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Lesson 7: Histograms Demystified…

April 2nd, 2011 · 1 Comment

This lesson will focus on reading Histograms. Histograms in editing software provide valuable information for making tonal and color adjustments whereas in digital cameras, they provide you with the information needed to make exposure adjustments for better results.  The ability to read histograms will serve you well both in the field and in post-processing your images.

A Histogram shows the distribution of tones in an image. In a standard 8-bit image each color channel shows 256 shades of gray. 0 (Black) starts in the lower left hand corner. As you move from left to right the graph shows pixel values from shadows to mid-tone and finishing in the lower right hand corner with 255 or pure white (highlights). The vertical part of the graph shows the amount of each pixel shade in the image. As a photographer you are mostly concerned with where your darkest pixels begin on the left and where your lightest pixels end on the right. Don’t be obsessed about the shape and vertical components of your histogram but be concerned about starting and ending points.

The image above is a fairly typical full range image. The shadows begin at the left hand lower corner and the highlights end at the lower right hand corner. The full range of available tones are represented here with no clipping or lost pixels in the shadows (left side) or highlights (right side). There is an even distribution of tones across the image.

The image above is another full range image but note how different the histogram appears. This is because most of the pixel values in this image are between the mid tones and the highlights. Notice however that the shadows begin in the lower left hand corner and the highlights end at the right hand corner. This is what you really need to focus on when reading histograms rather than the actual shape of the histogram.

The image above is a low key image. Most of the pixels are on the shadow end (left side). There are some highlight pixels in the lights of the ship but there are not enough of them to show up as a spike on the histogram. This is a situation where you would not want a full range image. This image was intentionally exposed as a low key image bringing most of the pixels to the shadow side to create the effect of ships passing at night. Perfect exposure is not always about a perfect full range image. Rather, it’s about the photographer’s intent or vision for his or her image.

The image above is a high key image. Most of the pixels are on the highlight side of the histogram. There should be no attempt to color correct or change camera exposure to force this to be a full range image.

The image above has clipped highlights. The tell-tale sign of clipped highlights is the gray tone of the histogram stacking up against the right hand corner of the histogram. When color correcting, this situation represents lost detail that you will never be able to get back. The DSLR camera will typically show a warning when highlights are being clipped, and the camera histogram will clearly show the clipping. The appropriate exposure compensation can correct for this, restoring correct exposure and detail to the highlights. This is why correct camera exposure in the field is so critical. Clipped highlights are THE most important concern to the photographer when it comes to exposure.

The image above has clipped shadows. Note how the gray tones of the histogram are stacking up against the left hand corner of the histogram. The human perception is more tolerant of loss of shadow detail, so clipped shadows are of secondary concern when compared to highlights. Always try to expose for the highlights, as loss of detail in the highlights is noticed much more by the human eye.

The image above shows shadow AND highlight clipping.  Note how the gray tones of the histogram stack up against both sides of the histogram. The image has a range of tones that is beyond the capability of the digital sensor to capture in one exposure. In this situation you should always expose for the highlights as the cloud detail is more important than shadow details. Other options with this type of image would be to bracket your exposure (basically exposing one shot for the highlights and the second shot for the shadows) and merge the two shots into one in an image editing program.

Let’s go over the “Top 5” summary points to take away from this lesson:
     1) Histograms provide the power to obtain better in-camera exposure
     2) Histograms provide critical info for making tonal & color adjustments
     3) Don’t worry about the shape of the graph, the left & right sides are key
     4) Perfect exposure is not always about a perfect full range image
     5) Always expose for the highlights

Stay tuned for a full wrap-up of the Bald Eagle trip I led in February, including participant images!

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