Exposure : Understanding Color and Grey Luminosity

Started Jul 12, 2007 | Discussions
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

Craig Robinson
Craig Robinson Senior Member • Posts: 1,211
Thanks, Ron

This is an informative and well constucted post....it belongs in a textbook. Again, thanks for sharing your knowledge in such a clear manner.
--
Craig

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The more he saw the less he spoke.
The less he spoke the more he heard.
Why can't we be like that wise old bird?

RomanJohnston
RomanJohnston Forum Pro • Posts: 18,806
One of the absolute BEST posts I have seen here.....

in QUITE a while.

Thank you for ALL your work on this.....I agree completly...

Roman
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eric burrows Senior Member • Posts: 1,501
Re: Excellent post Ron

Thanks Ron, very interesting and illuminating, (no pun intended). I really appreciate that you share your knowledge so freely on this forum.
--
eric burrows

Owen Dearing Regular Member • Posts: 175
Re: Exposure : Understanding Color and Grey Luminosity

Hi Ron, thanks for posting this very important information in such a clear manner. I have your e-book and also your dvd series. The combination of both have really helped me in both properly capturing the exposure and in post processing. Last summer when I was in Hawaii I had a print of your color chart (shown in your post) and used it in the field to help me understand the correlation of colors in the scene to what was showing up on the histograms and adjusting exposures accordingly. Powerful stuff, a real eye opener, and very cool once you get your brain working in a different way of perceiving the scene. Thanks again. Take care! Owen
--
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Bend, Oregon

Dave Wolfs Contributing Member • Posts: 664
Why the concern with luminosity values....

If your histogram already displays RGB values. Perhaps I missed the bus on your post but I am having difficulty in understanding why luminosity is important if you can detect channels at risk through an RGB histogram.

Please explain.

Thanks,

Dave

Derouyag
Derouyag Contributing Member • Posts: 824
Re: Why the concern with luminosity values....

Because your histogram lies. Please do a search on UniWB and read as many posts from Julia Borg. Green channel has two, Red has one, and Blue has one, then come the coeffiecients and all the other science... zoom over my head, but they know TONNES more than I do; so I'll trust them.

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OP Ron Reznick Veteran Member • Posts: 8,972
Re: Why the concern with luminosity values....

First: some cameras do not show separate channel histograms, so understanding color is crucial to achieving correct exposure with those cameras (that's how I had to learn, BTW).

Second: what happens when you have to make a critical exposure evaluation prior to taking a shot that has to be right the first time?

Third: the more you know about how this works, the more likely you will achieve consistent results. Do you check your histogram on each shot? Wouldn't it be useful to have an innate feel for color and luminosity so you would be less reliant on trusting a combination of camera-based decisions and fixing errors after the fact by examining the histogram and re-shooting?

Achieving correct exposure (based either on reality or the desired exposure based on your artistic expression) is of critical importance. While you can use a solid understanding of the histograms and the distance (in EV) between various luminosity values to fix an exposure error by re-shooting, getting the shot right the first time is better. It makes the overall experience more pleasurable, as well as reducing the amount of processing required, reducing the number of lost (or poor) shots, and increasing top-grade yield. It will also increase your confidence.

Where is the down-side to that?

Ron
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bill henry Senior Member • Posts: 1,392
Re: Why the concern with luminosity values....

I believe this is what forums should be, intelligent and informative. Not like the garbage so often seen.
Thank you Ron.

OP Ron Reznick Veteran Member • Posts: 8,972
Thanks, all

I'm pleased that you're finding this useful...

Ron
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Dave Wolfs Contributing Member • Posts: 664
Re: Why the concern with luminosity values....

Ron, I am trying to understand your post not criticize it. Perhaps an example would help to make things clearer.

Assume that the camera does have an RGB histogram, and suppose that you are shooting against a very green field with a bright blue sky in mid day light.

How would you use these charts to evaluate the scene and make sure that you get your exposure right the first time?

Jim Fenton Veteran Member • Posts: 4,432
My Take

I religiously use Ron's teaching relative to luminosity values and it works perfectly.

Being able to read luminosity correctly allows you to look at the histogram on your camera and see that the data for a particular color is recorded where it should be to be correctly exposed.

To me..."at risk" to some may mean "at risk of being blown" and while learning these values certainly helps you to avoid that, it also allows you to record the data properly, without the risk of over or under exposing it.

That's how I utilize this information anyway

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MK914 Senior Member • Posts: 1,202
Forgive this really dumb question...

Looking at square H-15, for instance... I see that the RGB value is 255 but its grey value is 64. Since this is sort of close to the 77 Grey value does this mean I can use a red rose in daylight like a grey card? Maybe stopping down 1/3 or so? Just trying to figure how to read all this.

Thanks.

Yves P. Forum Pro • Posts: 18,646
Very well done Ron ...

I was testing a few thing for fun yesterday and this might explain some of the results I got ...,

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RomanJohnston
RomanJohnston Forum Pro • Posts: 18,806
Re: Why the concern with luminosity values....

Well....for me more than just understanding what is in the histogram...if you become very familiar with these values...you will have a very solid understanding of what equals what in the sense of B&W and know what to meter off of and what to expect to compensate based on what its B&W luminosity would basicly be.

Goes a long way to using that nice spot meter that Nikon is awesome enough to put in every model.

Roman
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Dave Wolfs Contributing Member • Posts: 664
Everyone has an answer for everything yet also an answer for nothing....

Please contribute and share how these luminosity values help in capturing your image.

Perhaps you can go through your thought process and how you use these values in order to capture your images (you have some very good ones).

Dave

RomanJohnston wrote:

Well....for me more than just understanding what is in the
histogram...if you become very familiar with these values...you will
have a very solid understanding of what equals what in the sense of
B&W and know what to meter off of and what to expect to compensate
based on what its B&W luminosity would basicly be.

Goes a long way to using that nice spot meter that Nikon is awesome
enough to put in every model.

Roman
--
What will I be given today, and will I be open to see it?
Minor White
http://www.pbase.com/romansphotos/

RomanJohnston
RomanJohnston Forum Pro • Posts: 18,806
The camera we are using is digital...correct?

The more we understand how the camera sees....(digital language)

The more we can start to bridge that gap in our minds and turn that understanding into useful information.

Is that really that hard to understand?

It works the same for film....understanding how the camera sees is one of the top best tool you can have in your arsenal.

Roman
--
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mcd3 Regular Member • Posts: 225
Re: Exposure : Understanding Color and Grey Luminosity

Umm, could someone tell me how this information is applied? While informative, it doesn't really seem to have any value to me when I am in the field making exposures. Maybe I am missing something, but it seems to me that tone value - luminance - is what exposure is determined by. Is this a chart for those that cannot judge tone value of colors? Also, the term K refers to black ink in CMYK offset printing, it's not normally used elsewhere.

Dave Wolfs Contributing Member • Posts: 664
Re: The camera we are using is digital...correct?

I still don't understand how this chart is used. It's easy to post a chart and say its great. But if you can't explain how to use it (nobody in this thread as of yet has given a single example) then it is useless.

RomanJohnston wrote:

The more we understand how the camera sees....(digital language)

The more we can start to bridge that gap in our minds and turn that
understanding into useful information.

Is that really that hard to understand?

It works the same for film....understanding how the camera sees is
one of the top best tool you can have in your arsenal.

Roman
--
What will I be given today, and will I be open to see it?
Minor White
http://www.pbase.com/romansphotos/

Dave Wolfs Contributing Member • Posts: 664
Re: Exposure : Understanding Color and Grey Luminosity

With all the posts in this thread, unfortunately nobody has yet to be able to explain how to apply this.

Simply amazing!

mcd3 wrote:

Umm, could someone tell me how this information is applied? While
informative, it doesn't really seem to have any value to me when I am
in the field making exposures. Maybe I am missing something, but it
seems to me that tone value - luminance - is what exposure is
determined by. Is this a chart for those that cannot judge tone value
of colors? Also, the term K refers to black ink in CMYK offset
printing, it's not normally used elsewhere.

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