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Gerald Reisz
Participating Frequently
May 10, 2024
Answered

Need to Reset Over Exposed Pixels to Luminance 0

  • May 10, 2024
  • 2 replies
  • 1537 views

I have a number of total eclipse photos shot at different shutter speeds. I want to black out (set to luminance of 1 or 0) the totally over exposed pixels in each image before combining them. With the hundreds of tools in PhotoShop, I imagine there is an easy way to do this but I haven't figured it out. I am a beginner at PhotoShop. I normally use Lightroom. I am running Release 25.7.0.

Thanks for the help!

    This topic has been closed for replies.
    Correct answer r-bin

    Your image looks exactly like what I have imagined. I will try it. I also found a Threshold mode of Subtract which may do the same. Do you know if Subtract is equivalent to Invert plus Darken?


    quote

    Your image looks exactly like what I have imagined. I will try it. I also found a Threshold mode of Subtract which may do the same. Do you know if Subtract is equivalent to Invert plus Darken?


    By @Gerald Reisz
     
    Your method is much more convenient than mine. )
    No, Darken and Subtract modes are different things.
    In Darken mode, the result will be the darkest (per channel) color of the pixel from the current layer and everything under it.
    The Subtract mode is the subtraction of pixel values (in each channel). Since in your case the original treshold layer gives only white or black, then subtracting black (this is zero) does not change anything in the resulting pixels, and subtracting white (i.e. subtracting 255) just resets everything to zero (and even makes it negative, as it were, but in reality there is no less than zero)
     
     
     

    2 replies

    D Fosse
    Community Expert
    D FosseCommunity Expert
    Community Expert
    May 12, 2024

    I still dont't understand the end goal of this exercise.

     

    As for the original question ("I want to black out (set to luminance of 1 or 0) the totally over exposed pixels in each image"), that has been fully answered by c.pfaffenbichler. His procedure does exactly that.

     

    But what is supposed to come out of this? How does this make it any easier to combine these multiple exposures into one?

      D Fosse
      Community Expert
      D FosseCommunity Expert
      Community Expert
      May 12, 2024

      And one more thing: There is much more highlight information in the raw files than what you get straight up at default slider positions. This is where you would use the Highlight slider in ACR to recover those highlights.

       

      It may not be enough to cover the full dynamic range here, bit it should reduce the number of frames needed considerably.

       

      You say, "I can do far more editing in PhotoShop vs Camera RAW" - but that's not correct. It's the opposite. Opening into Photoshop throws away a lot of data, permanently.

       

      A raw file is linear sensor data. A modern sensor can capture up to around 14 stops dynamic range. A gamma-encoded RGB file can only hold about 7 or 8 stops and still retain a natural-looking contrast curve. But that extra data in the raw file, although initially hidden, can be brought out with the Highlight slider, and remapped into the 8 stop gamma representation.

       

      The golden rule with raw files is always to do as much as possible in ACR. Only when those options are exhausted, when you can't do anything more with the ACR sliders, do you move to Photoshop.

       

      (there are a few special exceptions to that rule regarding color corrections, but this isn't one of them).

        c.pfaffenbichler
        Community Expert
        c.pfaffenbichlerCommunity Expert
        Community Expert
        May 10, 2024

        Please post sample images. 

         

        A black Solid Color Layer with the appropriate Blend If-setting might suffice. 

         

        Edit: 

          G
          Gerald ReiszAuthor
          Participating Frequently
          May 10, 2024

          Thanks for the response. I have attached a sample file. The original file is a Fujifilm RAW file (RAF extension). I opened it in PhotoShop which opened the file in Adobe Camera RAW. I then saved the image file in the PSD format since it looks like I can do far more editing in PhotoShop vs Camera RAW. Now using PhotoShop I cropped the image to reduce the file size (113 MB to 14.4 MB). This is a typical total solar eclipse photo. It is one image from a set of nine bracketed images shot at various shutter speeds to capture the details of the solar corona. The dark circle in the center is the moon.

           

          The area around the moon is burnt out with luminance values of 250 to 255. It has no useful data to include in the final combined image. I want to reset the pixels in this area to a value of 0 or 1, completely black. I would repeat this process for each image, save these images with over exposed areas set to 0, and then combine the images to get a view of the total corona. The Threshold slider seems to be in the right area except it is doing the opposite of what I want to happen.

          R
          r-bin
          Legend
          May 10, 2024
          quote

          The Threshold slider seems to be in the right area except it is doing the opposite of what I want to happen.


          By @Gerald Reisz

           

          I don't know how this will help you, but you can do this.

          Make a copy of the layer and convert it to a smart object.

          Apply Image->Adjustment->Threshold 250 to the layer.

          Apply Image->Adjustment->Invert.

          Set the layer's blending mode to Darken.

          If everything is ok, flatten the layers or change the Threshold value by double clicking.

           

           

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