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CJBoghosian wrote:
JIMMY: Thank you for the links - I will check them out! In regards to your questions:
- I used my camera spot meter pointed at an evenly lit gray card, which completely filled the frame.
- I used the meter to determine "normal exposure," i.e., set at 0 on the Exposure Level Indicator.
- I then took a series of photos of the same gray card in the same lighting in 1-stop increments, 5 down and 5 up.
This is a link to my series of photos: A screengrab of my series of exposures.
What confused me was that ALL 5 down (underexposed) stayed within the histogram; however, only 2 up (overexposed) stayed within the histogram. The apparent imbalance in the range of latitude really confuses me. Why 5 down the range but only 2 up?
I suspect it has something to do with the way the sensor works, i.e., it assigns half the data to the brightest highlight in the scene.
...Show more →
Hi again, Christopher,
Okay, thanks for the clarifications! Here goes with some more homework for you...
The problem lies not with your camera but with our understanding with what our camera's histogram is actually telling us. Most folks (and a good portion of the internet) have this belief that 18% grey is middle grey to our cameras, or value 128 (on a scale from 0-255). Such is not the case. It turns out that our cameras use an sRGB-adjusted gamma for their histograms, not a linear gamma of 1. As a result the bit value for middle grey is more like 117 in sRGB. There's an excellent (read: understandable) write-up on this under the Simplest test to show obvious truth - Histograms show gamma data header a bit more than half-way down this page...
Histograms show gamma-encoded data:
http://www.scantips.com/lights/gamma.html
...explaining why you're seeing 2 1/2 stops up and 5-stops down. Your experiment, as it turns out, recreates the author's experiment and, not surprisingly, corroborates his findings!
Another page that utilizes this experiment, but where the author both struggles for but never figures out the math behind the effect is here...
Digital Tones and Exposure Zones:
http://www.rags-int-inc.com/PhotoTechStuff/TonesnZones/
And, another, still, under the Calibrating Your Brain to Understand the Histogram header, with no attempt at understanding or explaining the "why it is"...
Interpreting the Camera Histogram:
http://super.nova.org/DPR/Histogram/
FWIW, you can actually view the RAW images, and their RAW histograms, in Linear Gamma from your 60D using Canon's DPP software.
Hopefully, again, this is of some help!
Jimmy G
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