Dynamic range is a simple concept: it is the difference between the darkest and the brightest parts of your scene or image.
Let’s say the brightest part of your scene is a thousand times brighter than the darkest one. If your camera can record all of that, all is great, your photo will contain all the details that your original scene has. You would have nothing to worry about!
What happens, though, if your camera simply cannot record it all? And on top of that, your monitor cannot even display all the range that your camera did capture? When it’s just too much? The image below is an example of such an image.
The dynamic-range giraffe is a visual metaphor that explains what’s going on to you.
Is there a solution?
Of course, there is! Photographers and cinematographers have been using two approaches since the dawn of their trades. The first is, to modify the contrast ratio of your subject, to make the dynamic range of your scene fit the dynamic range of your negative, reversal film, digital sensor or your television screen. You need to be able to control light to achieve that and this is why so much photography is done in studios where everything is under your control.
The second solution for digital cinematographers is to use so-called gamma curves that modify how the dynamic range of your scene is ‘translated’ on your sensor. Instead of me repeating what much greater minds have said, here’s a link to a great introduction to the world of gamma curves (linear and logarithmic) by cinematographer Art Adams.
You still might be asking yourself: why do I need to know about this? Well, because this is your key to understanding the Histogram, the only tool in your DSLR that gives you objective feedback about the brightness values of your picture and whether you’re photo is too bright (overexposed) or too dark (underexposed).
And this is how it works.
Here is an image. It’s black and white (and grey).
And here is its histogram. On the left, you have black, on the right, there’s white. In between, you have all various shades of grey. On the vertical axis you see the relative amount of pixels that have that particular brightness.
In this case, because the image is made up of simple, grey rectangles, you see distinct spikes. On the very left, there’s the spike for the black rectangle and on the right, the spike for the white one. The tallest spike is the grey box surrounding the greyscale.
Here is a ‘real’ photo of a brick wall.
And here is the corresponding histogram. Don’t be misled by the color, it’s still about the brightness (luminance) levels of the image.
In this case the histogram is telling you, that everything is just fine. To stick with the earlier metaphor, the Giraffe fit perfectly into the container. No highlights are lost and no shadow detail is clipped.
Here is the same wall, overexposed. This time I did it on purpose, but you know how it is: overexposure happens all the time, sometimes by accidentally choosing the wrong manual setting, sometimes because we forget about a previous exposure compensations setting. And sometimes, because it’s just unavoidable.
Here is the histogram.
And here is the highlight warning that your DSLR would display. (Just not in red. What’s red here would be blinking in black on your camera’s display.)
And this is why the histogram combined with the highlight warning is the most important tool your camera offers to find the right settings for your exposure. In digital photography detail that is not recorded cannot be recovered. It’s gone forever. Overexposed parts of your image and white hole with no texture. Yuck. Avoid it. There’s no remedy for that. And so that you’d see why, here are the two pictures of the wall. First, the correctly exposed one and then, the overexposed picture darkened so that it would look as close to the original as possible.
Click on the image to see the difference!
See, how the part of the wall in the shade is almost identical? What’s really different is the sunny side. The overexposed part simply cannot be corrected in any kind of software.