:: Possibly the most useful tool available in digital photography is the histogram. It could also well be the least understood. On this page we will look at what a camera histogram tells the photographer and how best to utilize that information.

   Dynamic Range

The digital imaging chip in your camera is very similar to colour transparency film when it comes to its sensitivity to light.

Like slide film, if a part of the image receives too much light it becomes burned out, and if too little light it is rendered as black. A recognizable image is only recorded if the light hitting the chip falls within a range of about 5 F stops. (Remember — each F stop is a doubling or halving of the amount of light hitting the film). With digital things are much the same and even the dynamic range is about the same as for slide film; about 5 stops. Also keep in mind that the total range of brightness values encountered in the real world is only about 10 stops — from the dimmest light that you can read in to the brightest beach or snow scene in which you might find yourself).

 

I have labeled each of the five zones (or F stops) containing the dynamic range recordable by the cameras as Very Dark / Dark / Medium / Light / Very Light. But each of these 1 stop ranges contains within it just over 50 discrete brightness levels. (5X50=250 not 256, but who’s counting?) Seriously though, it’s a good idea to consider about 4- 5 points at the very bottom (black) and another 4-5 points at the very top of the scale (white) to be so close to the extremes as to not really be part of the image-forming segment of the graph.

                                                            

The Histogram

This is where the histogram comes in. It is a simple graph that displays where all of the brightness levels contained in the scene are found, from the darkest to the brightest. These values are arrayed across the bottom of the graph from left (darkest) to right (brightest). The vertical axis (the height of points on the graph) shows how much of the image is found at any particular brightness level.

So now things start to become clearer. The histogram shows us quite a bit, and just as a glance at the hands of an analogue watch instantly tell you the time without your even being conscious of the exact numeric values, similarly once you become proficient at “reading” a histogram you’ll be able to almost instantly evaluate the quality of the exposure that the camera is making. This is especially true when the histogram is superimposed on or just next to the image itself, making the graph that much more meaningful.

 

Not too long ago a histogram was something mysterious. Today it has become a valuable tool for the photographer who wants to gain mastery of their digital camera’s image quality. I hope that this has helped remove some of the mystery for you.  

Start using the histogram review feature of your digital camera. Set your camera to display a combined thumbnail and histogram for 5-10 seconds after every frame. Get in the habit of glancing at it. It's the greatest invention since the built-in light meter.

 

 :: There it's Easy!...........More great info on photography to come.......