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High dynamic range imaging (HDRI or HDR) is a set of methods used in image processing, computer graphics, and photography, to allow a greater dynamic range between the lightest and darkest areas of an image improving quality of details in an image.

HDR means ‘High Dynamic Range’ but what is Dynamic Range? Dynamic range in photography describes the ratio between the maximum and minimum measurable light intensities (white and black, respectively). In the real world, one never encounters true white or black  - only varying degrees of light source intensity and subject reflectivity. Therefore the concept of dynamic range becomes more complicated, and depends on whether you are describing a capture device (such as a camera or scanner), a display device (such as a print or computer display), or the subject itself.

In photographic terms

The Subject (Viewing with the Human Eye) - The human eye can actually perceive a greater dynamic range than is ordinarily possible with a camera. If we were to consider situations where our pupil opens and closes for varying light, our eyes can see over a range of nearly 24 f-stops.

Capture devices such as Digital cameras and scanners generally see a range of between 7 and 11 f-stops

And as for display devices your computer monitor has a range of 9 f-stops while your printer would only output a range of 7 f-stops.

So why then even bother to photograph scenes with higher Dynamic Range than your camera's or printer's  output range? The answer is that we can capture the scene's higher range and later map it to a lower dynamic range output. The key point here is that we don't lose any detail information during this process.

The process of reproducing the same dynamic range as the human eye with capture and display devices requires the capturing of a range of exposures and then blending them together using software to produce HDR images

Getting Started
In HDR photography we capture multiple different, but overlapping, exposures to cover the dynamic range of the scene. In general the exposures differ by 1-2 stops, this means the total number of needed exposures is defined by:

1) The Dynamic Range of the scene we want to capture
2) Dynamic Range the camera can capture in a single shot

Each additional exposure can add 1-2 stops (depending on your selected bracketing) option within the camera.

Creating the Image
Now we have to find out what we can do with these multiple exposures. There are quite a few methods:

1) Manual blending of different exposures (using what are essentially montage techniques) is nearly as old as photography. Even if Photoshop makes it much easier these days it can be a tedious process. We hardly ever use manual blending.

2) Tone Mapping used in most HDR software uses algorithms to map one set of colours to another to enhance the tonal range of the final output, however because it only maps colours the output in tone mapping software can sometimes appear flat.

3) Fusion Blending with Photomatix to process the bracketed images uses a complex algorithm blending the different exposures directly into the final output image, unlike Tone Mapping, Fusion blending preserves or has the ability to render a depth of field to the final output.


See our other tutorials on using HDR software and camera techniques to find out more about processing your images.

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