Exposure : Understanding Color and Grey Luminosity

Started Jul 12, 2007 | Discussions thread
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Ron Reznick Veteran Member • Posts: 8,972
Exposure : Understanding Color and Grey Luminosity

This is from a post from an image thread:
http://forums.dpreview.com/forums/read.asp?forum=1021&message=23970712

I thought it may be useful to folks who don't look at image threads, or who may miss the thread when it wanders into obscurity.

Many folks have trouble exposing when a channel is at risk (e.g. red/tan/gold in all sorts of situations, backlit green leaves in the forest, magentas and pinks when shooting flowers, etc.). To achieve consistent results when a channel is at risk, it helps a lot to understand color relationships and how the camera sees different colors, since the sensor reads grey values through a filter (most often a Bayer filter, which places a Red, Green, or Blue filter over each well in the pattern:

RGRGRGRGRGRG
GBGBGBGBGBGB
RGRGRGRGRGRG
GBGBGBGBGBGB
etc.

You can train your eyes to recognize luminosity values, which will give you reference points when looking at the histogram to evaluate exposure (e.g., if a large expanse of grass in your scene averages around 150, you should see a large more-or-less bell-shaped peak in the histogram centered on 150 if the exposure is correct). You can also predict when a channel will be at risk, and what luminosity value the camera should meter for a specific color if you understand how colors are composed in RGB color.

Take a look at the color chart below:

Note at the top of each patch -- I've put in RGB values for each color patch. Now, note the number at the bottom of each patch. This is the grey value that the camera sensor sees, based on the 25% Red, 50% Green, 25% Blue (Bayer Filter) over the sensor. Note esp. the Magentas, Reds, Pinks and Blues in the chart (e.g. E3-E5) and you'll see that, although Red or Red and Blue are nearing or achieving full saturation, the overall grey value can be quite low if there is little or no Green in the color. It is very useful to understand color relationships. By studying this chart and experimenting while using your split histograms (separate Red, Green and Blue), you will gain an understanding of how color can affect exposure. This will greatly assist in achieving better results, increasing your first-shot yield and your consistency of quality. This should get you started in the right direction to understand color relationships and how they relate to exposure issues.

For an understanding of grey values, see this chart:

It shows a 5% greyscale along with RGB values for each grey (plus the K-values -- showing % of black), as well as the grey values that correspond to the histogram dividing lines for Canon and Nikon cameras. Using this tool and a calibrated monitor (or an accurate print) you can train yourself to recognize various luminosity levels. This should help you a lot...

Ron

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