’t perceive. Most of the time, it is the photographer’s jobto manipulate the camera into seeing things a specific way. This is essentially the foundationof creative image-making.

Hopefully, you can see why the autopilot settings on your camera are really not as smart asthey may appear. When you set your camera to the Program mode, for example, thecomputer in the camera will use the built-in light meter to determine optimal shutter speedand aperture settings based on averages and “common scenarios” that have beenpreprogrammed by the manufacturer. In reality, the camera does not know whether youwant shallow depth of field, great depth of field, frozen action, or motion blur. The only“computer” that can make these decisions is your brain.

In conclusion, we suggest reviewing the topics in this lesson a few times. When it comes tomanual exposure, practice is the only way to improve your proficiency. So, if you truly wantto take photography into your own hands, and develop your own personal vision, then youmust, at all costs, avoid the Program mode whenever possible. With enough practice, youwill be able to manually set the aperture and shutter speed with more accuracy and morevision than any digital SLR can do on its own.

And remember to practice and have fun! That’s what photography is all about!



Understanding exposure

Art & Photos

Transcript of Understanding exposure