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High Dynamic Range (HDR) explained - The concept of high dynamic range processing has been around since 1997, but has started to increase pace in the digital photography world. Peter Bargh explains what it's all about and how you use it.
The concept of high dynamic range processing has been around since 1997, but has started to increase pace in the digital photography world. Peter Bargh explains what it's all about and how you use it.
High dynamic range (HDR) processing is a technique achieved using software that takes the best tones from several exposures and combines them in one HDR image.
The dynamic range is the range of brightness levels in a recorded scene from the darkest shadows to the brightest highlights. This is typically measured in f/stops and has always been a problem for film users shooting on colour transparency film, especially on films such as Fuji Velvia. The dynamic range is around six f/stops from the brightest to darkest points and while colour negative and black & white film users have always had a few stops to play with, even at a maximum of around nine stops most photographic systems struggle on high contrasts scenes.
With all the modern technology you'd be wrong to think a digital camera would resolve these issues. The CCD sensor can, like transparency film, actually only record about six stops of dynamic range, making it pretty poor at recording a scene with strong highlights and deep shadows.
The reason for this is simple, current film and image sensors are developed to record enough light that can be reproduced on paper or computer screen and neither are currently capable of displaying a dynamic range that we can see with our eyes. Until recently, to overcome this, digital photographers have either been using the old approach of ND graduated filters at the taking stage or combining the best bits of two different exposures on the computer, much like the dodge & burn technique used by experienced darkroom users.
New technology is on the way to record and display at higher dynamic ranges, but for now there's a software solution.
The job of HDR processing is to allow photographers to produce images with a much larger dynamic range. The photographer takes a series of different exposures of the same scene and then uses HDR software to automatically merge the resulting images into one image, referred to as exposure blending. This produces a file with the entire desired dynamic range, known as a radiance map.
How to take the photos
It's recommended that you take exposures one to two stops apart with enough shots to cover the complete dynamic range. This usually means you need to take between three and seven photos. This is a drain on the memory card space, but you can create images featuring well over 10 stops exposure range. Ideally the fewer image used the better, because it's less memory intensive and there's less chance of subject been out of alignment on each shot as a result of camera or subject movement. To find out how many you'll need to cover the exposure range take one meter reading from the shadow area and one from the highlight and fill in the gaps
Let's say that the shadow area is 1sec at f/5.6 and the highlight is 1/250sec at f/5.6. You could take nine 1 EV exposures at 1sec, 1/2sec, 1/4sec, 1/8sec, 1/15sec, 1/30sec, 1/60sec, 1/125sec and 1/250sec or five 1 2EV exposures at 1sec, 1/4sec, 1/15sec, 1/60sec and 1/250sec.
There are various ways to make the exposures
One RAW file
The quick route is to shoot one RAW file and process it using the RAW converter to create a series of different exposures. This allows you to shoot subjects that are moving, but the quality using this method is not as good as shooting different exposures in the options below. One of the reasons is because the noise is more apparent in the shadow areas when you increase the exposure for those darker shadow areas. If the program you are going to use to create your HDR uses EXIF data you'll need to remove this first otherwise any changes you make in the RAW processing will be ignored when the program reads the original exposure from the EXIF data.
It's better to have a series of exposures and these can be done using your camera's auto-exposure bracketing mode or exposure compensation if either option is available, or by using manual exposure mode. You should only adjust the shutter speed so make sure the auto-exposure mode doesn't vary the aperture. If the aperture changes, the depth-of-field of each shot will also change, making it almost impossible to get a sharp HDR. You'll need the camera on a tripod which must be rock solid to avoid any movement from one shot to another. It's also a good idea to lock focus and switch to manual exposure to ensure everything is consistent between exposures.
When shooting in RAW mode you have to convert the files first. Make sure the RAW converter settings are the same for each file and save as 16-bit TIFFs or PSDs. You'll need a fair amount of free RAM and, as the images are processed using 32-bit, our computer displays and inkjet printers display 8-bit, so it's impossible to see the full benefits.
The merged photographs are saved as 32-bit. This means there's very little you can do to edit the photos, they cannot be saved as JPG without reducing the bit-depth and you cannot view the superb dynamic range on a conventional monitor. One way around this is to apply tone mapping which converts the HDR image into a balanced exposure. Tone mapping looks wonderful when done well, but over steer on the sliders and you'll create a cartoon-like result which can, with the right subject, look good, but often looks like a badly processed digital filter effect.
What's software is available
There are various software packages around to help you create HDR photos. These include the following:
Adobe HDR - supplied as standard with Photoshop CS2 upwards. You can find this option in the menu File>Automate>Merge to HDR.
HDR Shop - an interactive graphical user interface image processing and manipulation system designed to view and manipulate High-Dynamic Range images. Two versions are available v1 is free, v2 has a single use license of $400.
Photomatrix Pro - a stand-alone program that runs on Mac OS X and Windows 98/Me/2000/XP. The Tone Mapping tool is also available separately as a plugin compatible with Photoshop CS2. More details here HDRsoft
Photogenics HDR - from Idruna Software the first 32-bit per channel High Dynamic Range Paint Package.
FDRTools a collection of tools supplied as a free Basic version and an Advanced version which has a trial option.
We will create several technique guides showing how to use some of these programs in the next few months.