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which means that the latest Fuji S5 camera has in fact a total of 6 million ‘combined’ photosites, but because of the way the photosites are laid out on the sensor chip and how the data is interpreted, the effective number of megapixels is rated as 12.1 MP. Fuji have had a problem convincing everyone that this can be regarded as a genuine megapixel rating, but wedding photographers in particular have been very impressed with the dynamic range offered by this range of cameras, which is especially important when shooting white wedding dresses alongside dark suits under what can sometimes be less than ideal shooting conditions. Unfortunately some wedding photographers have been sold the idea that JPEG shooting is the best approach for weddings and in turn lost all the advantages that can be gained from high dynamic range raw capture!

Alternative approaches

Other high dynamic range sensor technologies are in the pipeline. One method relies on the ability of a sensor to quickly record a sequence of images in the time it takes to shoot a single exposure. By varying the exposure time value for each of these exposures the camera software can extract a single high dynamic range capture. The advantage of this approach is that it might be feasible to capture a high dynamic range image using a fast shutter speed, although maybe not with a high speed strobe flash unit.

Bracketed exposures

Until we have true HDR cameras, we will have to rely on using bracketed exposures instead (see Figure 7.2). The aim here is to capture a series of exposures that are far enough apart in exposure value that we can extend the combined range of exposures to encompass the entire scenic tonal range as well as extend beyond the limits of the scenic tonal range. The advantage of doing this is that by overexposing for the shadows we can capture more levels information, which can result in cleaner, noise-free shadows. Exposing beyond the upper range of the highlights can also be useful when trying to recover information in certain tricky highlight areas. Shooting bracketed exposures is the only way most of us can realistically go about capturing all of the light levels in any given scene and merge the resulting images into a single HDR file. When this is done right you have the means to create a low dynamic rendered version from the HDR master that can reproduce most if not all of the original scenic tonal range detail.

Figure 7.2 This diagram illustrates how individual bracketed exposures, when merged to form a single high dynamic range image, can extend the histogram scale to encompass the entire luminance of the subject scenic range.

Displaying deep-bit color

It is hard to appreciate the difference between 8-bit per channel and 16-bit per channel images, let alone 32-bit images, when all you have to view your work with is an 8-bit per channel display. However, display technology is rapidly improving and in the near future we may see the introduction of displays that use a combination of LEDs and LCDs to display images at greater bit depths and over a higher dynamic range. Dolby already supply such specialist high dynamic range displays and the difference is remarkable. In the future, such displays may allow us to see the images we are editing in greater tonal detail and over a much wider dynamic range.

Capturing a complete scenic tonal range

The light contrast ratio from the darkest point in a scene to the brightest will vary from subject to subject, but in nearly every case it will certainly exceed the dynamic range of even the best digital cameras. Our human vision is able to differentiate between light and dark over a contrast ratio of 1:10,000, which in photography terms is equivalent to about 14 exposure values (EV). Meanwhile, most digital cameras can only capture a tonal range of around 6-8 EV. For the most part we have to choose our exposures carefully and decide in advance whether we wish to expose for the shadows, or for the highlights, or somewhere in between. We also know from experience that we don't always need to record every