Understanding Exposure

The Canon's Partial Metering mode will be use throughout this page so that it is easier for you to follow how the light meter is reacting to the exposure, and how I am making exposure corrections to compensate using the Exposure Compensation function on the camera. This central weighted metering mode will be reading an area of the viewfinder of around the seven auto focusing squares in the middle, an area of about 8-12% of the total frame. This area of meter reading sensitivity consideration feathers out fairly quickly towards the edges of the viewfinder.

Canon 5D MkII Viewfinder, Meter Reading Area

A light meter reading like this has a narrower field of view then that of the whole viewfinder. This gives you the option of selecting what part of the scene should have more importance for exposure consideration.

More importantly, what I use this narrow angle of view for is to exclude areas in the scene that might confuse me, and keep me from getting my main reflective subject or area in the scene exactly where it should be in the scene where I want it.

So with a narrow angle of view for the meter reading, I am reducing the number questions I have to ask myself.

In this first example below, I momentarily move in closer to the subject of the wood carving and then take a reading. I am filling the light sensitive meter reading area with just my primary subject to get a good base exposure. I am including the highlights, middle tones and shadows in this reading of the subject. The black background is being excluded from the light metering area. As just mentioned, this simplifies my understanding of what the light meter is reading.

This sculpture is a medium dark toned subject. The light meter will try to make this subject appear lighter than it really is, by rendering an exposure for a middle tone value.

To correct the light meter reading, you just need to dial a little negative exposure correction to place the tonal value of that subject back where it can resemble it's true medium dark toned appearance.

Image of wood sculpture, Artist : Mary Virginia Smith

In the picture of the Wood Bust carving, two different exposures are presented of three images.

The first image on the left was taken using only the exposure reading straight from the light meter. I move in close so that light meter would read only the light from the carving. But the carving is a nice rich dark warm tone, and is not a medium tone value where the light meter will place it. If you examine the highlights of this first image on the left, they are blowing out, over exposed, and this exposure makes the lighting look flat, loosing some of the drama and emotional quality of the light.

The second and third images are the correct exposure. The rich warm tones of the wood are showing the carving's true nature. The highlights have detail to the last peak brightness, and the black in this image are holding with no loss of pixels data (clipping), this includes the dark tones on the black velvet.

In the third picture, I placed a Kodak Gray Card in the shot for your examination so that you could see a true middle tone value under that same light and exposure. Note that the gray card tone value is just where it should be exposed, it is a true middle gray. There is a difference of about -1 1/3 stops of exposure here between image #1 and #2.

How did I set the exposure? I guessed at it.

Pretty easy, huh?

I envisioned a middle tone value in my mind, and compared that with the true reflective tone of the natural wood of the carving. I anticipated a little more than 1 stop difference in darker tone for the bust carving, so I reduced the cameras metered exposure by that amount using exposure compensation on my camera.

Intuitive Exposure!

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Not vary scientific I know. But when shooting photography you will be spending much of your time working with the esthetics of light as a technician and artist. I'm going to show you how I look at a subject or a scene and make a close guess and applying that to exposure compensation, to get from the light from the scene, to what I want.

Gray Card

There is a technically correct way to get a the right exposure of this scene with your camera, and that is to take an exposure reading off of an industry standard Kodak Gray Card, that we read under the same light that your subject is under. In a studio setting this would be common practice. Some people might use an incident light meter which can return the same exposure. What we are doing with the Gray Card, is giving the light meter the exact reflectance value it was designed to average light for.

This practice of metering from a gray card and then transferring the exposure meter reading to our scene, allows dark objects to be dark and light objects to appear light, so all objects are photographed with their natural reflective values based on the light of the scene. There are specific handling practices to follow to help you get consistent results, see the link Photography Light Meter for some guidelines. Or when you buy a gray card for your use, it will come with instructions on how to use them.

Getting back to our example

I just guessed at my exposure. I could be off by about 1/3 stop, or 1/2 stop from time to time. If I am, I can still take another shot after making an adjustment to exposure. The point is, my judgement of the exposure is likely to be closer to what I want from the scene than what the light meter is going to offer. So you can do this also, make an educated guess on the first shot, you will imporve with practice and in the future miss fewer shots.

The reason I did not use the gray card for setting our exposure in this example is because you likely are not going to have a gray card with you in the field when you are shooting, and many times you will not be able to stand with the gray card where you need to be in order to read the light that is actually falling on your scene. You need to be in the same light as your scene is lit by when using a Gray Card exposure reading.

We have already mentioned the important points at how I got this exposure, but let's go over the process.

The Process

Let the camera meter reading serve as a base exposure, then you apply your own rational interpretation of what the meter is reading, and how it is influenced by the scene and why, then deviate from camera's base exposure for your exposure correction.

It is a mental exercise I am presenting here that requires some practice, but it will become easier, and every good photographer develops some method of intuitive judgement of how their camera is going to read the scene, and they will make judgments based on their own experience.

With an image like the bust carving above, there is two major reflectance areas, the wood tone and the black background. I simplified my decision making here by excluding the dark background. What if I could not get any closer to this subject to read light directly off the subject. The light meter would have seen some of that dark black background. This would result in the light meter trying to give me an exposure that was even brighter than what I showed you in the first picture on the left. So my intuitive correction for this scene would have to consider just how much of that black background was influencing my metering area. I would have to add more negative correction to the exposure.

Lots of the scenes you will be shooting will have many natural middle tone values scattered throughout the scene. Those tones are not often gray. A middle tone value can be present in any color. Concrete, grass, some foliage, north blue sky are all brightness values vary close middle tone values.

I sometimes make it a game of it if there is nothing around I am inspired to shoot. I frame a shot, look at the tonal values of the scene and imagine how much the light meter is going to be effecting my exposure because of the dark tree to the right, or the bright snow to the left, and how much area do these subjects take up, and what area of my view finder is the light meter making judgements from.

After I got a guess, I just apply the exposure compensation to place real subjects in the scene at their correct reflected values.

It will help to know where middle gray is placed on your Histogram Graph. Let's go over that, and give you a feel for how much room there is for adjustment while adjusting exposure.

learning to place reflective tonal values

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We have already shown this graphic on the link Histogram Graph. Your image sensor can record light +3 stops above middle gray, and -3.5 stops below middle gray. In all you have about 7 stops of light before clipping at both ends.

This histogram graph shows the camera meter's interpretation of a white towel.
In the background of the histogram is a picture of the white towel.

This histogram bell cure is from an image of a white cloth towel, you can see the white towel in the background of this Histogram. It has some texture in the stitching and so the histogram curve is showing you not only the middle averaged tone for this towel's brightness, but also the ends of the toe of the curve reflect the subtle brightest and darkest values in the metered area.

This is a low contrast scene. Not because it's dark, but because the histogram graph does not stretch out to pure white and down to the pure black.

In the graphic bellow. I shoot multiple exposures of this same towel. I am adjusting exposure at 1/2 stop increments, so that I can see where these adjustments fall on the histogram and how they are spaced appart.

These Histogram curves were taken from multiple shots at 1/2 stop increments and superimposed on this one histogram so that you could see how the placement and shape of the graph for this same subject in those different exposures. This suggest the limits of over and under exposure that we can apply before we loose details due to clipping.

We already know from experience that a 1 stop adjustment in exposure can be performed by adjustment of shutter speed, aperture or ISO settings. This changes the exposure by twice the light or by half the light. So your subject tones within the scene will be effected by becoming twice their brightness, or half there brightness as rendered by your exposure adjustments.

If you shoot a picture of a gray card using an exposure that is +3 stops over exposed, you will have no details of the gray card, it is pure white. If you shoot a gray card at +2.5 stops over exposed, and you have carefully lit the card perfectly evenly with light, you should have details of the card without clipping at any of the brightest specks of reflection. In essence that gray card will look just like a white card in your photograph.

You actually know now, the high end limits of brightness adjustment.

If you shoot a scene of just snow in it with no other tonal values, then about +2 stops of light is as far as you want to go with exposure compensation. Remember you don't want any good details in the image to be effected by clipping.

If you look back at the above graphic of multiple histogram peaks, you can see that there is about -3 stops of adjustment room for this low contrast scene before we start clipping our shadow details.

If you are framing your picture of a Black Bear close up with a strong telephoto, your camera's meter is going to want to lighten up the image base on how much area in the frame the black bear actually fills.

This can be a little challenging and maybe imitating at first. But it is easier if you know what light metering mode you are using, and just what portion of the viewfinder is reading light, and what percent of sensitivity that part the of the viewfinder is.

One danger that I hinted at above is, if you ever have to pull up the shadows during editing, you can run the risk of bringing up the Noise levels in the darkest regions of your picture, and you don't want that.

You saw where the exposure bell curve peaks ended up on the histogram graph in the above diagram. This diagram shown below, shows you where those peak values are relative to the brightness values of histogram graph. These increment marks are at ±1 stop, and each mark represents a position on the graph that is twice or half as bright as it's neighbor. The green bar area is where the light meter tries to average light for every shot you shoot, it's the middle gray exposure.

By looking at this graph you can pretty much guess where you would like to see your subject's reflective tones placed when making a exposure correction. It does not matter much what brightness values your subject is, that is to say, if you place one item in your scene at the correct brightness value for that scene, all the rest of the brightness values are corrected for also.

You might have important bright highlights, or deep shadow details that need a little special attention, and so you might want to temper some of your adjustments to protect these areas. As mentioned before there are situations where your scene could have a larger dynamic range then your image sensor is capable of capturing, so in these cases we hold back on the amount of adjustment in order to protect those valuable shaping details.

A white swan on a sunny day might need only about +1/2 stop of light adjustment. On a cloudy day it might need about +1 1/3 stops of light added.

If you are photographing a relatively wide view landscape of the surrounding environment, like tall trees, ground, mountains without snow and not much sky present, you might think there is too much here to consider, how can I narrow my interpretation of this exposure. You may not have to. Most of these subjects are commonly close to middle tone subjects. The light meter averages all this for you and may give you exactly what you want.

Look at the scene and imagine the middle tone value, if your scene appears brighter than that to you, then add some exposure correction, if the scene seems darker than the middle tone, then reduce exposure.

People in pictures are a vary important subject matter. Skin tones around the world are mostly within ±1 stop of the middle tone values. You can take a bust shot picture of a person with vary dark skin and expose their skin as a middle tone value and be quite happy with the results of the image, especially if they take up most of the framed view. The farther back you are from them, the smaller they will be in the framed view, and if you have a broad dynamic range for a surrounding scene, then you'll want them placed closer to their natural reflective value, because the scene is also needs to be considered for exposure. A vary dark skin toned person will look just fine at -1 stop exposure.

Remember always to examine your histogram graph after the first image to make sure you are not clipping valuable brightness details that are needed in the scene. If you like what you see, and you continue to shoot from the same direction and cropping, and if the light is not changing, then there is no reason to readjust your exposure, concentrate now on composition and the action of your subject.

let's run though some examples so you can see how I might apply some exposure compensation. Then after that all you got to do is play a game of walking around, frame a shot, let the camera tell you the metered exposure, and then try to guess how much correction will be needed in exposure to insure the brightness values in your scene match exactly the same tones and colors that you see with your eyes, or what you desire from the shot.

Exposure Example Simmulations:

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Ping !

In these examples I picked up a little finger puppet to help us out. He is a practical toy for an adult to play with so don't be too embarrassed if you pick one up yourself. You can always use the following excuse.

Ping is just a puppet, so it can hang out in your camera bag. You can easily pull him out when you want to practice shooting under different lighting situations and scenarios. After all we are building a skill here so that you can be ready when the real live critters are around. Of course were not just talking about brightness tones on fur here, it is the luminance of any subject matter, and recordig their details is what is important.

You can practice using these techniques on whichever light metering mode you wish. Your camera manual will show you a diagram of what area of the viewfinder that the light meter will be sensitive to.

I used this light metering mode for all eleven examples on this page

Partial Metering Mode on the Canon 5D mark II.

This is the view finder sensitivity I mapped out for my camera's Metering Mode.

Mapping out your own light meter reading area is not something you have to do if you don't want to, your camera manual may serve you well enough with a graphic example of where the meter is reading from.

If you are curious how I tested my meter, I took a large white panel and glued a small flat black circle of paper in the middle. As you pan around with your camera lens in a tight crop, while on Auto exposure, you can see when the exposure meter is changing the exposure setting. It is important to have your flat panel vary large and evenly lit for this test.

Examples of metered readings and exposure correction
Exp. (1) Shot as metered, correction required	Our main subject is the bird on the snow. Most of the meter reading area here is snow, Ping is vary small relative to the meter reading area, so the meter is expected to return close to a middle tone value for the snow. The part of Ping that the meter is reading is his dark tummy, this influences the meter to expose the snow lighter than middle gray, but the exposure is still too dark. M.R. f7.1 @ 1/4000sec.; ISO 1000 E.C. = 0
Exp. (2) EC +2 stops	I adjust EC +2 stops of the metered exposure. I know this is going to make the snow brighter as snow should be. I have to be careful about not clipping the important white fur details. I really want to keep the white fur in the head below 92% or so, above that and it begins to be hard to see any details in those bright areas. Note: that the finger tracks in the snow are almost gone. As you increase exposure the contrast that defines those tracks will decrease some. However if this was an image of a real baby bird on the snow, you would favor the exposure of the bird, over the tracks in the snow. M.R.: f7.1 @ 1/4000sec.; ISO 1000 E.C. +2 stops
Exp. (3) Shot as metered, correction required	This scene is the opposite situation that was shown in the first image. Here the White Spruce needles on the branches fill all of the metered area, and these branches are darker than the middle tone value that the light meter is tuned to, so the image is going give a brighter image than it should be. White Spruce needles are about -2/3 stop darker than a middle toned gray card. The foreward branch is a little brighter than a background branch and it takes up more of the meter reading area, so this exposure would need to come down at least one stop. The histogram shows the white fur on Ping's head is clipping. M.R. f7.1 @ 125sec. ; ISO 1000. E.C. = 0
Exp. (4) Shot as Metered, Good Exposure	This shot does not show an exposure correction. The meter reading area includes all the brightness tones of our subject,"Ping". The spruce needles that are being read by the meter are almost exactly offset with Ping's lighter tones. So average of this meter reading area is close to the light meter's middle tone value, even though the full frame of the viewfinder area clearly shows as much as 70% dark spruce branches. You have to keep in might what area the light meter is is reading. Right now this image is -1 1/3 stops darker from the image above. The histogram shows the brightness levels spread out nice over the dynamic range, and I would consider making an editing adjustment in this image to turn exposure down a little less than a 1/3 stop, which should give me the shaping of details in the white highlights on Ping's head that I want. In comparing this shot to the last one, both of these pictures were shot as the light meter reading suggested, yet there is a huge difference between them. This demonstrates the importance of knowing your meter reading area within the viewfinder, and how the meter will react. M.R. f7.1 @ 320 sec.; ISO 1000 E.C. = 0
Exp. (5) Shot as metered, correction required	In this shot Ping is resting on a Paper Birch branch. This bark is vary light in tones whose area is about 3/4 of the total light meter's reading area, so It will dominate the results of the exposure reading. Shooting this as the light meter suggest results in an underexposure. Looking at the histogram for this shot you can see shadow details are okay but the highlights could be brighter. M.R. f7.1 @ 1000 sec.; ISO 1000 E.C. = 0
Exp. (6) EC +2/3 Stops	This image is the result of the +2/3 stop correction, but the nice thing about this correction is the shadows remained at the far right end of the histogram, and the mid range and highlights moved to the right, where they should be placed. We now have a nice full dynamic range for this shot compared to the flat looking image before. If you are close to the correct exposure, you can make corrections without too much trouble, just don't error by clipping your highlights or shadows. This shot almost looks like a brighter change then is stated what is stated in the EXIF data, sometimes the clouds can thin and it's gets brighter. It is still a perfect exposure. M.R. f7.1 @ 1000sec.; ISO 1000 E.C. +2/3
Exp. (7) Shot as metered, good exposure	This background is a middle tone value and Ping is not significantly effecting the light meter's reference for exposure. Every one of the above shots were taken within a few minutes of each other and under the broad diffused light of cloud cover. M.R. f7.1 @ 400sec.; ISO 1000 E.C. = 0
Exp. (8) Shot as metered, correction required	Generally, shooting under direct sunlight increases the dynamic range and so the amount of latitude available for exposure correction is now smaller depending on how your frame the shot. If the shot includes brilliant specular highlights and deep shadows within it, then there is a risk of loosing details at one end or the other of the dynamica range. This shot is not too much different then Exp. (3), except this exposure is in direct sun light. You can see between the two histogram examples that the dynamic range in this histogram is expanded, so be sure to check your highlights after applying a correction. I usually don't let white objects with detail run all the way to the far right of the histogram, if you do that you don't have much shaping or details in those areas. It is okay to let specular highlights go the far right, there is no details in those. M.R. f7.1 @ 320 sec.; ISO 1000 E.C. = 0
Exp. (9) -1 1/3 EC correction	The spruce branches in the above examples are under diffused light, and they needed about -2/3 stop correction. Ping is just inside the meter reading area but as a subject it is fairly small. On my first try I used a -1 stop EC, and found the exposure to be within dynamic range limits without clipping. I shot this again though using -1 1/3 stops EC to protect the shaping quality of the fur in the brilliant highlights. M.R. f7.1 @ 1000sec.; ISO 1000 E.C. -1 1/3
Exp. (10) Shot as Metered, correction is an option	It is easy to anticipate that snow is going to under expose your image if it has a large presence in the meter reading area. This was shot as the light meter read the scene. This exposure is fine if you want the look of late evening sunlight. f7.1 @ 2000sec.; ISO 1000 E.C. = 0
Exp. (11) +1 stop EC	The brightest highlight I have to be concerned about is the white fur on Ping. It is usually a pretty safe bet that if snow is filling the frame that a +1 stop exposure increase is a good place to start. You can see the long shadows here. The more exposure you add to this shot with a low Sun angle, the less it will look like evening light, remember that to under exposing a little helps to saturate colors more. If you want to protect that evening look then use a little less exposure adjustment. I live up north, and it is obviously winter here. This light just happens to be at mid afternoon, and the flatter looking dynamic range from the example #9 was not going to look as good to me. So the above picture is not actually wrong for an exposure if you prefer that kind of look of early morning or late evening. f7.1 @ 2000sec.; ISO 1000 E.C. +1

If you have found that getting the correct exposure result has been a bit perplexing, then put into practice this activity of seeing what the light meter is reading. Be aware of the area the meter it is reading in the viewfinder, and know that the light meter is programed to provide a middle tone exposure for what it sees. This should be all the clue you need to alert you to what kind of accuracy you might expect from your exposure reading. You then consider the real reflective values of your main subject in your scene, and compared those to the middle tonal value. Then use Exposure Compensation to place those values where they should be for the best success of the image.

Light is the inspirational motivator of photography. With more careful analysis of exposure, you will find yourself beginning to experiment more creatively. You can set exposure to saturate colors, or pastel them. Before long, light will become one of the main subjects you are shooting, and not so much object centered pictures. The added confidence you gain by understanding exposure will empower you to explore more with your photography.

Return to Photography Exposure from Understanding Exposure.