Exposure Compensation

(This is lesson 8 in my “How It’s Done” Series)

In this lesson we’re going to try to untangle the tricky subject of exposure compensation.  If you stick with me through this lesson you can be proud of yourself.  This is the lesson where we weed out the weak.  The reward for completing this lesson is you will be able to take successful photos in natural light.  Once you understand exposure compensation you can call yourself a photographer.

Any discussion of exposure compensation must first start with light metering and metering modes.  We’ve covered setting aperture and shutter speed in the previous lesson, but glossed over light metering.  You must have a basic understanding of metering so you know when to trust your meter, and what type of lighting will trick your meter.  Exposure compensation is all about compensating for nonstandard lighting and subjects.  Your high tech light metering system is great, but like all things mechanical it sometimes has a hard time operating in our random world.  Once you understand the basics of metering you will be able to predict when your meter is seeing the world correctly and also know when you’re meter needs some help.  You’ll be able to add some exposure compensation to create great photos even in tricky lighting situations.

Inside your camera is a fairly sophisticated light meter.  This light meter looks at a scene and calculates what combination of shutter speed and aperture is required to properly expose your camera’s sensor.  The meter works almost instantaneously.   Try it out.    Pick up your camera.  Put it in Aperture priority mode (Av mode for Canon).  Now set your aperture to f/5.6.  Point your camera around the room and watch how the calculated shutter speed changes.  Point it into a dark corner and you get a slow shutter speed.  Point your camera out the window and you get a fast shutter speed.

Depending on your camera model you should have several different metering modes.  The two modes common to most cameras are evaluative and spot metering.  Evaluative is the mode I use almost exclusively.  In evaluative mode your meter looks at the light in the entire frame and makes its exposure computation based on the entire scene.  In spot metering mode the meter reads one specific spot and calculates exposure based on the light at that spot.  Spot metering is useful when your primary subject is in lighting that is not typical for the rest of the frame.  For example, a ballerina lit by a spot light on an otherwise dark stage.

For the purpose of this lesson we are only going to talk about evaluative metering.  The concepts we’ll discuss will work for both spot and evaluative modes, but it will keep things simpler if we stick with the more commonly used mode.

So what exactly is your meter thinking when it calculates exposure?  In the olden days of the 1990s that would be an easy question to answer.  In older cameras the basic principle of evaluative metering was that your camera thought the entire world was neutral gray.  When you pointed your camera at the world your meter assumed it was looking at neutral gray and it would set exposure values so that the tones (lightness/darkness) in the resultant photo averaged out to neutral gray.  Now, with modern digital cameras the engineers have gotten more bold.  Today’s cameras still think in terms of neutral gray but the exposure calculation has been augmented in many ways.   For instance, Nikon’s advertising literature states that new Nikon DSLRs evaluative metering system compares the scene being photographed with 10,000 programed exposure scenarios.  From this information an accurate exposure is instantly calculated.  That’s great.  Modern DSLRs do a great job calculating exposure in complex lighting situations.  I have personally observed the progression in exposure technology on the Canon side of things.  My current camera does a much better job calculating exposure than my first DSLR purchased back in 2002.  However, even with all these advances there are still many situations where my camera gets the exposure wrong.  You still need to understand exposure compensation if you want to be a successful photographer.  Let’s start with the basics.  Let’s go back to neutral gray.  Even with all the new ways of calculating exposure your camera still thinks mostly in terms of neutral gray.  We’ll start with gray and work up to the ways your modern camera tweaks its exposure calculation.

Below is a sample patch of 18% gray.  Without any other cues your camera thinks the entire world is made up of this 18% gray.

When you press the shutter button the metering system will let just enough light into the sensor for the tones in the resultant photo to average out to the gray color.  You don’t believe me?  Here are some examples.

What you are looking at in the above diagram are the three worst photos ever posted on this blog.  The middle photo is my black suitcase and a pad of paper.  The gray mess of a photo on the left is an unfocused closeup of the pad of paper.  The photo on the right is an unfocused photo of the back of the black suitcase.  All three pictures were taken in aperture priority mode at f/5.0.  When the frame was filled with the black suitcase the shutter stayed open for 1.3 seconds and the resulting photo is a blob of gray.  With a white piece of paper filling the frame, the shutter stayed open for 1/15th second.  The photo that resulted is also a mass of gray.  With no other cues the light meter assumed that both scenes should be neutral gray.  Take a moment to think about the shutter speeds in both the gray pictures.  All three of the photos were taken in the same crumby lamp light.  Why did the camera calculate a different shutter speed for each picture?  The answer is the camera doesn’t know what it’s looking at.  It wants to make everything neutral gray.  To turn a black suitcase gray the shutter had to remain open 1.3 seconds.  To turn the white piece of paper gray the shutter stayed open only 1/15 of a second.  In the middle photo the camera’s exposure computer worked correctly.  The paper is white and the suitcase is black.

The close up of the white paper is underexposed.  If we wanted to render the paper white we’d have to order the camera let more light into the sensor (longer shutter speed) during the exposure.  The opposite is true for the close up of the black suitcase.  It is over exposed.  To render the suitcase a darker black, we’d have to order the camera to reduce the light (shorter shutter speed) during the exposure.  How do you do that?  Exposure compensation!

Exposure compensation is set differently on each camera.  You are going to have to check your manual.  I use Canon cameras and the exposure compensation is set using the thumb wheel on the back of the camera.  Luckily for us teachers, the exposure compensation display is the similar on most cameras.  To make your photos darker, move your exposure compensation to the left, towards negative numbers.  To make your photos lighter, move the exposure compensation to the right, towards positive numbers.  Each click of compensation is equal to 1/3 of an f stop.

The above animation shows four different exposures.  I overlaid an exposure compensation scale and I also show you the aperture and shutter speed used in each shot.  This scene is typical of the type of situation where you would use exposure compensation.  As I was setting up for the picture I thought to myself that the landscape in the frame was probably slightly darker than neutral gray.  It was a tough call to make because in the frame are some pure white clouds and bright buildings.  However, the dark lagoon is taking up a large swath of the foreground and the dark colored hills in the background were darker than neutral gray.   I was guessing this scene’s tones averaged out to darker than neutral gray, but wasn’t sure how much.  To hedge my bets, I took the four different shots.  Each one has a slightly different exposure compensation.  This method of shooting multiple exposures of the same scene is called bracketing.  The hope is that one of the frames comes out with the correct exposure.  To my eye I like the third shot the best, the one shot at EV -1/3.  As you get more familiar with your camera’s metering system you get better and better at guessing exposure compensation.  You won’t end up with a hard drive jammed full of wasted bracketed frames.  If I wasn’t making a tutorial out of this, i probably would have only shot two frames instead of four.  One would have been at EV-1/3 and the other would have been with no compensation, EV-0.  Practice bracketing and you’ll quickly learn to make this type of judgment call.

The above picture of an egret is an example of a time I used positive exposure compensation.  I was following this egret along a white sand beach on the Sea of Cortez.  The bird is obviously white, and the background was lighter than neutral gray.  The scene was screaming for plus side exposure compensation.  The tricky part about setting exposure compensation when shooting white subjects is the possibility of blowing out the highlights.  The trick is to take some test shots and use both your camera’s histogram and over exposure warnings.   I shot images from EV +1/3 all the way up to EV +1 and 1/3.  After checking the histograms I settled on EV +2/3.  At that setting the brightest parts of the white bird weren’t overexposed (blown out), but the white did come right up next to the right side of the histogram.  At EV +1 the birds head starting flashing on my LCD.  That’s the warning that I over exposed.  I knew I had to have an exposure compensation less than EV +1.

Are these concepts making sense?  If your frame is filled with tones that are darker than neutral gray you need negative exposure compensation.  Dial the needle to the left.   If your frame is filled with tones that are brighter than neutral gray you need positive exposure compensation.  Dial the needle to the right.   How much is enough?  You have to experiment and bracket.  After awhile you will get a feel for your camera’s evaluative metering system and you’ll be able to pick out the correct EV with very limited frames wasted to bracketing.   To drive the point home below are lists of times you might need exposure compensation.

When you would need negative exposure compensation:  close up photos of tree bark, a frame full of blue sky, your friend’s black trench coat, a photo of your navy blue car, a portrait of a buffalo, any time your frame is full of really dark stuff

When you would need positive exposure compensation: a snowy scene, a foggy scene, close up of a white sheet of paper, photo of white linen, a white sand beach, a photo of your white car, any time your frame is full of really light stuff

It is important that you practice these concepts and get to really know your camera’s metering system.  Like I said earlier, modern cameras do more than calculate a neutral gray exposure.  Some manufacturers obsess over not overexposing highlights.  Maybe your light meter is underexposing every photo that contains bright white clouds because your camera manufacturer doesn’t want even the tiniest speck of cloud to overexpose.  If this is the case, simply nudge your exposure compensation a click or two to the right.  In my day job I am a pilot and we have a saying, “Fly the Airplane!  Don’t let the airplane fly you!”  I believe that thought is applicable to photography also.  Know what your camera is doing, but also know what results you’re looking for.  If your camera’s auto exposure isn’t doing what you want it to do, change something.  Control your exposure!  Don’t let your exposure control you!

Because I am not completely knowledgeable in all the magic inside my camera’s exposure computer I do a lot of chimping.  Chimping means to check the camera’s LCD often and make adjustments based on what the LCD is telling you.  The chimping thought process is not very deep.  Here is a sample inner dialogue I might have while taking photos.

“Hmm self… looks like the forground is a little dark.  Maybe I should dial in a little compensation”

-Click- takes another picture and checks LCD

“Hmm self… looks like you dialed it up too high.  Now the foreground is too bright and the highlights are blown out completely.  Better dial it back a 1/3 stop”

-click- takes another picture and checks LCD

“Hey self… this one looks good!  The foreground is just right.  Some of the brightest highlights are blown out slightly, but who cares.  Sometimes white needs to be all the way white.  I think this is the best exposure for this scene.  I’m gonna stick with these settings till the light changes”

That’s really all there is to it.  It’s a thought process and some trial and error.

Now that you know what you’re doing get out there and practice these concepts!  Take a bunch of bad pictures around the house and play with the exposure controls on your camera.  Learn this stuff so that you know what you’re doing when you have the opportunity to capture a really great photo.

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