What is Tone Mapping?
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What is Tone Mapping?

With tone mapping adjustments you can add a dreamy dramatic effect to your HDR photos. Find out what tone mapping is.

If you're interested in High Dynamic Range images, you've probably heard the term “tone mapping” at least a few times. If you've created HDR images, you've probably tone mapped them in the process, although you may not have realized it. That's because it's often applied as a preset that you selected. It's also the reason that many photographers mistakenly believe that tone mapping and HDR are the same thing.

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Understanding tone mapping will help you create more stunning HDR images and give you a better grasp of how your camera records photos and how your display devices render them. All of these devices are limited in comparison to your eyes and that's the reason tone mapping is important.

Understanding HDR

Before you can “get a handle on” tone mapping, you need to understand exactly what High Dynamic Range means. The concept isn't new and it isn't unique to digital photography. Film, like your digital camera sensor, has limited dynamic range when compared to your eyes.

This limitation has always challenged photographers to find ways to approximate the way our eyes perceive tones, whether on photographic plates, film or digital sensors. In the darkroom, blending of images in the enlarger and dodging and burning are the basic methods used to manipulate dynamic range.

The “range” is the number of steps in which a medium can record or display luminance, which translates into contrast – the difference between no light and all available light. These differences, applied to all colors of light are the basis of tone. Our eyes can distinguish an incredible number of differences in luminance, because they constantly adjust. Image recording and display devices are limited by their native technology.

In other words, your camera, your monitor, your smart phone screen and even your printer can only record or display a limited number of steps in contrast from high to low. That's why we often see blown out highlights or loss of detail in shadow areas of a standard image. There simply aren't enough steps in the range of tones available.

HDR imaging attempts to overcome that by blending several images taken at various exposure levels, to take advantage of overlapping steps in tonal values. This increase in the number of tonal values displayed is High Dynamic Range. Tone mapping determines how the values are blended.

Global vs Local Tone Mapping

Alright, so if you've been keeping up so far, you should realize that HDR imaging adds more steps to the tonal values in an image, which actually reduces contrast. This is most noticeable immediately after you merge photos into an HDR file. That's because the algorithm used to create that initial image created the greatest number of tones possible within the image based on the overall, or global values.

Global tone mapping generally results in duller images. To compensate for this, we can use local tone mapping to analyze and adjust the differences between neighboring pixels. Enhancing the image this way helps bring out details by increasing contrast in local areas.

This localization of contrast is the reason that halos around objects and other effects often appear in HDR images. It can also create surreal skies and other effects that can be beneficial or detrimental, depending on your preference for the final image. Generally, creating an HDR image with a natural appearance is preferred, but more challenging. That's why high-end photo editing applications like  Aurora HDR software, Lightroom® and Photoshop® offer presets that can generate the desired effect for you.

The Importance of Shooting RAW

You've probably noticed that most tutorials about HDR imaging emphasize saving your photos as RAW files. Tone mapping is one of the most important reasons for doing so.

Remember, your camera's sensor is only capable of recording a finite number of steps in contrast. When you save the capture as a RAW file, it retains all of the recorded contrast information is retained. When you allow your photos to be converted to JPG format, it uses a “lossy” algorithm to increase contrast in areas of the by discarding many of the steps in contrast.

In effect, the JPEG conversion does some tone mapping of its own, but the mapping is irreversible because data is permanently deleted. This, in turn, reduces the effectiveness of your bracketed exposures. For this reason and more, most pros agree that RAW files are worth the extra storage space.

That's a Wrap!

There's not much more to know about tone mapping, other than there are very few limits to how you can do it. Most high-end photo editing applications allow you to tone map manually with various sets set of controls for Gamma, Exposure, Highlight Compression and many other aspects of the image.

Tone mapping is often combined with other effects, such as vibrance and saturation adjustments, sharpening tools and other modifications to achieve the best results. I highly recommend  Aurora HDR's video tutorials for some awesome ideas and instruction.

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