Histograms

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Fishermen in Fog — Jordan Lake © jj raia

There was some discussion in a recent post about the usefulness of histograms in making informed judgements when metering scenes and the “best practice” of not having tonal values pushed up against either side of the histogram display. The intention was to illustrate how stretching those tonal values across the entire histogram, yet not touching either edge, might improve the overall information contained within a RAW file, which can then be adjusted to the photographer’s taste in a way that should produce better results in the final file or print. Points were made to keep values away from either side to maintain information that can be used during processing. If highlights are overexposed, there is no information to adjust (exposure, color, etc), it is just empty space in your photo. The same holds true for shadows, or dark values; if they are grossly underexposed, there is not much that can be done to recover them.

However, there are instances when you may want the final file to be pressed against either edge of the histogram. But it has to be considered only in the final result, and not how it may appear when you initially take the shot. For the image above, the histogram of the original RAW file (below) is much different than that of the final post-processed file. The majority of values lie just right of center in the RAW file, while intentionally pushed up against the right side in the final to accurately portray the light values of the fog that day. But even more importantly, is that the small amount of middle tones (thin line along the bottom edge of the post-processed histogram) are being stretched out across a much wider section of the display, indicating that there is a greater separation of those values to work with. If the exposure was brightened globally by simply adding two-stops, the overall image would be brighter with the exact same graph moved to the right, and the mid-tones would have moved to the right along with every other one, to a higher value. And they might then be lost.

Why is that important for the image above? Only for the two fishermen in the boat who, through the fog, still have some color in their clothing, and upon close inspection, can be seen as it is in the 1:1 of them. If the colors were brightened, they’d get lost in the fog and would not stand out as much. Although basically a Black & White image, there has been some color recorded by the sensor as seen in the histogram as well.

1:1 Magnification


The same can be said of an image that is almost all black as the one below. I wanted the dark tones to be almost completely black, and pushed them to the left edge, with an almost imperceptible thin strip of complete black adjacent to the brighter middle on the right (That thin strip may not be visible on your screen). And the two other basic gray tones can be seen in the histogram as well, with the brightest just off the right edge.

Sculpture Abstract — NYC © jj raia
Histogram – Sculpture Abstract

Once you have an image with full tonal range and values shown throughout the histogram, you are then free to move certain values around with global adjustments, or locally with the tools available in your preferred processing software.


Reflections — NC Museum of Art © jj raia
Histogram — Full Range Post-Processing

The image above with the accompanying histogram represents one containing the full range of tones in most every color. Highlights remain just inside the right edge, with just a tiny amount of shadows touching the left edge, while the density of color is high showing full saturation. Remember, for the fishermen, the mid-tone colors were along the bottom edge and showed as almost gray with just a hint of color. Here, with the colors reaching toward the top of the graph, the colors are richer and more vibrant. The Hue, Saturation and Luminance panel shows that there was actually slight reductions made to the Aqua and Blue saturation values while there was no increase in global saturation.

Histograms can be a useful informational tool by using them at the time the photo is taken in making judgements for the intended final file. If the shadows are the most important to your image, overexpose them a bit to keep them from being blocked up. There will be more information in those values to make adjustments later making them as dark as you wish but with some detail if desired. Just make sure you’re not blowing out the highlights at the same time you’ve increased the shadows. If that is the case, then again, two frames are needed to be combined later.

The same is true for highlights if they are the most important in your photo. By not exposing to the right, and keeping them well off the right edge, you can then stretch them out, expanding the number of values within the highlights rather than having them compressed into a narrow slice of the graph.

I’m afraid that trying to describe in words what we really want to see, is difficult at best, and I have struggled to best illustrate these points. Ansel Adams may have described these ideas best referring to changes in chemicals and length of developing times when processing his negatives, but we now have graphs to physically see where light values lie from your particular photograph, from the darkest black to the brightest white. And I have to admit, that I do not use this tool nearly as much as I should. But just writing about this subject has given me more clarity of their usefulness, and determination to use them more in the future, and a better understanding of how to use a Histogram to create a better file.

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