EVERYTHING ABOUT HISTOGRAMS - A HISTOGRAMS PRIMER

Tuesday, July 18, 2006

Histograms are a vital tool that most digital photographers tend to use when taking pictures. This is a short and easy essay on understanding and using histograms in the daily life of the digital photographer.

HISTOGRAM, WHAT IS IT?

To say it shortly, a histogram indicates the brightness level of an image. Most of the advanced digital cameras, and probably all digital SLR cameras have the feature to display the histogram on the LCD. On my Canon EOS 350D, I need to press the info button twice to see it. It may be different in your camera. You can also see the histogram of an image after downloading it to the computer, using software like Photoshop.

Any digital image is represented by thousands of small dots called pixels. In a color image, each of these pixels will have two important characteristics - the color of the pixel and the brightness of the pixel. Let us focus on the brightness part. A pixel can have a brightness level from a pre-defined set of values. For simplicity, let us say we have brightness level zero to brightness level 255. Then, 255 represents the brightest possible pixel and zero represents the darkest possible pixel.

Now, the histogram is simple graph that represents this brightness level(zero to 255) on the horizontal axis and number of pixels for each brightness level on the vertical axis.

HOW TO INTERPRET IT?

It is easy. If your image is dark, then you have more pixels with low brightness value, which means that your graph will be tall on the left side. Similarly if your image is very bright, your graph will be tall on the right side. For an image that has lot of mid-tones, the graph will have its peak in the middle.

To demonstrate how histograms look, I took three different shots of the same object. The first was properly exposed. Second image was deliberately under-exposed and the third was deliberately over-exposed.



The above images are of a white wall lit by florescent light and its histogram. To the right of the image is part of a poster included deliberately to demonstrate the histograms. This is a properly exposed picture. Since the image is properly exposed, most of the pixels are concentrated to the middle of the histogram. The pixels related to black poster are concentrated to the left.



The above image shows the under-exposed image and it's histogram. See that the peaks of the histogram have shifted to the left.



The above image shows the over-exposed image and its histogram. As you can see, peaks are to the right as the image has many bright pixels.


Above images demonstrate that a histogram can be used to check the brightness level of the image. But it does not necessarily mean that all images with histogram skewed to the right are over-exposed, or skewed to the left are under-exposed. Some images may naturally have many bright elements in it. For example, snow being bright by itself, histogram of an image with lot of snow in the frame will have more bright pixels and yet does not mean that it is over-exposed. See the following image that has lot of snow. It is properly exposed, but its histogram shows many bright pixels, which is how it should be. Similar theory applies to dark images too.



BURNT IMAGES

If you have any pixels recorded at the right most level of the histogram, it could indicate a problem. It means that you have some pixels that are fully white or at level 255. Typically, it could mean that the image is very over-exposed and any details at this part of the image are completely lost, and all that remains is of white colors. This could happen often if you are photographing an area with snow, or bright sky. Some cameras indicate this with a blinking region on the images when you play back on the LCD to warn that the image is over-exposed. In my Canon camera, I need to press the info button twice to see this blinking display. The exact way to see it may vary in your camera.



In the above image the details of the grass on the earth outside the window are lost since it is completely over-exposed. Notice that there are some pixels on the extreme right of the histogram.

WHY IS IT NEEDED?

The histogram gives a reliable feedback on the exposure level of your image. The LCD display is not a reliable way to interpret this because what you see depends on the brightness of the LCD and not just the captured image. But the histogram will give away any areas that are too bright or too dark. It especially serves to warn about completely burnt pixels.