Opposite colour to Blue?

Discussion in 'Film and Processing' started by curtis_lowe, Dec 16, 2008.

  1. I have a colour printing chart which states if the cast is blue, dial out yellow.
    This is confusing as I allways understood red filters will darken blue skies more than yellow, so I thought red was the opposite of blue?
     
  2. color opposite basics:
    Red against Cyan
    Green against Magenta
    Blue against Yellow
     
  3. Line up RGB against CMY (K) as Ellis says.
    To remind me I printed this out and put it above my monitor. I also printed a graphic depiction of L*A*B*
     
  4. I find a "star of David" handy: put the 3 primaries: red, green and blue in the corners of one of the trianges, and then fill in the 3 secondaries: magenta, cyan, and yellow in the corresponding corners of the other triangle. I've got a graphic at home I can dig up tonight.
     
  5. "I thought red was the opposite of blue?"
    It is on a color wheel for subtractive color, such as pigments. Primary (thalo or cobalt) blue's compliment is a secondary red (cadmium crimson or vermillion), but not in additive color, such as RGB. They have diffenent primaries. Green is not a primary in subtractive colors, yellow is.
     
  6. The "K" in CMYK refers to black ink in the offset printing process, which is subtractive color. Here's "the Star of Light" (additive color).
    Gray is in the center because equal amounts of any color pair (magenta and green) of light you'll get a neutral gray. Add equal amounts of light of all colors and you get white. Absence of light = black.
    00Ropq-98213684.jpg
     
  7. Thanks Don,
    Yeah, subtractive and additive colours get confusing.
    Depends if you are an artist (oil,pastels, water colours),
    or a physicist (photographer, television engineer).
    /Clay
     
  8. Happy חנוכה‎ (Chanukah), Ellis and everyone ;)
     
  9. It's easier to see in GIMP than with the star of David.
     
  10. Here's JPEG because GIF was undisplayable.
    00Rp19-98315584.jpg
     
  11. "I allways understood red filters will darken blue skies more than yellow"
    You are correct, but that doesn't mean that red is the opposite of blue. Part of the problem is that blue skies aren't really blue. They are a combination of blue and cyan. Since cyan is a conbination fo blue and green you could also descsribe blue sky as a combination of blue and green with more blue than green.
    The other problem is the difference between additive filters (RGB) and subtractive filters (CMY). A yellow filter blocks one third of the spectrum (blue) and passes two thirds (green and red). A red filter blocks two thirds of the spectrum (blue and green) and passes one third (red).
    A red filter blocks everything that a yellow filter blocks and it also blocks green as well.
     
  12. Filters you throw on the lens work differently than printing to color negative paper.
     
  13. "Filters you throw on the lens work differently than printing to color negative paper."
    You are correct from the view of the final print in a neg-pos system.
    I like to think of the physics involved. On a physical level, a filter is a filter and does the same things whether it is in front of a camera lens or an enlarger lens. A CC 20Y filter in front of the camera will absorbe a certain amount of blue light. This reduces the amount of blue exposure and (in a color negative) decreases the amount of yellow dye that is formed in the negative. If you put that same CC 20Y filter in front of (or behind) an enalrger lens, the exact same thing happens--you get less yellow dye in the print.
     
  14. "I allways understood red filters will darken blue skies more than yellow"
    Yes, but that applies to Black & White film photography when shooting (not post processing). When shooting color (film or digital) a polarizer is used to darken the sky.
     

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