Understanding Histograms

Preview screen at the back of the digital cameras is a wonderful tool to assess the exposure. Though present in all the good cameras, very few people use it. Histograms when used properly can be a very simple tool to take care of all your metering related problems.

Histograms on Nikon camera bodies

(Histograms as seen on a Nikon preview screens)

Most of the photographers I have seen use the preview screen just to take a quick look at the captured image to assess the overall. This is termed chimping in the photography circles and it really is a bad habit.  If you have it, try not to discontinue it. Do not peep at the preview screen after every shot. (To Chimp or Not To)

Histograms are different. These are very powerful aids provided to photographers. To access the histograms on Nikon cameras, try using the big navigator buttons and scroll up or down to reach the histogram preview screen.

Simply put, histograms are graphical representations of the number of pixels from a photograph, on a scale starting from complete dark to complete white. The left most part of the scale is the darkest black that the camera can capture. Similarly, the right most part is the lightest white that can be captured by the camera. The middle region represents the mid-tones with the center most part corresponding to the 18% grey. The total scale ranges from 0 to 255 where, 0 corresponds to the blackest black and 255 to the whitest white (8 bits).

 

Photograph with all tonal values and well exposed

An ideally exposed photograph should have all the values centered in the histogram with tapering ends of the histogram just touching the either ends. What it means is that the photograph has all the tonal values spread across the the tonal range available with almost no areas devoid of information in extreme black or white areas. See the histogram given below –

Histogram

(Histogram with most tonal values in the central region, on a Nikon D60 preview screen)

Low Key Photographs

Low key photographs or photographs with predominantly dark tones have histograms concentrated towards the left of the region. (Low and High Key) There is nothing wrong with it. This kind of histogram just shows the overall low number of pixels with light tints in the photograph.

Dark Photograph - Histogram

(Histogram gathered around the left side of the area indicating dark tonal values)

High Key Photographs

On the other hand, high key photographs or photographs with predominantly light tones have histograms which appear to be shifted towards right. Once again these kind of histograms show that the there are very few dark areas in the overall photograph.

High Key - Histogram

(Histogram of a high key photograph as seen on a Nikon D60 preview screen)

Clipping

Now let us understand the histograms where the tonal values appear to touch the extreme left or the extreme right end of the values. These give an indication that there are values outside of the tonal range. In other words, some areas have been captured as the lowest value (black) or the highest value (white) with no information inside these areas. This is called ‘clipping’. The areas which have been clipped are devoid of any information and no amount of post-processing can recover this information.

Histogram showing clipping

(The histogram shows a lot of values on the extreme left typical of clipping in the shadow areas. Overall the histogram is shifted to the left showing under-exposure)

Additional features

Camera manufacturers have now gone a few steps further. The histograms when activated represent the area that is being previewed on the preview screen. This is very useful. Now it is possible to enlarge an area and study the corresponding histogram.

Another additional feature in some cameras is that the histograms for the three primary colors (red, blue and green) are also available along with the overall tonal values.  These are especially useful when the main subject contains a predominance of a particular color. A histogram showing all the tonal values might show a well exposed photograph but the color specific histogram may bring out the clipping of shadows or highlights for a particular color.

Blue Photo - Histogram

(Preview of a photograph on a Nikon D200 with predominantly blue hues. The average tonal value histogram or the RGB histogram shows a low key image. The individual color histograms show the blue color almost to the right showing the correct exposure for this color without getting blown-out)

An interesting thing to note is that the green color histogram quite frequently looks similar to the overall tonal histogram. The reason behind this is that the camera sensors have double the number of green color specific cells than any of the other colors, in an attempt to replicate how our eyes see the colors. This is called Bayer’s pattern.

 

How to use the histograms

  • Cross check the exposure after taking a shot using the histogram. Do not depend on how the image looks in the preview screen. Depending on the histogram, decide on exposure compensation or changing the basic settings.
  • Avoid as much clipping as you can. Even in low key or high key images, avoid clipping. Try to capture information even in the dark or light areas. When clipping occurs, textures disappear and the image might loose its impact.
  • Review the color specific histograms too for accurate information about reds and blues as well. In most cameras, reds tend to get blown out and blues tend to be very dark. Avoid clipping where possible.
  • The histograms also reflect the changes based on some of the camera settings even if you are using raw files. These are, after-all, calculated from the jpg image generated in the camera. The histograms may be misguiding at times. Try to use the camera settings which you might use after post-processing even if you are clicking raw. This will help in keeping histograms as true to the image as possible.
  • In all the post-processing and image-editing softwares, histograms are included. Do keep an eye on these even while working on your photographs on your computer.

 

Blue Abstract

(A blue abstract showing an hour-glass form. I used a water bottle that had this shape reminiscent of females. Placed it in front of an overpowering blue background and faint yellow light from a side to show a kind of cloth wrapped below the waist The exposure retained the textures in the upper part of the bottle without getting overexposed. The exposure compensation used was based on a preview of the histogram)

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