Using Exposure Bias To Improve Picture Detail

Using Exposure Bias To Improve Picture Detail

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AV +- Exposure Bias can be a very handy tool on a digital camera but most people aren’t aware of its use. Most DSLRs have had the feature for a while and now more and more compact digital cameras are adding it. For those who have never experimented with this useful feature, Peter Carey delivers this quick primer demonstrating how it can help improve your photographs.

Let’s face it, today’s digital cameras come packed with a LOT of features and buttons we never use. Snow Scene Mode, flash bracketing, infinity mode and the list goes on. One feature that deserves a bit more attention is automatic exposure bias. It goes by various names depending on the brand of camera you own but the premise is the same; adjust the exposure a certain amount of f-stops either over or under the camera’s metering. Its use is usually quick and easy and the results can make the difference between a washed out photo or one with plenty of depth to it.

There are times when your camera will have difficulty picking the right exposure. Some scenes that cause this are:

  • Snow pictures with a dark subject in the foreground
  • Bright, cloudy skies
  • A single light source such as a candle


The common theme here is high contrast. While the human eye and brain do an amazing job of adjusting to high contrast situations, digital cameras have to compromise much more so. When using a general evaluative mode, your camera looks at the whole scene and picks a median setting to compromise between the highs and lows. The problem with using the camera’s default settings is highlights tend to get washed out during this compromise, which are hard to recover correctly in post-processing (as compared to dark areas where there is less loss of data).

To overcome this limit, use your camera’s Exposure Bias adjustment to either underexpose or overexpose what the camera believes are the correct settings. Below is an example showing the difference in detail of the mountain Cho Oyu on the Nepal/Tibet border. Both images are straight out of the camera and cropped as close as possible in exactness. The left image has no bias. The right image is underexposed by 1 2/3 stops. Following the images are the histograms for each zoomed picture.

Cho

081016-110739-4049

Zoomed

Zoomed

pic1

pic2

Click on each image to see a larger view.

While the second shot is notably darker, the amount of detail retained is the important factor. The histogram for the first shot shows evidence of some slight clipping while the brights are pushed near their limit. The second shot is far more balanced and will be an easier image to pull detail from.

If you happen to be shooting in RAW mode, you already have about 1 decent stop of latitude and will not need to use the Exposure Bias as frequently. But if you’re shooting in JPG mode most of the time, it’s vital to make sure the details are retained for crispness and clarity. So experiment a bit the next time you find yourself in a high contrast situation. You might find your brain is sometimes a better judge of the lighting in a situation than your camera.

Peter is an avid photographer currently traveling through South America who enjoys travel, portraiture and wildlife photography. A travel related blog of his past and current shenanigans can be found at The Carey Adventures. He also hosts a Photo of the Day RSS feed found here.

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Peter West Carey leads photo tours and workshops in Nepal, Bhutan, Seattle, Portland, Los Angeles and beyond. He is also the creator of Photography Basics - A 43 Day Adventure & 40 Photography Experiments, web-based tutorials taking curious photographers on a fun ride through the basics of learning photography.

Some Older Comments

  • Antara Kundu September 13, 2013 12:46 am

    Quite a helpful post for beginners like me. I wasn't aware of what 'Exposure Bias' means - and this article offered me exactly what I needed. The comment thread (esp. the comment by Chudez) has been extremely helpful as well.

  • mrpinto December 13, 2008 03:46 pm

    @Bilka (and all the other traditionalist naysayers out there),

    The point about allowing gadgetry to trump artistry is well taken, but any camera is just a gadget: why not be a "real" artist and paint a picture?

    For those who do product photography, portraiture, landscape, or other disciplines where time isn't critical, your point about taking the time for perfect composition and exposure is valid.

    For those like me who shoot a lot of sports, or really any one who does event photography, these tools can be really helpful. The reality of sports photography is that conditions change. Half of the potential subjects are wearing dark clothing the other half are light. Parts of a field or arena might be in harsh light, parts might be in shadow. The players won't always be the same distance away from you. In situations like this, any help that your camera can give you before you shoot and any help that your software can give you after a shoot are totally worth it. It's enough challenge for this amateur to just get the focus sharp and the composition close enough that I can crop it the rest of the way in post.

    To each their own though - just because your camera has a feature doesn't mean that you have to use it.

  • John December 12, 2008 06:15 am

    I'd go further than Joost and say that in this case, if you didn't plan on doing further post-processing the first image is probably a better exposure. Yes, you've lost detail, but at least the snow is white.

    I find that the more usual use for EV in winter conditions is the opposite. The camera sees a white dominated scene and underexposes to bring the average back to a middle grey. But of course snow should be white, so the average should really be balanced towards the bright end. Adding between 1/3 and 1 stop using EV is my usual snow setup.

  • Joost van der Borg November 15, 2008 08:18 pm

    In the article, the following is said:
    "While the second shot is notably darker, the amount of detail retained is the important factor. The histogram for the first shot shows evidence of some slight clipping while the brights are pushed near their limit." This directly below the two histograms. While it's true that some (minor) highlight clipping occurs in the first, the second does not retain more detail. Most of the information is retained in the brightest fifth of the histogram, and underexposing like this results in the loss of most detail. This is much more than the tiny bit of highlight clipping in the other image.
    A good explanation can be found here: http://www.luminous-landscape.com/tutorials/expose-right.shtml

    While I agree you should prevent clipping highlights, I would recommend exposing so you're as close to the 'bright' edge as possible, to retain maximum detail. Underexposing in post provides much better results than overexposing (which would be necessary with the second example, to get a properly exposed image).

  • Bilka November 14, 2008 05:06 pm

    I think I am going to coin a new term to replace the term "photographs" in this digital world we live in and call them "Engineered Images" instead. Push this button, enter that EV, compensate .0000325 stops, power setting to +/- 0.054, Phasers on Stun, allow for a battery power factor of .0947 percent, beam me up and bias your hot mirror to optimise the image. Of course this is all nonsensical information but I am tryin to make a point. We are dehumanizing the art of photography.

    This all seems like an overly complex method of good old manual exposure bracketing. Why bother with another useless function to fiddle with on your camera? Concentrate more on composing the image and less on button pushing. Just open or close the dag-gone Iris as you see fit and go make art.

    Bilka

  • John November 14, 2008 02:26 pm

    Just use Photoshop Elements: Enhance; Adjust Lighting; Shadows/Highlights. This allows you to lighten the shadows and darken the highlights to reveal what is hidden in those shadows and to show the detail in the burned-out highlights.

  • chudez November 14, 2008 01:22 pm

    This is the way I understand exposure and exposure compensation. Please let me know if I missed the mark in anywhere.

    1. The camera uses it's best judgement as to what a "correct" exposure looks like.

    2. To adjust the exposure, the camera will automatically play with the factors that affect exposure (shutter speed, aperture and ISO) depending on the camera mode (and the camera).
    2a. If it's one of the automatic modes (full auto, portrait, scenery, etc.), the camera will freely play with all three factors.
    2b. In aperture priority, it can only play with shutter and ISO, while in shutter priority, it will only play with the aperture and ISO.
    2c. In P* mode (I forget what P* stands for), you can play with one and the camera will adjust the other; i.e., play with the shutter and the camera will adjust the aperture and vice versa.
    2d. In full manual, the camera is kicked out of the sandbox and doesn't get to play with anything at all.

    3. To judge what a "correct" exposure looks like, the camera uses the built-in light meter and aims for a preset exposure setting. It basically looks at the scene, measures the brightness and tries to adjust the exposure so that the average brightness of the scene will match an arbitrary benchmark (often quoted as "18% grey").

    4. One way you can influence how the camera judges what a "correct" exposure will look like by telling the camera *what* to measure:
    4a. measuring the brightness of the entire scene (average weighted metering) works well with low contrast scenes
    4b. measuring the brightness of a portion of the scene (center weighted metering) works well with "contrast-y" scenes like at the beach
    4c. measuring the brightness of a tiny spot within the scene (spot metering) is useful for scenes with extreme bright or dark spots or for instances when the photographer decides to bring out the detail in a particular part of the subject (like the eyes on a face or the frieze on a wall). the photographer is basically saying i want *this* part "correctly" exposed even if it fraks the rest of the picture.

    5. After all is said and done, the camera's judgement of a "correct" exposure is completely ARBITRARY and you, the photographer, may freely disagree. If you think the camera is making the exposure too dark, you can compensate (which is why it's called exposure compensation) by telling the camera to underexpose (brighten it up). If you think the camera is making the scene too bright, you can tell the camera to overexpose (darken it).

    Is this correct?

  • John November 14, 2008 08:02 am

    Exposure bracketing (bias) is all well and good, but a much simpler way is to shoot in "RAW mode"! Then Post processing will correct any under or over exposure, WITHOUT any loss of details. Almost all professional photographers use RAW mode, as it acts as a huge safety net.

  • Howard November 13, 2008 10:09 pm

    Thanks Neutralday. I'm not sure why this concept seems to evade me. Probably because of my many years with film, when I'd look at a negative and see it as too light when underexposed and too dark when overexposed. Opposite of digital concept. Can't get used to seeing the final product rather than a negative in the camera, I guess. Modern life!

  • Chris November 13, 2008 06:38 pm

    Doesn't this not work when you're on manual? I seem to remember that. Bracketing's a good feature though :)

  • Neutralday November 13, 2008 10:59 am

    Howard,
    you got it right over relates to "+" and under relates to "-". Need more light (image too dark/underexposed) then adjust to the "+" side. Need less light (image to bright/overexposed)then adjust to the "-" side.

    it helps to just add the word "light" (in your head) in between the "+" and "-".

    Just conceptualize: " + <<>> - "

  • Thomas November 13, 2008 10:37 am

    This is something I'm still a little unsure of. Thanks for the article!

  • zack November 13, 2008 09:51 am

    I used to use EV on my camera (pentax k100d) until I realized that using it sets my auto iso to 200. That means if I am shooting at auto iso 800 and I need to make my picture a little brighter, I have to manually set the iso to 800 and then use EV. Just something to be aware of because I only recently realized that.
    -Zack

  • Deirdre November 13, 2008 09:07 am

    Sorry, this should read +0.7:

    "I keep my Nikon d40’s exposure on +.07.

    I was having a lot of problems with overexposure, was playing with this button a lot, and then read the suggestion to do just this Ken Rockwell’s site. It helps a lot."

  • Deirdre November 13, 2008 09:04 am

    I keep my Nikon d40's exposure on +.07.

    I was having a lot of problems with overexposure, was playing with this button a lot, and then read the suggestion to do just this Ken Rockwell's site. It helps a lot.

  • Howard November 13, 2008 05:06 am

    I always have a problem with explanations like this. On the camera, the Exposure Bias is done with either a Plus (+) or Minus (-), yet the explanation deals with under or over exposure. That's normally the way the explanation goes, without ever relating the terms. Is Under the same as a (-) bias? And Over the same as a (+)?

  • Dan Rode November 13, 2008 04:02 am

    I use EV compensation (Exposure Bias) less and less these days. There are two main reasons. First, I shoot RAW, so I have a good 2/3 to 1 stop latitude in post processing. The second and more important reason is that I usually switch to manual for difficult exposures. I read the meter, take a shot and review the image on the LCD. Then I adjust (usually shutter speed) and take another shot. Lather, rinse, repeat.

    It is still very hand when in Aperture Priority mode when light conditions are constantly changing.

    One note. EV/EB DOES have the same effect in manual mode. EV sets a bias on the cameras light meter, not the image.

  • The Floating frog November 13, 2008 02:14 am

    Using exposure bias on a three stop exposure bracketing is a good technique for capturing a well exposed shot. These three images can also be used to create a HDR photo with software like Photomatix. Exposure compensation is a very nifty tool that I'm starting to use more and more.

  • Image-Y November 13, 2008 01:15 am

    For those who would like a different way of understanding EV Compensation, here's another link for you:

    http://yanikphotoschool.com/tips/understanding-the-ev-compensation-button/

  • Joakim November 13, 2008 01:09 am

    Thanks for the article. I was hoping to learn a bit more about what the exposure compensation function actually does. I thought the camera changed the shutter speed. But you say that it "adjust the exposure a certain amount of f-stops either over or under the camera’s metering". If it changes the aperture you'll get a slight change in the DOF...? If so, it's probably better to take control of the camera manually.

  • Les Stockton November 13, 2008 01:06 am

    Great explanation. Short and sweet.

  • sanders November 13, 2008 12:15 am

    I always have difficulties with properly understanding this.

    What If you for example say that you want to underxpose 1 stop (using this function) and then you set your flash to overexpose 1 stop?
    Which of the functions have precedence?