Friday, 20 July 2012

Exercise: Highlight Clipping

The purpose of this exercise is to look at the effect of overexposing an area of the photograph which results in highlight clipping. This is a digital problem – ‘when the highlights go, they go suddenly, as if they’ve been cut or clipped’ (Freeman 2009, p34) and thus it’s important to understand why it happens, in what situation it might occur, and how to avoid it (both whilst taking the photo and if that’s not possible, then in post-processing afterwards).
We are advised in the notes to pay particular attention to the following issues relating to highlight clipping:
  • Complete loss of visual information
  • Visible break in the form of an edge between nearly-white and total white
  • A colour cast in a fringe along the clipped white highlight
  • The colour saturation

The Exercise:
A scene with a wide range of brightness was chosen. The image below shows the scene photographed with the highlights just below clipping point.
Highlights just clipped (on-camera)

I had my camera set on manual exposure, so then increased the exposure by one stop (changing shutter speed) for the next photo, and then took 3 photos with progressively lower exposure, resulting in a total of five images, as shown below. In addition, I took photos 1/2 stop over and under the mid-point image. I have chosen an outdoor situation and used a tripod in order to keep the framing constant for this exercise.
Comparison between images above and below the 'Highlights just clipped' image

When zooming in, the shot that is one stop over shows some areas that look ‘blown-out’, and the shot with highlights just clipped shows some bright areas that may show some clipping problems also. The three decrease in exposure images don’t’ appear to have problems but are generally very dark (and in fact look like they may show clipping at the other end of the spectrum.

I have then analysed the photos in Lightroom, the screenshot of 1 stop over shows the highlight clipping turned on (this is the only image with any highlight clipping according to Lightroom).
1 stop over showing clipping of highlights in Red (Lightroom)
Zooming in on this area, with and without highlight clipping displayed.
1 stop over showing clipping of highlights in Red (Lightroom)

1 stop over (Lightroom)

I would have to say that there is a complete loss of visual information in the centre of this screenshot, though perhaps not to the detriment of the whole image. There is no visible break clearly seen, nor a colour cast along a fringe observed. The screenshot below shows the same area covered when the photo is taken at 2 stops over.
2 stops over (Lightroom)
In this image, the histogram (top right) shows clear clipping of highlights, and there is considerable loss of visual information in the overexposed areas. I can't clearly identify the other problems, though this is not to say they are not present! 

I then opened the 1 stop over image in Photoshop RAW Converter and used the recovery slider to 50%. This has brought back some of the lost detail in the overblown areas, as seen in the two screenshots below. The image on the left has had recovery applied.
Recovery applied to left image (1 stop over)
Recovery applied to left image (1 stop over) zoom into highlight area
Freeman, M. (2009), Perfect Exposure. East Sussex: Ilex

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