High Dynamic Range Photography

Although the human eye can perceive a brightness range equivalent to more than 17 f-stops, most digital cameras have a dynamic range less than half as wide. Consequently, while your eyes can adjust dynamically to enable you to see details in highlight and shadow areas, your camera will usually record only the middle tones in a scene. Bright highlights and deep shadows – where your eyes can see details – will be ‘clipped’ and devoid of detail.

The dynamic range of a digital camera is directly related to the bit depth of the analog to digital (A/D) converter that processes the digital signals. Most digicams have 8-bit A/D converters that restrict their dynamic range to less than eight stops (six stops of dynamic range is typical). In DSLRs, A/D converters are typically 12-bit or 14-bit, the former covering a maximum of 12 f-stops and the latter, 14 stops; so even they can’t record the full range of tones your eyes can see. The dynamic range of printing papers is even more restricted, the best covering roughly seven f-stops – but most being lower.

Consequently, the experience of seeing the original scene, then capturing it and reproducing it, involves progressive loss of dynamic range. Lost highlight and shadow details are gone for good. The aim of High Dynamic Range (HDR) photography is to capture as much as possible of the tonality present in the original scene in order to reduce the tonal range to a usable level that allows you to reproduce an image with a more accurate interpretation of the scene’s tonality and detail.

A number of solutions exist to achieve these objectives. The oldest of them was invented by Gustave Le Gray, working in the 1850s, who discovered that he could print seascapes with details in both sky and sea by using two negatives, one for the sky, and another with a longer exposure for the sea. Combined in one picture, this enabled him to cover a wider brightness range than existing technologies could encompass.

However, HDR photography has only become widely popular with widespread use of DLSR cameras and computers with adequate processing power. Today, it is commonly used for real estate and architectural photography, where it enables photographers to produce pictures of interiors containing a natural-looking balance between window and room areas. It is also useful for landscape photography where a wide brightness range is involved.

Shooting HDR Pictures
Your aim when shooting a set of images that will be combined to make a single HDR picture should be to cover the full brightness range in your subject, capturing detail all the way from the brightest highlights to the deepest shadows. You can generally achieve this with three shots, although some subjects may require up to seven shots to cover their entire brightness range. For our examples, we will use three shots.

The first shot is normally exposed to record the mid tones.

The second shot is underexposed to record highlight details.

The third shot is overexposed to record shadow details.

The order in which the shots are taken is irrelevant; what matters is to capture details across the entire subject brightness range. It doesn’t matter whether you shoot raw files or JPEGs, although the latter will be simpler as no post-capture processing is required before you combine them. Even though other file formats won’t necessarily span any more of the subject’s dynamic range, they will give you more data to work with and may contain more highlight and shadow information.

Set the camera’s ISO to 100 (or lower) to minimise the effects of noise. Set the white balance to whatever the scene requires; don’t use auto in situations where the light is variable. Use autofocusing to obtain a sharp image but disable it before taking your shots. If autofocus is left enabled, a different focal point may be selected between one frame and the next. This can create image combination problems.

Step 1: Mount the camera on a tripod before taking your shots. Because the HDR software needs to map the luminance values at each corresponding pixel from the series of input frames, any movement of the camera between shots will cause alignment problems when they are combined. Set the shooting mode to aperture priority (A or Av) and the lens aperture to between f8 and f16 to provide adequate depth of field.

Step 2: Using your camera’s AE bracketing function, set the bracketing to at least 2EV over and under the metered exposure. If your camera doesn’t offer bracketing across this range, your objective can be achieved with the camera’s exposure compensation control, or by setting the camera to manual and adjusting shutter speeds to provide the desired degree of over- and under-exposure. (For subjects with an extreme brightness range, you may need bracketing by as much as 4EV.) Do not change the aperture setting. If shutter speeds are slow, use the mirror lock-up function on the camera (if it has one).

Step 3: Trigger the camera with the self-timer and make sure three images are recorded.

HDR Processing in Photoshop
HDR processing was introduced in Photoshop CS2. It works by merging a set of images using floating point 32-bit/channel algorithms, which result in a single, 32-bit image file with a huge dynamic range (typically 10 f-stops or higher). The process is straightforward: you simply upload the images to the Photoshop desktop and select File > Automate > Merge to HDR.

This calls up the screen (above), which lets you select the images for merging. If you click the Add Open Files button, the images on the desktop will be selected.

Clicking on OK initiates the merge, which can take several minutes. The merged image is then displayed, along with thumbnails of the three source images plus an adjustable histogram.

Adjust the histogram until you can see detail in the highlight areas. (Failure to do this can lead to blown highlights in the final image.) Don’t worry about shadows at this point; they can be dealt with in the next step.

While the merge provides a usefully large amount of image data for you to work with, your monitor display and printer are 8-bit devices (although high-end printers can handle 16-bit files). Consequently, the image must be compressed from 32-bit to 16-bit for printing (or 8-bit for images that will be displayed on a computer screen). Select Image > Mode > 16 Bits/Channel.

This opens the following dialog box with four compression settings for reducing the dynamic range to something you can use: Exposure and Gamma, Highlight Compression, Equalise Histogram and Local Adaptation. Click on the Toning Curve and Histogram button to display the full suite of adjustments.

Exposure and Gamma (the default) lets you adjust image brightness and contrast manually.

Highlight Compression automatically compresses the highlights so they fall within the luminance range of the 16-bit image file.

Equalise Histogram is a fully automatic control that compresses the dynamic range of the image with minimal reduction in contrast. No fine-tuning is available.

Local Adaptation is the most flexible option as it lets you adjust image tones selectively by using the toning curve. It’s similar to using the Curves function for image adjustment. This is the control we will use.

The default setting for the Local Adaptation control makes the screen image look rather flat. Set up some anchor points along the curve then adjust them to bring out detail in the highlights (the right hand side of the histogram) and shadows (the left side). When you’re happy with your fine-tuning, click the OK button.

Now select Image > Adjustment > Levels and use the Levels control to restore contrast to the image and, if necessary, fine-tune brightness levels.

The end result (bottom shot below).

(Bottom shot) The HDR image; and (top) a correctly-exposed shot of the subject contains blown highlights and blocked-up shadows, which are corrected by HDR shooting and processing.

Footnotes:
Photoshop is by no means the only application that can be used for processing HDR images. We’ve used it here because it’s popular with most photographers. Unfortunately, Photoshop Elements doesn’t support HDR processing, although you can achieve a similar result (albeit much less easily) with Layer masking. Take two shots, one exposed for highlights and the other for shadows. Superimpose them and mask or erase the shadow areas of the highlight layer to reveal the correctly-exposed shadow layer below.

Other popular HDR applications include Photomatix (www.hdrsoft.com) and the freeware EasyHDR (www.easyhdr.com/download.php). PaintShop Pro also contains an HDR Photomerge function and Media Chance produces Redynamix (www.mediachance.com/plugins/redynamix.html) , a plug-in for Photoshop Elements, which creates ‘HDR Like Images’ from a single original.

Regardless of which software you use, HDR processing requires a light touch. Over-processing will result in excessive compression, creating a surreal low-dynamic-range rendering of a high-dynamic-range scene.