Project: Highlight Clipping

With the sensor output using just a limited number of bits to represent the intensity of light falling on it, it is inevitable that there comes a point when the light is too great to be properly represented. Film and our eyes have a natural roll-off in their response so that the progress to extreme brightness is gradual and not easily noticed. Digital sensors have a linear response. This makes the transition abrupt. The purpose of this exercise is to investigate this effect by taking a series of pictures of the same scene at different exposures and comparing the area where highlight clipping occurred.

My first attempt at the series used a scene with too much detail and possibly not enough contrast to show the effect successfully so I chose a black dog against a white background.

This is the first image. This was taken with no exposure compensation using evaluative metering:

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This shows where the highlights are clipped:

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I took the next shot with +1 stop exposure compensation:

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And here it is with highlight clipping shown:

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The sudden transition is not obvious so I tried another one with +2 stops exposure compensation:

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And showing the clipped areas, firstly with Canon’s Digital Photo Professional:

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Then with Adobe Camera Raw:

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I’m not sure why there are differences. It might be due to the software interpreting the digital data differently, or perhaps ACR is more sensitive to clipping. Whatever the reason, it shows that you should not rely on the software but use your own judgement; it’s not whether highlights are clipped or what is clipped, it’s the effect on the overall image that matters. The software and its functions are only there to provide more information.

This version has much more dramatically clipped highlights. Here is a crop of the area to the left of the dog’s shoulder:

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I have adjusted the curve to darken the midtones which shows the transition a little more clearly. Particularly noticeable is the loss of detail in the clipped area.

ACR has the recovery tool mentioned in the course notes. This is graduated on scale to 100 and the first 31 points on this scale had a noticeable effect:

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Here it is without the clipping warning:

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I’m not sure what I was meant to do with the other images the exercise asked for, the ones that were underexposed. These showed no highlight clipping so I have not used them in this analysis.

So what have I learnt from this?

· Highlight clipping occurs when the intensity of light exceeds the sensor’s ability to record and process it.

· The sensor’s linear response results in a sharp drop-off as opposed to e gradual roll-off we experience in our own vision.

· This sharp drop off can result in a particularly noticeable transition.

· The clipped area is characterised by a loss of detail.

· The recovery tool in ACR can result in a reduction of the degree of clipping.

· A further understanding of how the camera sensor reacts to light and what processing can be done to the resulting image.

· Different software may show highlight clipping differently so the results should be judged on the effect on the image.

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