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Histograms and Safari Photography - Part I

Histograms and Safari Photography - Part I

Many beginner photographers are intimidated by the bar chart(s) that appears when they accidentally press too many buttons on their digital cameras. But, as daunting as the histogram may appear (and it is, after all, a concept borrowed from statistics), it is a fairly simple tool that is easy to understand and use.

A digital camera's histogram is a visual representation of the brightness values of all the pixels in any given photograph. The horizontal axis represents 256 brightness values from pure black on the left to pure white on the right. The height of the histogram (the vertical axis) represents the number of pixels at each brightness value. Specific numeric values on the horizontal axis are unimportant, as is the shape and height of the histogram on the vertical axis, all that one really needs to pay attention to is the two sides of the chart.

Point 1 - there is no such thing as a perfectly shaped histogram for all photos. Each photo will have an "ideal" histogram that is unique to the tonality values for that particular scene. So don't get hung up striving for a perfect bell curve, or be concerned if there are lots of spikes and gaps in your histogram.




Image 1 - "Normal" histogram showing no "clipping"

Image 2 - Another "normal" histogram with no "clipping"


 Point 2 - avoid "clipping". The most useful application of a histogram is to ensure that parts of the image are not under- or over-exposed. A histogram that bumps against the left side of the graph (Image 3) indicates that portions of the image are severely under-exposed, a condition known as "blocked shadows"; a histogram that touches the right edge (Image 4) indicates that parts of the photo over over-exposed ("blown out highlights"). In both instances, the portions of the image that are either "blocked" or "blown out" will show no detail, but will appear as nondescript areas of pure black or pure white on the photo. This, as a rule, is undesirable.




Image 3 - A histogram for an underexposed image showing blocked shadows

Image 4 - A histogram for an overexposed image showing blown out highlights


 Point 3 - the height of the histogram (vertical axis) is unimportant. As long as they don't occur at either the left or right extreme of the histogram, don't be concerned about spikes in the histogram, even if they reach (or surpass) the top of the vertical axis - e.g. the histogram in Image 2 is perfectly acceptable. This is not "clipping", it just indicates that a high number of the pixels share a brightness value, and is of no concern.

To slightly complicate matters, most cameras will show two types of histograms, the first (typically white) is an aggregate histogram of all colors; the next is a series of three histograms representing the red, green, and blue color channels. While it is possible that one of the color channels might be "clipping" while the aggregate histogram does not show this, unless you are an intermediate to advanced photographer, keep it simple and focus on the aggregate (white) histogram.

What relevance does all of this have for safari photography? In the next edition of Classic Africa News we will look at the histogram's practical application on safari, and how it can be used as an aid to achieving correct exposure.


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