Sunday, October 31, 2010

Ex. 3 Histogram

Whilst I understand the rationale behind trying to manage the shape of a histogram in a digital image, it is not something I tend to worry about too much during shooting, although I do take care to ensure that I have a properly exposed image, actively using the different metering modes on the camera and using an external exposure meter when unsure.  A good photo is a good photo, whether technically perfect or not.  However, a good photo that is technically perfect can become a great photo.

Having said all of this, this has been a useful exercise for the first time thinking about different subjects based upon the degree of contrast in the image.  On a bright sunny day, high contrast is provided by a mix of sky and shade, whilst low contrast can be found in any evenly coloured object in full shade.   Finding the medium contrast scene was the most difficult, what is medium contrast, how does that look?

For each of the 3 scene types I have imported the photographs into light room and then taken a screen shot of the resulting image alongside it's histogram.  In each case I present the +1 eV first followed by the cameras exposure prediction and then the -1eV shot.  In all cases I have left the highlight and shadow clipping on, highlights are red, shadows are blue.  Other than this I have not done any processing.

Starting with low contrast the bark on a tree trunk provided a good example. The properly exposed image shows a nice bell curve in the center of the brightness range, all colours being exposed pretty equally.  Over or under exposing has not lost any detail from the image.  The histogram can be seen to visibly shift up and down the brightness range.

For medium contrast I photographed a local gas station.  Even in the correctly exposed middle image there is a degree of clipping, however, it is limited and there is a distinct curve to the histogram, with most of the pixels in the mid-range of the graph.  The overexposed image shows clear clipping where the sun reflects from the steel building in the background, whilst the underexposed image shows loss of shadow detail, but definitely the best contrast in the bank building at the left.

Moving to a high contrast image I have selected a hotel that overlooks a small local park.  The hotel has a white frontstrongly relflecting the sun, whilst the trees at either side contain dark areas of shade. In this case the histogram is fairly evenly distributed across the entire brightness range with clipping at both ends of the graph. Under and over exposing shift the histogram as in the earlier studies.


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