Today’s Post by Joe Farace
“Let your words be few and your exposures many…”—anonymus
A histogram is a graphic representation of the distribution of exposure data in an image file. It was first introduced by Karl Pearson (1857-1936,) an influential English mathematician. Those who are mathematically inclines will tell you its an estimate of the probability distribution of a continuous variable (like image exposure.)
An photograph’s histogram is a graphical representation of the tonal distribution in a digital image, plotting It plots the number of pixels for each tonal value. By looking at the histogram for a specific image file you should be able to judge the tonal distribution—underexposure or overexposure—in a single glance.
The histogram’s horizontal axis indicates the level of brightness while the vertical axis indicates the pixel quantity for the different levels of brightness. If the graph rises as a slope from the bottom left corner of the histogram, then descends towards the bottom right corner, all the tones in the image should be captured.
If the graph starts out too far in from either side of the histogram so that the slope appears cut off, then the photograph is missing data in and in fact, the image’s contrast range may be beyond the camera’s capabilities to record it. When the histogram is weighted towards either the dark or bright side of the graph, detail may be lost in the thinner of the two areas. If highlights are important, for example, be sure that the slope on the right reaches the bottom of the graph before hitting the right side.
From all this comes rules like Exposing to The Right (ETTR) and counter agruments such as Exposing to The Left (ETTL.) My take on both of these tropes is that I ignore them. I don’t think you should be a slave to or either of these two rules or a perfect histogram. That’s because while the classic histogram features the famous bell-shaped (Gaussian) curve, not every photograph you’ll make fits this distribution of exposure values. A high or low-key image can have a lopsided histogram. Does that mean exposure correction is needed? Nope, it just means that the histogram is appropriate for the image that you’ve just captured.
Along with photographer Barry Staver, Joe is co-author of Better Available Light Digital Photography that’s available from Amazon for $21.87 and used copies starting at $7.00 as I write this. The Kindle price is expensive for some reason (not Barry or me.)