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Author Topic: expodisc/gray card  (Read 10023 times)
hdegroot
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« Reply #20 on: August 09, 2007, 11:36:02 AM »
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White balance is actually a fairly complicated topic.  The fact that accomplished photographers using extremely advanced digital single lens reflex cameras continue to have white balance issues speaks to the fact that white balance is not simple, and a quick and reliable solution has not yet been discovered.

White balance is a process of measuring color temperature, and applying a correction to the data that comes out of the image sensor to remove color casts.  In order to do this, the camera needs to be provided with an external reference, an internal reference, or it needs to use an algorithm to search the actual image data for a neutral reference.

The automatic white balance AWB feature on simple and sophisticated cameras is a process where the camera uses an algorithm to process the actual image data and extract a neutral reference or extract the color temperature.  There are a number of different implementations of the automatic white balance feature.  The quality of the automatic white balance depends on the processing power of the circuitry in the camera and on the underlying assumptions that are built into the algorithm.

The Expo disc White balance tool works on what is called the "grey world" algorithm.  This algorithm assumes that if you average all of the light in any given image, it will balance out to a neutral point.  The Expo disc has an acrylic layer built in that scrambles, and diffuses the image, and in essence averages the light in the image.  Provided that the scene you are pointing the Expo disc at does indeed have an overall neutral color temperature.  

Obviously, there are going to be scenes that will not balance out to neutral, that do not conform to the " gray world" paradigm.

Using a gray target or digital gray card is a better method because it is a direct measurement of color temperature, rather than an assumption or approximation.  The digital gray card (http://httP://www.digitalimageflow.com - my company's product)  acts as a color temperature mirror.  It simply reflects the incident or illuminant light back into the cameras sensor, where the cameras electronics or the postprocessing software's algorithms can use it as a reference.  Since in using a gray card you are in fact performing an effective measurement of color temperature, you will get a more accurate white balance.

Naturally, if you use a gray target that is not spectrally neutral, it will not perform as a faithful color temperature mirror, but instead it will add or subtract or somehow alter the reflected light, giving you an abnormal color balance.   In a related way, if you do not place the great card where it will be illuminated by the light illuminating your subject, you will not achieve correct color balance.

Sorry about the long winded post , as I mentioned, this is actually a fairly complicated topic.
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digitaldog
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« Reply #21 on: August 09, 2007, 11:42:20 AM »
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And lets not forget that defining the color of white using correlated color temperature (the K scale) is mildly inaccurate even with the best targets. I'd love to see the day when I have a tiny embedded Spectrophotometer in the camera that would measure the illuminant and write the necessary information into EXIF data for the Raw.

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Photographers have been taught for years that tungsten film has a color temperature of 3400K (Kelvin; a unit of temperature). Lower Kelvin values appear more red, and as the Kelvin values get higher, the color becomes more blue. In actuality, a color temperature is a range of colors correlated to the temperature of a theoretical object known as a blackbody radiator. The blackbody reflects no light and emits energy in shorter wavelengths as it is being heated. Imagine a black cast iron pan on a very hot stove. As it is heated, it begins to glow dark red. As the temperature increases, shorter wavelengths are emitted, causing the color of the light emitted by the skillet to appear orange, then yellow-white, then blue-white. The tungsten filament of a light bulb behaves similarly to the blackbody and radiates energy in the form of light because its temperature is so great (around 3200K).It's somewhat dangerous to use color temperature to define what you want because the reality is if all light sources were true blackbodies a particular color temperature would produce the same color of light. Because natural materials are not theoretical blackbodies, heating them to a specific temperature creates deviates from the theoretical color from magenta to green. It's really much safer to use the term correlated color temperature (CCT) because many colors of white may correlate to the same blackbody color temperature. Different illuminants can have the same correlated color temperature.

 This is one reason why the CIE defined the Standard Illuminants. These illuminants are defined spectrally meaning a certain amount of energy at each wavelength across the spectrum. This is an exact and non-ambiguous description of color. D65 is an exact color, it is not a range of colors. If you have a color meter that reports color temperature of a light source many light sources that appear different could read the same, that's kind of a problem!
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Andrew Rodney
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Gregory
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« Reply #22 on: August 09, 2007, 11:48:19 AM »
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Dear hdegroot,

what happens to the White Balance adjustment if for example the grey card is placed in a scene lit by a yellowish light? the way I see most people applying the dropper, the yellow hue would be lost producing an image not like the original scene.

regards,
Gregory
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« Reply #23 on: August 10, 2007, 12:09:07 AM »
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Dear hdegroot,

what happens to the White Balance adjustment if for example the grey card is placed in a scene lit by a yellowish light? the way I see most people applying the dropper, the yellow hue would be lost producing an image not like the original scene.

regards,
Gregory
[a href=\"index.php?act=findpost&pid=132346\"][{POST_SNAPBACK}][/a]

Exactamundo !

To me it seems sensible to calibrate your camera either using Grey or Macbeth cards (dependant on your botheredness level) in neutral conditions - grey day car park - sunny day car park - studio with nothing much in it

And then paste on those settings in RAW to images as a starting point for working the raw

This is of course dependant on the look you are going for - I like colour casts they are part of the environment - grass is green and trees cast green shadows

If however you are trying to accurately photograph a white product under tungsten light in an orange room a different approach will of course be needed

S
« Last Edit: August 10, 2007, 12:09:57 AM by Morgan_Moore » Logged

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sergiojaenlara
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« Reply #24 on: August 10, 2007, 02:01:29 AM »
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What about the cheapest one?

The qpcard!!!

http://www.qpcard.se/BizPart.aspx?tabId=28
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hdegroot
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« Reply #25 on: October 25, 2008, 07:35:43 PM »
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Quote from: Gregory
Dear hdegroot,

what happens to the White Balance adjustment if for example the grey card is placed in a scene lit by a yellowish light? the way I see most people applying the dropper, the yellow hue would be lost producing an image not like the original scene.

regards,
Gregory


Quote from: Morgan_Moore
Exactamundo !

To me it seems sensible to calibrate your camera either using Grey or Macbeth cards (dependant on your botheredness level) in neutral conditions - grey day car park - sunny day car park - studio with nothing much in it

And then paste on those settings in RAW to images as a starting point for working the raw

This is of course dependant on the look you are going for - I like colour casts they are part of the environment - grass is green and trees cast green shadows

If however you are trying to accurately photograph a white product under tungsten light in an orange room a different approach will of course be needed

S


The answer to the first question is:  A grey card (http://www.digitalimageflow.com) placed in a scene illuminated by light with a yellowish color temperature will reflect that light faithfully (without any alteration) into the camera, so that the image of the grey card will effectively encode the color temperature of the scene.  When the image is processed, setting the color temperature to the reading off the grey card will correct the scene so that it appears as it did when it was photographed.

For the second question:  I'm not sure what is being recommended there.  It seems that the writer recommends taking three calibration readings in various "neutral" scenes.  I imagine that would be three readings from a grey card.  He then recommends you set put of those into your RAW converter as a starting point.  Maybe I'm missing the point.  The way I see it, if you need highly accurate color, why not do a grey card shot in the scene you are shooting, and then use that?  Shooting a grey card image is as easy as giving your model the grey card to hold at the beginning and the end of each shot series (if you change your lighting between series).  Why bother to shoot "neutral" scenes?  

The point about color casts is well put - sometimes color casts are desirable, and using a grey card may reduce or eliminate them.  For example, indoor shots with tungsten lights shot on film and developed at the drugstore typically look a bit yellow.  A perfect way to add an "indoor snapshot" mood to a creative piece would be to try for a bit of a yellow cast.  Color encodes meaning, says John Paul Caponigro, so when I'm shooting a scene, I try to make sure the color that means something to me stays where it belongs.  Trying to neutralize every color, "correct" all colors, may lead to loss of valuable information.

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