The Value of a Histogram

This is less a rant or even a clarion call. Think of it as a peer review…
...concerning Histograms.

“When you can measure what you are speaking about, and express
it in numbers, you know something about it.”-  Lord William Thomson Kelvin (1824 -1907)

Lord Kelvin, scientist, philosopher and extraordinarily curious man.

But relating to photography, I say yes and no to his premise.  Yes, I can express what I shoot
in numbers, but no – a graphic display of any given image is just that and cannot be construed
as being anything other than a dubious post mortem chart. However, there is one exception, and
that is the “Characteristic Curve”. A histogram is not a “Characteristic Curve”. This short series will
try to make sense of and interrelate waveforms, curves, the zone system and histograms.

A histogram is a metric looking for a friend.

We’ll start first with these 3 images and their histogram value. To me, the informational content of this
system may only be of value in these 3 histograms. Though, specifically the 18% gray panel with
respect to calibrating a meter. Shoot a gray card and if the lines spike in the middle of a histogram
then your meter is good. If the line falls to either side of the middle, then an adjustment is required.

Qualitative v Quantitative

Determining an exposure is a Qualitative process that for me involves
5 steps: exposure index (ISO), shutter speed, f stop, and the meter reading
process that ultimately relates to the story tone. In that calculus I will often
glance at the waveform (qualitative) which gives me a real time, across the
board assessment of ALL of the luminance values present in the frame.
This is a helpful tool to gauge extreme over or under exposed parts of the frame.
Notice all of the data in the waveform in the lower picture. See if you can find
Janet’s face in the display? Now, compare the waveform to the histogram . A
histogram is a quantitative tool. Compared to the waveform, what does this
histogram tell us? I say - not very much.

This image is made up of mostly medium to darker tones in the range of z
ones 2,3,4 and 5. Or generally in the histogram realm of 0-150. So this histogram
depicts an average all of these tones in the image. But in fact, this image is far
more complex than our histogram indicates. Where are the highlights on the roof
or the bright side of Janet’s face? If you studied this histogram you might conclude
that the entire image consisted of gray or black tones. Not so.

The Histogram - telling us what we should already know.

Each of these photos consists of two distinct luminance values and
the histogram has identified them nicely. I’m curious about the double
spike at zone 6 and 7 in the grey panel. I’m not convinced that the
slight dapple in the gray could generate that strong of a spike.

In each of the panels, Vader’s head is rendered almost  identically.
The other common read in the three histograms is the weak thin line
that runs along the bottom of each graph. These lines are the highlights
found on the surface of my highly coveted Darth Vader piggy bank.
The highlights are of varying densities, thus the long line across at least
6 zones on the histogram - barely measurable, and almost invisible on the graph.

While shooting the middle panel, I knew that the green screen was one stop
overexposed. On the histogram, the placement of its spike at zone 7 proves
that – yes - it is one stop over. So as far as useful data, that would be my only
analytical takeaway. (But then, I already knew that.)

Where’s the rest of the data?

I pulled three frame grabs from Salem for Histogram analysis. I must say
that the results for each image left me wanting a little more. Take a look
at the top panel. Seth is lit by a nicely by a single candle, resulting in a
reasonably bright highlight on his face. Why isn’t there any evidence of
that on the histogram?

The middle panel renders the most tones of the three but I’m a little surprised
that the bright exterior is reading more as middle gray. To me the sky feels like
zone 7 and perhaps it is on the histogram in the form of that little bump at the
far right end of the data chain.

The bottom frame finally renders the bright area in the shot, (far right spike),
but where is the spike representing the key side of Janet’s face? Study this image
carefully. How many tones can you count? I see at least 8. Do you feel that this
histogram accurately reflects the complexity of this image?

Trust your Meters

We live in an over measured, quantitative world. As DP’s we exist on the qualitative
side of things. Our job is to measure light with our eyes, with meters and with the help
of a technical crew - an F Stop will be set that will capture the image in a way that
supports the story at hand. The histogram of that image is an anemic, post script
from a purpose driven image, that is inconsequential in every way.

Are all quantitative measurements as useless as a histogram? MRI’s, CAT Scans,
XRAYS, Depth Soundings and many, many more examples have benefited us all enormously.

But alas, the lowly histogram should be placed in the quantitative hall of shame that
includes suggestion boxes and political polls.

I hope that the photographic examples in this series have helped you understand the
problems I have with this metric.

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