High dynamic range (HDR) imaging

From Wikipedia:

In image processingcomputer graphics, and photographyhigh dynamic range imaging (HDRI or just HDR) is a set of techniques that allows a greater dynamic range between the lightest and darkest areas of an image than current standard digital imaging techniques or photographic methods. This wide dynamic range allows HDR images to represent more accurately the range of intensity levels found in real scenes, ranging from direct sunlight to faint starlight, and is often captured by way of a plurality of differently exposed pictures of the same subject matter.

In simpler terms, HDR is a range of techniques geared toward representing more contrast in pictures. Non-HDR cameras take pictures at a single exposure level with a limited contrast range. This results in the loss of detail in bright or dark areas of a picture, depending on whether the camera had a low or high exposure setting. HDR compensates for this loss of detail by taking multiple pictures at different exposure levels and intelligently stitching them together so that we eventually arrive at a picture that is representative in both dark and bright areas.

The two main sources of HDR imagery are computer renderings and merging of multiple low-dynamic-range (LDR) or standard-dynamic-range (SDR) photographs. Tone-mapping techniques, which reduce overall contrast to facilitate display of HDR images on devices with lower dynamic range, can be applied to produce images with preserved or exaggerated local contrast for artistic effect.

example of HDR image

example of HDR image including images that were used for its creation. Photo: Igor Iric

In photography, dynamic range is measured in EV differences (known as stops) between the brightest and darkest parts of the image that show detail. An increase of one EV or one stop is a doubling of the amount of light.

High-dynamic-range photographs are generally achieved by capturing multiple standard photographs, often using exposure bracketing, and then merging them into an HDR image. Digital photographs are often encoded in a camera’s raw image format, because 8 bit JPEG encoding doesn’t offer enough values to allow fine transitions (and also introduces undesirable effects due to the lossy compression).

 

When creating an HDR image from multiple you need:

  1. A digital camera that allow you to set the exposure settings manually. A digital SLR will be perfect, but  some compact camera (see listing below) have also got manual settings.
  2. A tripod or some other sturdy support for your camera.
  3. Software that can merge the image files. (Photomatix, Photoshop etc)
List of compact cameras with
manual exposure settings:
Canon PowerShot SX240 HS
Canon PowerShot SX260 HS
Canon PowerShot G1 X
Fujifilm FinePix S4500
Fujifilm FinePix F660EXR
Kodak EasyShare Z5120
Leica V-Lux 3
Nikon Coolpix P310
Nikon Coolpix P510
Panasonic Lumix DMC-ZS20/TZ30

When using manual settings you can adjust several things, including aperture and shutter speed, which together makes the exposure value (EV). The exposure is the total amount of light allowed to reach the image sensor during the process of taking a photograph.

A low aperture value such as f/2.8 is equivalent to big opening inside the lens(1). More light will pass through it. When using a high aperture value such as f/16, the opening inside the lens is small(2).

aperture

Photo: Mohylek

When creating an HDR image, it is common to set a fixed aperture and then vary the shutter speed for at least three shots.

But first, find the exposure settings that gives the best result (remember: use manual settings). Shoot a test photo and study the image histogram.

Try to get shoot an image that have a bell shaped histogram, or at least no peaks at the ends. In other words; a picture with no completely black or completely white areas.

Let’s say that the settings are f/8 (aperture) and 1/250 seconds (shutter speed). Then, with these values as a starting point, shoot three photos:

1. Same aperture. Set the shutter speed to 1/60 seconds
2. Same settings as test image; shutter speed 1/250 seconds
3. Same aperture. Set the shutter speed to 1/1000 seconds

Picture number one will be a bit dark, but it will have more details in the lighter part. The third picture will be bright, but with more details in the darker parts.

If you have a digital SLR (DSLR), you can use the Auto Exposure Bracketing feature together. Then you can set the number of +/- stops very easily.

After you transfer the image files to your computer, you can combine them to an HDR image using Photomatix or Photoshop.

This brief blog post was not meant to cover everything. For more information about HDR imaging, check out these resources:

Cambridge in Colour – High Dynamic Photography

Stuck in Customs – HDR Tutorial

Thinsite tutorials

Beforethecoffee.com – Post processing

Photoshop tutorial – HDR using layer masks

Some HDR images from Globitude:

Globitude photo album

Vegard's HDR photos

One last tip: If your intention is to create a image that looks real, be careful when you do the tonemapping. If you pull the sliders too far, you can end up with a scene that looks like a painting or a cartoon drawing.

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