Digital Photography: Optimum Exposure

“I’ve missed more than 9,000 shots in my career. I’ve lost almost 300 games. 26 times I’ve been trusted to take the game winning shot and missed. I’ve failed over and over and over again in my life and that is why I succeed.”  – Michael Jordan

For many years, Michael Jordan was, by far, the world’s best basketball player. He was capable of phenomenal physical maneuvers and accuracy previously thought to be impossible. That is why I find this quote so impressive, both for its humility and for its wisdom.

My last post was about thoughts and feelings. This post focuses on technique. Specifically, it explains how to get the best detail in the low light and shadow areas, and another adjustment for maximizing color detail in the highlights of your photos.

Digital sensor limitation: In a digital camera, the actual picture is taken by a sensor which is made of millions of pixels. It is these pixels that actually “see” the light. When bright light is falling on a pixel, much more information about the light is recorded than when it is being exposed to low light. As a result, hundreds of times more gradations of light are recorded from the bright areas of a digital photo than from the low light areas. As a result, shadows show relatively little detail.

Correcting for limited detail in darker areas:  There are two stages to the correction. First, slightly overexpose your initial image. Second, use your editing software to decrease the exposure an equal amount (if it appears too bright). You end up with an image correctly exposed, but with more detail in the low light areas and shadows.

A drawback to this overexposure technique is that gradations of color will be lost in the highlights, resulting in a blotchy appearance of very bright areas of color. If your primary interest is fine detail in color, please skip down to “Optimum exposure for color”.

Correction Stage 1: Controlled overexposure is the key. But how much overexposure is appropriate? (If there is too much overexposure, detail in the highlights will be lost or “blown out”). The precise amount of overexposure is determined by using the histogram (The dreaded histogram… but wait, don’t go. This is not terribly difficult).

A histogram is simply a graphic representation of the light that the sensor is “seeing”.  The histogram looks like a mountain range. The right side of the mountain range represents the brightest areas of your photo. The higher the peaks, the greater the number of pixels sensing bright light. (“The right is the bright”). Conversely, the left side represents the dark areas and shadows in the photo. The higher the peaks on this left side, the greater the numbers of pixels exposed to low light. (Since this histogram is representing light intensity, it is called a luminance histogram.)

How to display the histogram: All DSLR’s (of which I am aware) provide a histogram. Many, but not all, other types of digital cameras offer this feature (Check your camera’s operation’s manual). Most cameras display the histogram only in the playback mode. To have the histogram display, first go to the playback menu. Find histogram. Set it to on (If you have a choice between luminance and color histogram, choose luminance).  Now, playback any image so that it shows on the screen. If the histogram does not show on your screen, use the display options button (located on the camera back, usually near the screen and labeled “disp”) to cycle through the display options until the histogram shows . (If you have an option for a histogram display in your pre-shot viewfinder or “live view” screen, it can usually be turned on in the recording menu. You can make your adjustments directly without using the playback mode).

Determining correct overexposure: (For most lighting conditions, exposure compensaton is going to be +1/3 to +2/3 stops). We will be moving the mountain peaks of your histogram toward the right, approaching the far right end of the baseline, without going so far that any of them abruptly disappear.

  • Turn on automatic playback so that your image shows on the screen immediately after your shot is taken.
  • With exposure compensation set to zero, take a photo in your usual manner.
  • In playback mode, check the histogram for your image.
  • If the peaks on the right end of your histogram do not extend almost completely to the far right side of the baseline, then set exposure compensation up by +1/3 to +1/2 stop.
  • Repeat the same shot. Again check the histogram.
  • The peaks will extend somewhat more to the right (If they extend almost to the far right end of the baseline, then you are done with this stage). If there is still a lot of black space between your peaks and the far right baseline, set exposure compensation up another +1/3 to +1/2 stop.
  • Repeat the shot and check histogram again.  (Probably you are done now, but repeat if needed).
  • For most lighting conditions, in my experience, an exposure compensation adjustment of +2/3 stop is correct.
  • Here are two tips for remembering  this technique.  First, “The bright is on the right”. Second, “Moving (the histogram) to the right” is abbreviated Mttr, pronounced “Meter” as in light meter)
  • The above technique gives you correct exposure for that specific subject and lighting condition. Whenever lighting conditions change, check your histogram and, again, adjust exposure compensation with this technique.
  • A word of warning: Do not set it and forget it. Re-evaluate exposure for every new lighting condition.

Correction Stage 2: This is done with your editing software. Any editing program will work for this. It does not need to be elaborate (The program that came with your camera will work fine). Open the image that you just overexposed (If you like the way it looks, you can skip this stage).  If you image appears too bright, slide the exposure slider to the left to decrease exposure by the same amount that you increased it when taking your photo. (While you have your editing program open, you may wish to adjust white balance, contrast or any other parameter that improves the appearance your image)

Results: Now you have an image with correct exposure, but with more detail in the darker areas and shadows than it would have had.

Optimum exposure for color: A drawback of the above (luminance) overexposure technique is that fine gradations of color will be lost in the highlights, resulting in a blotchy appearance of the color in these areas.  For best detail in color highlights, an image usually needs to be underexposed a small amount. To determine the appropriate amount underexposure, use the color histogram, not the luminance histogram.  (If your camera does not offer a color histogram, I recommend “guesstimating” with exposure compensation of -1/3 in subdued lighting and of -2/3 in brightly lit conditions). Color histogram display during playback is usually set in the same way that the luminance histogram was set above (Set it in the playback menu and also with the display options button on the back of the camera).

  • Take a shot with exposure compensation zero. Check the color histogram.
  • Probably at least one color will extend all the way to the far right end of the baseline and abruptly cut off (No black area between the tips or all the colors and the far right baseline).  If that is what you see, decrease exposure compensation by 1/3 to 1/2 stop. (If the color channels approach, but do not abruptly disappear off the far right side of the baseline, then your exposure (for color) is correct already.
  • Repeat your same shot and again check the histogram.
  • If any color channel still abruptly cuts off at the far right, decease exposure compensation another 1/3 to 1/2 stop.
  • Probably you have good results by now, but repeat this procedure once more if needed.
  • Generally the correct amount of underexposure will be -1/2 to -2/3 stop.
  • Your color highlights will now be the best that your camera can offer.
  • However, whenever lighting conditions change, re-evaluate the histogram and adjust exposure compensation if needed.
  • A word of warning: Do not set it and forget it. Re-evaluate exposure for every new lighting condition.

I welcome comments, corrections and suggestions for future blog topics.

I am available at NaturePhotoRehab.com to assist clients with transforming their images into impressive works of art that they will be proud to either hang on their own wall or to give as a gift.

J. Michael Harroun(c)2012  NaturePhotoRehab.com

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12 comments on “Digital Photography: Optimum Exposure

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