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Recovering Over/Under exposed colour channels

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  • Recovering Over/Under exposed colour channels

    Hiya All,
    Is it possible to decrease the luminous intensity of say the 'Red' colour channel if it's overexposed. Same with underexposed parts is it possible to bring the information back into the image if its a touch underexposed in a 'RGB' colour channel.
    Thanks
    Ryan

  • #2
    Technically yes, but practically no. For an image to be overexposed all three (red, green and blue) channels need to output full, or 255. You can decrease one channel only and the image technically won't be overexposed, but it will completely screw your colours up. White balance is basically the balance of these three channels and when you adjust WB in RAW you're shifting these channels around, so if you reduce just the red channel then you'll start getting an imbalance that will look unnatural. Technically you're image won't be overexposed but it will look pretty rubbish! The reverse (but same principle) applies to overexposure.

    Hope that helps!
    Seeing the world with a 3:2 aspect ratio...

    My images on Flickr

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    • #3
      Originally posted by PMN View Post
      Technically yes, but practically no. For an image to be overexposed all three (red, green and blue) channels need to output full, or 255. You can decrease one channel only and the image technically won't be overexposed, but it will completely screw your colours up. White balance is basically the balance of these three channels and when you adjust WB in RAW you're shifting these channels around, so if you reduce just the red channel then you'll start getting an imbalance that will look unnatural. Technically you're image won't be overexposed but it will look pretty rubbish! The reverse (but same principle) applies to overexposure.

      Hope that helps!
      Thanks Paul,
      I was thinking about this are surely if you get the exposure spot on across the channel would that mean that the photo has no cast at all?
      RT

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      • #4
        Originally posted by Rtyrpics View Post
        surely if you get the exposure spot on across the channel would that mean that the photo has no cast at all?
        Nope, you're actually confusing exposure and white balance. Exposure is essentially the sum of how much the red, green and blue channels output, whereas white balance is the relationship of those three channels with each other. You can have fairly accurate exposure but still have a slight imbalance of the individual channels and it's this imbalance that gives colour casts. You can see this on a RAW image when you adjust the colour temperature. Make it warmer and you can see the red information in the histogram move to the right and the blue move to the left. Make it cooler and the reverse happens, blue moves to the right and red to the left. In doing this you're not changing the exposure as such but changing the relationship between the red and blue channels, so you can see that exposure doesn't really relate to colour in the sense you're thinking, but top marks for at least thinking about it. Knowledge is good.
        Last edited by PMN; 2010-04-18, 20:05.
        Seeing the world with a 3:2 aspect ratio...

        My images on Flickr

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