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Technical

November 2008

The brief you've received is to photograph a stylish restaurant interior with panoramic views of the city from the window. By day or by night such a commission is a challenge. Not only do we need depth of field but also, on a bright day, the dynamic range of the image exceeds 14 stops, making a single capture impossible, and add internal lighting into the mix and you have the added complication of reflections. So, how should you tackle such an assignment?

Combining images in post-production with heavy retouching is one way to achieve a result for such a commission but, with limited budget, High Dynamic Range (HDR) capture can provide a good, creative solution for a fraction of the cost and time.

The limitations of extreme contrast have always been with us. The human eye can perceive around 18 stops without adaptation or movement of the pupil, allowing us to enjoy scenes with huge contrast. The camera has always been far more limited. A good negative film, hand-printed, could render around 16 stops of dynamic range and the best digital sensors can now work to around 11 stops.

Photographers are a creative bunch and have always been able to find ways to get around these limitations - it is perfectly possible to process a RAW file several times to maximise the information in highlights and shadows and then manually combine the result. But it’s better to utilise specialist software that’s dedicated to High Dynamic Range photography.

© Nick Wilcox-Brown

A clear example of the impact of HDR photography. The image on the left is a normal exposure of Clifton Suspension Bridge, Bristol; the image on the right is an HDR file made up from a single straight RAW file of the scene put through HDR software, bringing out tone-mapping.

The concept of HDR is straightforward: capture a sequence of identically composed images (ideally on a stable tripod) at different exposures and then combine them in specialist software. The aim is to give detail in both shadow and highlight regions by compressing the tonal values of the image to allow reproduction on normal monitors and in print. Used correctly HDR can give some beautiful looking results that enhance a photograph. Used to extremes HDR can result in unfamiliar images that look wrong to the eye and challenge the sense of reality.

Getting started

Capturing HDR requires no sophisticated equipment, but it is important to follow certain basic rules:

  • A tripod is not essential, but it makes the whole process so much simpler.
  • Ensure that Auto White Balance (AWB) is turned off (it should be anyway), as it leads to colour variations between frames.
  • Changing aperture affects depth of field and will prevent the images aligning. All exposures need to use a fixed aperture and should be controlled by changing the shutter speed. Ideally Manual mode is most effective or, alternatively, use Aperture priority (Av) mode.
© Nick Wilcox-Brown

Three exposures were combined to produce the result shown here. The longest exposure was 1/120sec at f/8.

The EOS-1 series SLR cameras allow for up to nine frames of autobracketing, so in conjunction with exposure compensation there is potential to automate the capture, but much of the pleasure can be lost by such automation.

For long exposures of certain scenes Canon’s TC-80N3 remote release and timer works well in conjunction with mirror lock, particularly when your exposures start to go into minutes.

Perfect exposure of highlights

For the best results, an initial exposure needs to be determined that yields perfect exposure of the highlights without any clipping, even of specular points. The flashing highlight warning on Image Playback on your EOS camera’s LCD panel is very useful for checking this.

Once the initial exposure has been determined, you need to capture a sequence of images that covers the whole dynamic range of the subject. Increasing the shutter speed for each exposure by a consistent amount will achieve this. This would normally be a stop at a time but, after some experimentation, I have found that a two-stop increment works well, with the added advantage that fewer frames are needed to cover the dynamic range of a scene.

A single RAW file can, at a push, be used as the source for an HDR image. More normally between three and six images are used, but there is no fixed number. How many images are needed is determined by the contrast range of the scene to be recorded - the higher the contrast range in the scene, the greater the number of images required.

The key here is to expose correctly for highlights and then increase exposure, two stops at a time, to ensure that the darkest shadow areas are exposed as mid-tones or brighter (think mid-grey or Zone V to VI in Ansel Adams terms). This will ensure the shadows are noise free and will render out nicely when the image is tone-mapped later on in the process.

HDR software options

© Nick Wilcox-Brown

An HDR image of city lights at night, shot from the roof of a hotel. The HDR shot is comprised of five source images, shot two stops apart on a tripod with the help of a TC-80N3 remote release. No attempt has been made to eliminate ghosts or light trails from the resulting file. The normal file on the left is the middle frame from the sequence, shot at f/8 for four seconds, and the resultant composite HDR file is shown on the right.

HDR images work most reliably when captured on a tripod. Most HDR software has the capability to re-align images slightly by checking for vertical/ horizontal movement or aligning common features. With luck and a steady hand it is possible to capture images hand-held, but don’t count on it.

In earlier generations of software, the presence of ‘ghosts’ or moving objects, only visible in one of the images in the sequence, was a major cause of retouching. Recent software versions have improved matters, giving options to remove or determine a specific image as the source for features like people that will appear in the final rendered file.

Once images have been captured they need to be merged into a single HDR image. Photoshop CS2, CS3 and CS4 Extended all have the capability to create and work with HDR images, but how to use them is far from obvious. Once explained, the results can be good, but control functionality is more limited than some other dedicated products. Most people that have spent any time working with HDR images tend to migrate towards using HDRsoft’s excellent Photomatix or Photomatix Pro – both are all-round applications that are intuitive and flexible.

There are other options. Another application I have used successfully is Andreas Schömann’s FDRTools, which offers some different options to Photomatix Pro. Key amongst these is its early support for ‘xDOF’, or the combination of multiple images for extended depth of field.

Whatever your choice of software the basic steps for HDR are similar:


  • Images are selected. These can be RAW, TIFF or JPEG. Because the images will be combined, there is no great advantage to having 16bit images or RAW shots. Whichever format is chosen, no sharpening should be applied to source images.
  • The source images are combined to form a 32bit (or higher) HDR image. The image is then saved before tone-mapping to generate the desired result and those settings are then saved.
  • Finally, an 8bit or 16bit output is made to a Low Dynamic Range Image (LDR) format such as TIFF.

Virtually all resulting LDR images will need some form of post-production to clean up artifacts and apply contrast, sharpening and colour corrections. This must be considered as a part of the process and not simply bad photography.

Photoshop CS4

Most photographers have a copy of Photoshop and it is a good way to generate your first HDR image. New with the Photoshop CS4 package is better colour management for HDR images and a substantial increase in performance if you have a supported graphics card of over 128Mb.

© Nick Wilcox-Brown

The Merge to HDR option in Photoshop CS4.

© Nick Wilcox-Brown

Option to select Local Adaptation in CS4.

Images can be selected in Bridge and then choose Tools > Photoshop > Merge to HDR. From Photoshop, go to Automate > Merge to HDR. Alternatively, you can use Lightroom 2 and in the Library or Develop module choose Photo > Edit In > Merge to HDR in Photoshop.

© Nick Wilcox-Brown

Once the image has been rendered out to 16bit, it can be manipulated as normal in Photoshop CS4.

Once images have been merged and aligned automatically a 32bit composite image is generated. This is as far as most people get with Photoshop. Save the image, but then how do you proceed? With Photoshop CS3 and CS4 Extended a wide range of painting tools can be used on the 32bit image – these include the clone and spot removal tools, but not the curves or burn and dodge tool.

To do further work on the image, it is necessary to go Image > Mode > 16bit. This will bring up the dialogue box that's shown below.

Choose method: Local Adaptation - this is the only way to get a good result! Select the Toning Curve and Histogram drop down at the bottom of the dialogue and adjust to suit your image. Be sure to experiment with the Radius and Threshold sliders.

Photoshop CS4 has a handy script lurking in Bridge CS4: Stacks > Auto-Stack Panorama/HDR. By default the time interval between images for stacking is 18 seconds - image content is also examined for matching features at pixel level. To change the default time, it is necessary to go into the JavaScript with the Adobe ExtendScript Toolkit. If you need to do this, check Adobe forums for information.

© Nick Wilcox-Brown

Toggling the preview in CS4 for conversion to 16bit with Local Adaptation gives a striking before and after view of the image.

Photomatix Pro

Photomatix Pro has a straightforward interface and very capable controls that allow for detailed setting of tonal range compression, micro contrast and a host of other fine-tuning controls to optimise the image appearance for screen or print. It also offers a choice between two different tone-mapping methods - Details Enhancer or Tone Compressor.

© Nick Wilcox-Brown

Options for merging images to HDR include the ability to align the images and reduce ghosting from moving objects or people that appear in only one frame of a set of images

© Nick Wilcox-Brown

Selecting source image for HDR creation

Details Enhancer increases local contrast and boosts shadows, but needs to be used carefully as it can create haloing and noise artifacts.

The Tone Compressor method is intended to give a more photographic look to images.

Like most HDR software, source images are imported with the useful option of alignment by vertical/horizontal correction or feature matching.

It may be a surprise to some, that even with a stable tripod, images can still end up out of alignment by one or two pixels.

© Nick Wilcox-Brown

By using the Details Enhancer method of tone compression to extreme, images can look surreal and non-photographic.

© Nick Wilcox-Brown

Using the Tone Compressor method renders an image more photographically but allows foreground detail to be revealed.

The initial view of any HDR image tends to be a disappointment; like a RAW file, the final result is dependent on the processing, or in the case of HDR, the tone-mapping method chosen.

© Nick Wilcox-Brown

Slightly surreal looking; a sunset captured from a hotel window. The four images were captured at two stop intervals. This image is a good test of software as there is potential for trouble from a number of sources, with moving water and trees.

The beach sunset images shown here give you an idea of how differently an image can be used, depending on the tone-mapping method used.

Once the image has been tone-mapped to suit the required use, processing generates an output file in 16bits that can be saved as required.

Photomatix is good on colour management and, usually, little major colour correction is needed after processing. However, the process of blending images frequently has a softening effect, making careful sharpening of the final image a necessity.

FDRTools

© Nick Wilcox-Brown

Crowds of people meant that using a tripod was not an option. This single, hand-held image, was shot on a bright day, without any flash and then tone-mapped from the RAW in FDRTools. Close examination reveals plenty of shadow noise.

For many photographers, Photomatix Pro is an excellent application that will cover all of their needs. There are always some, though, that want to dig deeper. Because HDR was originally developed for computer graphic applications and later, the movie industry, there are very scientific command line tools available, such as PFS Tools, with a range of tone-mapping operators not suited to photographic images. Photographers who want to experiment will find FDRTools to be a good package that offers an excellent balance of control and usability.

The main advantage of FDRTools is that its Compressor tone-mapping control (only available in the Advanced version) is far more powerful than that in Photomatix and the application also allows control of the way that the source images are utilised for the creation of the HDR image. Additionally, the software also adds early support for combining xDOF images; sequences of images that have been focused at different distances to be combined to give wider depth of field.

Although not quite as straightforward to use, the manual and quick start are excellent and it only takes a few minutes to get up to speed.

Like Photomatix Pro, FDRTools allows for the tone-mapping of a single image. The image shown above of the arch at Granada’s Alhambra Palace had a dynamic range of 14.7 stops, yet the result is still acceptable with a little post-production.

© Nick Wilcox-Brown

FDRTools allows detailed setting of alignment and tone-mapping of source images.

To work with a group of images, the files are selected, imported and aligned like the other applications and an automatic tone-map is created. It is then a matter of working through the main image tabs from left to right (Open, Alignment, HDRI, Tone Mapping and Save), making the software logical to use.

By selecting alignment, it is possible to manually adjust the alignment of source images. Next, selecting HDRI creation allows you to choose the way that source image tones are mapped into the HDR file.

© Nick Wilcox-Brown

FDRTools compressor tone-mapping mode gives powerful control and the ability to use a familiar-looking curve tool for mapping tones.

Within the HDRI creation there are a further four tabs giving you control over the HDR creation process. The main ones of interest are Creative, which allows for the blending of non-matched images, and the experimental xDOF that allows differently focused images to be mapped together.

Once the HDR image as been created, tone-mapping is carried out using one of three sub tabs. The only one of real interest is the powerful Compressor.

Once options have been set, saving the image will render it out and open the LDR image in the application specified in the dialogue.

High Dynamic Range imaging is a complex subject. This article has been designed as a taster, yet it barely scratches the surface. There is a great deal of information on the web and a few excellent books. Additionally there is a lot of software for Mac, PC and Linux, some of it Open Source and shareware. Why not click through on the links provided in the right hand column with this article to find out more, or do some of your own research on HDR as it can provide some stunning image results with just a little bit of thought and creativity.


Recommended HDR reading
The HDRI Handbook: High Dynamic Range Imaging for Photographers and CG Artists by Christian Bloch
Mastering High Dynamic Range Photography by Michael Freeman
Practical HDRI: High Dynamic Range Imaging for Photographers by Jack Howard