This is based off the concept that a histogram curve that is predominately in the left edge of the histogram is indicating a scene that is underexposed. And a histogram that is all the way to the right is likewise overexposed.
Unfortunately, this is patently false! While this is correct in the literal interpretation of the image’s tonal range, it is not a correct evaluation of correct exposure. That is the key difference.
Why? Because there is no “perfect histogram” to base all of your exposures when shooting.
To understand why this approach is false — and learn how to correctly use the histogram reading — let’s start with understanding what the histogram is doing, first via a technical definition:
The camera’s histogram is a translation of the tonal range of the scene, represented in a two axis interpretation: a horizontal axis that shows the brightness levels, from black shadows (0) on the left to white highlights (255) on the right; and a vertical axis that shows the amount of pixels that are in each brightness area, as pixels of equal brightness are stacked vertically one upon the other to create lines of varying heights. The result is a graph that rises up vertically in the form of jagged lines, smooth curves, or a combination of both.*
In layman terms, the histogram shows if the brightness range of the scene, from the highlights to the shadows, will fit within the camera’s dynamic range — what your camera sensor can “see” and capture. Learning how to read a histogram relative to the scene you are photographing is key to using the histogram correctly.
Which brings us back to why this common thought of keeping the histogram’s curve in the right 2/3rds of the histogram is incorrect and can result in an improperly exposed photo!
As we said before “there is no perfect histogram.” That’s because every scene you shoot, regardless of if it is people, landscape, urban or wildlife, will be made up of uniquely different blend of tones, which will result in a uniquely different histogram every time.
VARIATIONS OF CORRECT EXPOSURE HISTOGRAMS
For example, the scene above from Yosemite Valley’s Cook Meadow, with a green pine tree-lined valley, gray granite cliffs with storm clouds in the sky, is a mid-tone scene with a dark shadows without detail (the trees in the center of the photo) while the rest of the histogram forms a bell curve to the right of the middle of the graph.
But the same scene taken in winter, by HSW workshop participant Bob Melgar, when the valley is blanketed in white with little mid-tones in the scene, the curve is considerably flatter on the left 2/3rds, with the graph peaking sharply on the far right of the graph, a measurement of the snow.
Likewise, at a favorite workshop shooting location at Yosemite’s Bridalveil Fall, with the falling light, dark shadows and warm light on the falls, the scene yields a histogram with the graph rising sharply in the far left 1/3rds of the histogram, depicting a scene that is primarily black without any detail, with a few highlights appearing at the right end of the graph.
In this last example, with heavy overcast light in Yosemite Valley just prior to a winter storm, the flat light allows for less contrast and greater range, as evidenced by the flat graph in the histogram, with the sole exception of one blown-out highlight that spikes at the far right of the histogram.
In all three of these examples, the scene is exposed correctly to faithfully render the sunlit portions of the scene. Yet, in all three the histogram does not fall into the “correct right 2/3rds area rule” that is used incorrectly.
That is because the histogram is simply displaying the tonal range/brightness/contrast of the scene — and nothing to do with evaluative exposure! How do you truly determine the scene’s technically correct exposure? By using a light meter — be it the camera’s light meter or a handheld incident or spot meter.
A handheld incident light meter measures the light falling on the meter/subject, thereby providing an accurate exposure. A white snow scene would render faithfully and accurately while using an incident light meter. Even the in-camera meter (which is for all intents and purposes a ‘spot’ meter) can be fooled by high-key (bright) and low-key (dark) scenes. A handheld incident meter can never be fooled.
It is important to recognize that the histogram isn’t displaying proper exposure, but it is showing you valuable information about where the tones are falling in dynamic range of the camera’s sensor — and what will be recorded and what will not!
In the third photo above, the histogram displays a curve that is flush to the left, due to the overwhelming amount of dark shadows in the scene. While the scene is faithfully rendered and exposed, the dynamic range of the camera doesn’t allow for any shadow detail. As such, the histogram shows ‘clipping’ in the blacks, translated by the full vertical rise of the graph punctuated to the extreme left end of the histogram. If the left end of the graph was near the bottom of the histogram, then it would be translated as minimum black without detail in the scene. But not the case in this photo.
And this is where correctly reading histograms can save your photos! When your blacks or whites are ‘clipped’ — showing no detail — it is easier to see that in the statistical analysis of the histogram, versus the tiny LCD screen on the back of the camera. Checking the histogram after you have taken the photo, not for how the curve necessarily looks on the histogram chart, but rather whether you have blacks or white that are ‘clipping’ in areas that you want to retain detail.
NOTE: Keep in mind, that if your photo is correctly exposed — faithfully rendering the scene as per the your visual interpretation and/or the meter reading — adjusting that exposure simply to retain details in either ‘clipped’ highlights or blacks, will result in an exposure that truly is, technically, over- or under-exposed.
Another area that understanding and correctly using the histogram is when you intentionally over- or under-expose the image, to generate a high-key or low-key scene. While you could observe your exposure change and how it looks in the camera’s LCD screen, only through the histogram can you correctly evaluate if the high-key tones are where you want them in your tonal range.
In the above photo, taken by Joan Hammond during the Yosemite in Winter workshop, Joan overexposed the photo intentionally to create an artistic interpretation of a bright snowy scene. By using the histogram, the photographer can watch as the tonal curve shifts to the right as the picture becomes over-exposed, ensuring that the scene retains any white details desired without any clipping.
WORKING THE LIGHT METER AND HISTOGRAM TOGETHER
A histogram cannot tell you about lighting ratios, the best flash-ambient mix, or if the subject is properly exposed. That is why using a light meter, along with your camera’s histogram is the best way to assure optimum results that can be easily reproduced.*
In conclusion, as best said by Sekonic in their PDF publication about light meters and histograms, “Photography would be much easier if histograms had instantly recognizable good shapes and bad shapes and if there were “ideal shapes” that guaranteed perfect results. The truth is, they are just graphs. All of the histograms in this example accurately describe the pictures … Their shapes are all very different because the tonal distribution of the image they represent is also very different. At best, a histogram is a guide, it is up to you to determine how to use it.”
*Source: Sekonic PDF publication “Histograms & Light Meters: How They Work Together“
Have you ever seen the beauty of Yosemite Valley under a blanket of fresh-fallen snow? Or witnesses the numerous thunderous and roaring waterfalls as they pour over the rim of the valley during the annual spring season? Have the photos above whet your appetite for a journey into one of America’s most beautiful national parks?
Then come join us for either our Yosemite in Winter or Yosemite in Spring workshops, and enjoy an educational professionally-guided photography tour of both the iconic and hidden photographic gems of Yosemite Valley. Workshops size is limited to small groups only, to ensure a one-on-one teaching environment between yourself and the workshop instructor, in order to maximize the educational opportunities as you improve your photographic abilities.