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High Dynamic Range (HDR)

 

High Dynamic Range (HDR) Photography

 

Dynamic Range (DR) is the range of luminance (brightness) values of a scene from the darkest to the brightest. In a real world situation the DR is that which can be perceived by the eye or captured by the imaging sensor of a digital camera and these two are by no means the same. A typical sunlit landscape is likely to have a DR of 100,000:1 or ~17 stops, but the human eye can detect a DR of only 10,000:1 or ~14 stops and a digital camera a DR of only 400:1 or ~8.5 stops. It is clear therefore that it may not be possible for the scene as viewed by eye to be captured and reproduced for others to see in the same way as the original viewer saw it due to a progressive loss of Dynamic Range.

We all know that any exposure made by a camera, digital or film, is a compromise – an average; multi-matrix metering is a great asset in determining where the optimum average exposure lies in terms of the brightness range of the scene, but even so some highlights will be burnt out and some shadows blocked up even when shooting RAW. The limitation is the DR of the camera sensor.

High Dynamic Range (HDR) photography is a means of capturing the full range of brightness in a scene with high perceptual accuracy and faithfulness. In simple terms a series of images of the scene captured at varying exposures to faithfully record the highlight and shadow detail are merged together in HDR software to produce a single HDR image file which, after processing, produces a final image close to that seen by the naked eye. HDR software is included in CS2, CS3 and CS4 and there are also third party programs, e.g. Photomatix Pro available. HDR photography comes into it’s own in capturing scenes which have a wide tonal range such as building interiors, especially if a window is included, or a landscape and other outside shots.

The basic technique in capturing the image files to be processed using HDR is straightforward; with the camera mounted on a tripod either focus the scene manually or if autofocus is used set the lens to manual focus once focus has been achieved and set the exposure mode to Aperture Priority. Both of these settings are important as the focus and aperture setting – which controls the depth of field, must remain constant throughout the series of bracketed exposures. Set the White Balance to Auto or to a preset, but do not alter it during the sequence of exposures. Take a minimum of 3 bracketed exposures at -2, 0, +2 stops in quick succession either by using the Autobracket function or by varying the shutter speed manually. Examine the histogram of the over and underexposed images on the back of the camera and check that the shadows are not blocked up or the highlights burnt out; if they are repeat the series of exposures extending the exposure range, i.e. -4, -2, 0, +2, +4 stops and check again and repeat the sequence yet again if necessary using a wider exposure range. For most situations -2, 0, +2 stops will be adequate and stepping the exposures in 1 stop increments may be more appropriate for some situations.

Once the files have been downloaded to the computer it is not advisable to make any corrections to them whether they be jpeg or RAW and to process them as they are. Processing is in two stages – firstly the images are merged to a single 32 bit HDR file (this will look awful on the monitor as the graphics do not have the ability to resolve the high dynamic range in the image) which is then tone mapped to a 16 bit image in a further process.

Now for the processing procedure – open CS3 (similar in CS2 & CS4) and go ….

File > Automate > Merge to HDR – the Merge to HDR dialogue box opens. Use the Browse button to navigate to the folder in which you saved the series of exposures and highlight them and click on OK. Ensure that the ‘Attempt to Automatically  Align Source Images’ check box is checked and click on OK, Fig1.

Fig 1 Merge to HDR

Photoshop will now process the images which can be quite a lengthy process depending on the speed of the computer. Eventually a preview of the HDR image will appear on the screen with thumbnails of the component images down the left hand side. By checking or unchecking the tick box by each image the effect of that image on the overall HDR image can be determined, Fig 2.

Fig 2 HDR Preview

Click on OK and the process of producing the 32 bit HDR image file is completed. At this stage it does not look very promising! However the image must now be tone mapped which is achieved by going to Image > Mode where the image is flagged as 32 bit and selecting 16 or 8 bit (this example continues in 8 bit) which brings up the HDR Conversion dialogue box, Fig 3.

Fig 3 HDR Conversion dialogue

From the Method dropdown list select Local Adaptation and click on the Toning Curve and Histogram arrow to reveal these items.

Use the curve (Fig 4) in conjunction with the Radius and Threshold sliders to adjust the image and click on OK and the HDR image is processed – again this will take time! Save the file as a PSD and adjust further in Photoshop using the normal techniques.

Fig 4 Curve Adjustment

 

               

Fig 5 Average Exposure (as metered) & HDR Image (both unmodified in PS)

The difference between the straight shot and the HDR image processed from a series of bracketed shots is clear, although further processing of the latter in Photoshop is clearly needed, but all the detail is there!

 

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