Saturday, March 17, 2012


Dynamic Range

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What is Dynamic Range?

Dynamic range in our everyday lives, is the difference between the darkest and the brightest part of our vision that we can see. Dynamic range is working in our everyday lives. For example, have you ever walked into a dark room, and found yourself temporary blinded by the dark for a short time? It's because the darkness of the room exceeds our dynamic range of eyes, and our pupils dilate (open up) to allow more light to pass through into our eyes, enabling us to see darker objects brighter, and brighter objects brighter as well. Comparing our eyes to camera sensors, our eyes have an amazingly wide dynamic range!

Dynamic range in photography is simply the difference between the darkest and brightest part of a photo. It is usually expressed in a graph such as the one below. Dynamic range gives us a clue to whether a photo is properly exposed.

How to read a Dynamic Range graph?

Dynamic Range graph, is more commonly referred to as "Histogram". On the extreme left of the graph, the "0" represents pure black, and on the extreme right of the graph, the "255" represents pure white. Anything that lies in the middle is a shade of grey somewhere between black and white. I'm going to keep this simple and talk about dynamic range on the RGB (Red, Green, Blue) channel only.

The amount of a specific shade of grey in the photo is represented by the height of the "Mountain" in the graph. A higher peak shows more shade of that grey in the picture.

I like to quote this example when explaining dynamic range.

1) Take a photo.
2) Convert that photo to greyscale (black & white).
3) Now each pixel in the photo is of a certain shade of grey (or it could be pure black or white, but mostly grey for sure)
4) Take each pixel and arrange it on the graph from pure black to pure white.
5) There you have it, a dynamic range graph!

How can Dynamic Range help in photography?

In photography, we generally want to have a correctly exposed picture. A correctly exposed picture should have a nice peak in the center.

However, do note that every photo is different and there is no perfect graph for dynamic range. Depending the the scene, the graph could have it's peak more towards the left or right, and still be correctly exposed. Or depending on the photographer's creativity, under or over exposing the photograph deliberately can make the picture look better.

Dynamic range of a very overexposed photo
Dynamic range of a very underexposed photo

Before you call the above dynamic range graphs "trash", they are actually part of a group of photos used to create a HDR (High Dynamic Range) photo.

Using HDR can drastically improve the dynamic range of photographs by combining the useable parts (those closer to the center) of the dynamic range to enhance the dynamic range of a correctly exposed photo.

Understanding Dynamic Range for use in Photoshop

This photo needs a little correction. The reason why the graph is skewed to the right is because of the very overexposed background. The subject is of the right exposure, but is lacking a little contrast, and the background is very overexposed. So how can we correct this problem? Now, here is a extremely simple cure for this problem. Navigate to the Levels menu by going to: "Image > Adjustments > Levels" (or CTRL + L). With the levels menu open, you have 3 options to get the right levels for your picture.

Photo edited using auto settings method.

1) Auto - Auto is a no brainer. Photoshop will decide for you what's best and then automatically determine the black and white point. However, it may not be accurate all the time, nor it can create any creative effects you want on the photo.

Photo edited using the level sliders method.

2) Level Sliders -You can manually adjust the white and black point by moving the sliders. Moving the black slider inwards gradually turns darker colors to pure black, and moving the white slider inwards gradually turns lighter colors to pure white. The grey slider in the center can go both ways and it can make greys darker or lighter depending on the direction it's slide towards.

Photo edited using the sample tool method. Sample tools circled in red

3) Sample tool - Consist of a black, grey, and white point sample tool. For example, using the black point sample tool, you can quickly and easily sample a spot on the picture that you want it to because pure black (0 on the dynamic range graph). Anything darker than the spot sampled will be pure black as well. I normally use this tool sparingly, and use it only if I find a known pure black object doesn't turn out pure black when shot on camera. Similarly, the white point sample tool samples a spot on the picture and turns it pure white.

Or if you wish, you can use a combination of 2 or more of the methods, such as using auto before manually adjusting the level sliders. Any method that makes a good photo is a good method. If you noticed little difference in the photos, that is normal. Changes like this are subtle, but after a series of other post-processing, the difference will show and people will be sure to notice it.

There are some other functions on photoshop that uses the dynamic range, but they are generally similar to the levels function. If you know levels, you shouldn't have any problems when you encounter a tool that has something that looks like a dynamic range graph on it. Good luck and have fun shooting!