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Thread: Hello and question

  1. #1
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    Hello and question

    Hi Everyone,
    I've been lurking here trying to learn more about digital photography and image processing. I'm working on a time lapse project where we are watching how colors change over time, and I was wondering if anyone has had experience calibrating photos. I'm hoping to describe color changes independent of any changes in lighting. For example, right now I'm in Mendocino and smoke is changing colors quite a bit!

    We'll be taking photos once a day (RAW) around noon using an intervalometer and the same manual exposure setting. I was planning on putting a color chart out that will be seen in each photo, and then adjusting the photos to correct for smoke, cloud, and seasonal changes in sun position.

    As someone new to the forum I feel guilty asking for help when I have not contributed, so thanks ahead of time!
    Best,
    Mike

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    Administrator Bob's Avatar
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    Re: Hello and question

    Hi Mike,
    Don't feel guilty at all.
    Color rendition is a slippery thing, and tough to do on an absolute basis.
    By your post I understand that color is changing, but the smoke in the air and the angle of the sun, your latitude, time of year, and atmospheric conditions all change the color of light.
    There are international standards for illuminants, such as D50, which attempt to describe "standard" which is supposed to be daylight correlated standard "day". I don't know if I have seen one yet.
    Without light there is no color, and color is always perceived as it is influenced by the available light. That is why I am struggling with measuring color independent of light. The important thing to nte is that these standard illuminants all have a continous spectrum that contains all visible colors, just to varying degrees. You might look up references to "black body radiation" to see where the terms like 5000 degrees Kelvin came from.
    What you can try to do, is to take pictures of a test target, such as the Macbath color checker. It contains several neutral patches that you can use to adjust the white balance of your shot. Although this will to some degree "standardize" each shot to the same reference, the changes in the spectrum of illuminating light will affect the relative apearance of the color patches.
    Fo example, if you take pictures of the target under sodium lamp illumination, although you can set your white to look white (good luck BTW), blues will be black because there is no blue light in the illuminant.
    -bob

  3. #3
    Sr. Administrator Jack's Avatar
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    Re: Hello and question

    THe other thing to consider is setting the camera to a fixed WB instead of auto. While this won't necessarily be accurate color per Bob's explanation, it should at least show the changes in a relative fashion. Obviously if you also shoot in raw mode, you'd need to process each image identically.
    Jack
    home: www.getdpi.com

    "Perfection is not attainable. But if we chase perfection, we can catch excellence."

  4. #4
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    Re: Hello and question

    Thanks Bob and Jack for your help!
    I had thought about taking the photos in the dark, with a known light source to standardize things, but we are looking to quantify color changes over a large scale (60 sq. meters). I have a Macbeth type color chart, and was planning on getting the red, green, and blue squares to be the same between photos in photoshop somehow. I'll have measurements of lux at the site, but nothing else. Good point about the white balance, I'll make sure everything auto is off.
    Thanks,
    Mike

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