{"id":1374,"date":"2009-02-28T09:00:57","date_gmt":"2009-02-28T09:00:57","guid":{"rendered":"http:\/\/www.markwilson.co.uk\/blog\/2009\/02\/understanding-how-histograms-are-used-in-digital-photography.htm"},"modified":"2009-02-28T09:01:25","modified_gmt":"2009-02-28T09:01:25","slug":"understanding-how-histograms-are-used-in-digital-photography","status":"publish","type":"post","link":"https:\/\/www.markwilson.co.uk\/blog\/2009\/02\/understanding-how-histograms-are-used-in-digital-photography.htm","title":{"rendered":"Understanding how histograms are used in digital photography"},"content":{"rendered":"

The backlog of photography-related posts for this blog is now almost as long as the virtual pile of half-written IT-related ones but I’ve been meaning to write something about histograms<\/a> in relation to digital photography ever since Andy Gailer<\/a>‘s presentation on working with raw digital camera images<\/a> at my local camera club<\/a> last year.<\/p>\n

For the first few years that I owned a digital camera, I ignored the histogram – quite simply because I didn’t understand it. Since Andy explained it to me, I find it an incredibly useful tool for helping me ensure that I’ve captured the maximum amount of data in my images.<\/p>\n

The histogram, quite simply, shows 256 levels of light (tones) that are present in an image. The peaks indicate a large volume of pixels in a particular tone and the troughs appear where there are fewer pixels. On the left side of the histogram are the dark areas and on the right side are the lighter areas.<\/p>\n

\"ImageTake, for example, the accompanying picture of a post box outside a gift shop. As can be seen, I took this after a snow fall and I didn’t compensate for the light reflected by the snow. Even though the camera’s automatic white balance would have attempted to make some corrections, the snow shows as grey and the histogram tells me that most of the pixels occur in the darker areas to the left of the graph.<\/p>\n

\"AMy camera will show me a histogram but if I view this in Adobe Photoshop, I can see a little more detail. As well as some statistical data about the mean levels and the standard deviation, I can hover my mouse pointer over the graph and see how many pixels appear for a given level. In this case I can see that at level 247, there are only 24 pixels (out of more than six million in the entire image) – effectively there is very little happening in the highlights.<\/p>\n

\"AdjustingBy adjusting the levels, I can alter the highlights (white slider), midtones (grey slider) or shadows (black slider) and improve the overall exposure of the image. Effectively, I set the black and white points and adjust the contrast.<\/p>\n

The ideal histogram is evenly distributed with no breaks and a gradual tail off for shadows and highlights. If I hold the Alt key (Option key on a Mac) as I preview the adjustments, I can see the pixels which are being clipped from the image and I should stop just before this become noticeable (Adobe Camera Raw will show clipping as blue or red pixels when this option is enabled). A few points to note are:<\/p>\n