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How luminance works

From: Photoshop CS6 One-on-One: Fundamentals

Video: How luminance works

In this movie, I'll introduce you to a few terms and ideas so that you understand how luminance works inside of a digital image. And these ideas will not only help you understand how to correct luminance throughout this chapter, but also when you're working on your own images in the future. I'm working inside a file called luminance demo.psd, it's found inside the 07_luminance folder. Now even though we think of an image as being full-color, it's really a combination of grayscale images working together.

How luminance works

In this movie, I'll introduce you to a few terms and ideas so that you understand how luminance works inside of a digital image. And these ideas will not only help you understand how to correct luminance throughout this chapter, but also when you're working on your own images in the future. I'm working inside a file called luminance demo.psd, it's found inside the 07_luminance folder. Now even though we think of an image as being full-color, it's really a combination of grayscale images working together.

And I'll show you what that means in just a moment, but in the meantime know that every pixel has a luminance level, from black at the darkest to white at the brightest, and these ranges of luminance have general names. The darkest luminance levels are known as the shadows, the brightest luminance levels are known as the highlights and then the luminance levels in between are known as midtones. Now there's no specific place at which highlights end and midtones begin, or midtones end and shadows begin.

These are just general ranges of luminance. Now as I was saying, what we see as a full-color image is actually multiple grayscale images working in concert with each other. These grayscale images are known as channels. This image, like all digital photographs contains, three channels, we have a red channel, we have a green channel and we have a blue channel. Where the bright colors in the red and green channels intersect you get yellow. Where the bright colors in the green and blue channels intersect, you get cyan, and where the highlights in the red and blue channels intersect, you get magenta.

Just to give you a sense of how these channels mix to form the full-color image. If you have highlights in all three channels you get white, if you have shadow in all three channels you get black. Now let me show you what the channels look like where this specific image is concerned. I'm going to go up to the Window menu and choose the Channels command in order to bring up the Channels panel which by default lives next door to the Layers panel. And notice that we're seeing what's known as the RGB Composite; that is red, green and blue working together and that the red, green and blue channels are all selected because they're all turned on.

However I can click on any one of these channels to view it independently. So for example, I'll click on the red channel and as you can see, it is a grayscale image. This is what Photoshop sees as it evaluates a full-color image, because Photoshop sees and addresses the image one channel at a time. And as you can see, where this image is concerned, we have tons of highlights inside the red channel. We have a few midtones here and there, but we really don't have anything along the line of shadows.

And just for reference, I'm going to turn that gradient back on, and you can see that the darkest luminance level inside this channel is somewhere around here inside the gradient. So it's by no means black, which is why we have such a washed out image in the first place. Now let's take a look at the green channel, and you can see that things darken up but still not enough, and then here's the blue channel, darker still, but also very bright. All right now, I'll go ahead and switch back to the RGB image and I'll go up to the Image menu and I'll choose a command called Auto Contrast.

And this is one method for correcting the luminance levels inside of an image. And notice that Photoshop darkens up the image considerably and again it does so on a channel-by-channel basis. So every one of these channels is darker. And in fact what Photoshop has done is it's taken the darkest pixels inside the image, which were quite light, and turn them black and then stretch the other luminance levels across the gradient spectrum. And so if I take a look at the red channel now, you can see we've got some very dark shadows inside the pupil, in the eyelashes, and around the iris, and so forth.

The same goes for the green channel, which is darker still, have some very rich shadows going on, and then in the blue channel the same is true except we have more shadow detail than ever. And that friends is how luminance works here inside Photoshop.

Show transcript

This video is part of

Image for Photoshop CS6 One-on-One: Fundamentals
Photoshop CS6 One-on-One: Fundamentals

100 video lessons · 55628 viewers

Deke McClelland
Author

 
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  1. 19m 15s
    1. Welcome to One-on-One
      2m 27s
    2. Opening from the Windows desktop
      4m 7s
    3. Opening from the Macintosh Finder
      4m 9s
    4. Opening from Photoshop or Bridge
      2m 45s
    5. Opening an image from Mini Bridge
      1m 16s
    6. Opening through Camera Raw
      2m 32s
    7. Closing one image and Closing All
      1m 59s
  2. 38m 14s
    1. Navigating your image
      40s
    2. The dark vs. the light interface
      3m 12s
    3. Navigating tabs and windows
      4m 32s
    4. Panels and workspaces
      4m 27s
    5. Zooming incrementally
      4m 29s
    6. Zooming continuously
      2m 43s
    7. Entering a custom zoom value
      2m 25s
    8. Scrolling and panning images
      2m 31s
    9. Rotating and resetting the view
      2m 11s
    10. Cycling between screen modes
      3m 10s
    11. Using the Navigator panel
      3m 38s
    12. Adjusting a few screen prefs
      4m 16s
  3. 45m 58s
    1. Digital imaging fundamentals
      1m 45s
    2. Image size and resolution
      3m 3s
    3. The Image Size command
      3m 27s
    4. Common resolution standards
      3m 20s
    5. Upsampling vs. real pixels
      4m 36s
    6. Changing the print size
      6m 16s
    7. Downsampling for print
      4m 12s
    8. Downsampling for email
      3m 11s
    9. The interpolation settings
      5m 22s
    10. Downsampling advice
      4m 36s
    11. Upsampling advice
      6m 10s
  4. 53m 17s
    1. The layered composition
      1m 40s
    2. Introducing the Layers panel
      4m 12s
    3. Adding, scaling, and aligning layers
      5m 27s
    4. Dragging and dropping layers
      4m 36s
    5. Stack, reveal, and rename
      2m 58s
    6. Opacity, history, and blend mode
      6m 5s
    7. Duplicating a selected portion of a layer
      5m 32s
    8. Applying a clipping mask
      3m 58s
    9. Blending inside a clipping mask
      4m 10s
    10. Finishing off your artwork
      3m 13s
    11. Creating a new layer and background
      4m 24s
    12. Layering tips and tricks
      7m 2s
  5. 26m 19s
    1. The art of saving
      54s
    2. Four things to know about saving
      6m 0s
    3. Saving layers to PSD
      6m 38s
    4. Saving print images to TIFF
      4m 48s
    5. Saving an interactive image to PNG
      3m 41s
    6. Saving a flat photo to JPEG
      4m 18s
  6. 19m 36s
    1. Honing in on your image
      1m 43s
    2. The new and improved Crop tool
      3m 35s
    3. Editing your last crop
      3m 1s
    4. Straightening a crooked image
      2m 29s
    5. Filling in missing details
      6m 44s
    6. Using the Perspective Crop tool
      2m 4s
  7. 42m 6s
    1. First, there is brightness
      2m 12s
    2. How luminance works
      4m 18s
    3. The three Auto commands
      3m 27s
    4. Automatic brightness and contrast
      3m 19s
    5. The Brightness/Contrast command
      2m 47s
    6. The dynamic adjustment layer
      4m 5s
    7. Editing adjustment layers
      3m 52s
    8. Isolating an adjustment with a layer mask
      3m 31s
    9. Introducing the histogram
      4m 58s
    10. Measuring an adjustment
      3m 34s
    11. Using the Shadows/Highlights command
      6m 3s
  8. 44m 33s
    1. And second, there is color
      1m 31s
    2. Identifying a color cast
      3m 34s
    3. Correcting a color cast automatically
      3m 57s
    4. Changing the color balance
      6m 10s
    5. Compensating with Photo Filter
      3m 11s
    6. Adjusting color intensity with Vibrance
      3m 29s
    7. Correcting color cast in Camera Raw
      5m 46s
    8. The Hue/Saturation command
      5m 26s
    9. Summoning colors where none exist
      4m 8s
    10. Making more color with Vibrance
      4m 27s
    11. Making a quick-and-dirty sepia tone
      2m 54s
  9. 55m 46s
    1. Making selective modifications
      1m 10s
    2. The geometric Marquee tools
      6m 1s
    3. Aligning one image element to another
      4m 59s
    4. The freeform Lasso tools
      3m 59s
    5. Polygonal Lasso tool and Quick Mask
      5m 19s
    6. Cropping one selection inside another
      6m 15s
    7. Creating rays of light
      4m 44s
    8. Quick Selection and Similar
      4m 11s
    9. Making it better with Refine Edge
      4m 56s
    10. Integrating image elements
      2m 39s
    11. Magic Wand and Grow
      5m 17s
    12. Refine, integrate, and complete
      6m 16s
  10. 53m 49s
    1. Your best face forward
      1m 0s
    2. Content-Aware Fill
      6m 11s
    3. Using the Spot Healing Brush
      5m 36s
    4. The more capable "standard" Healing Brush
      5m 55s
    5. Meet the Clone Source panel
      3m 53s
    6. Caps Lock and Fade
      4m 57s
    7. The Dodge and Burn tools
      5m 1s
    8. Adjusting color with the Brush tool
      6m 35s
    9. Smoothing skin textures
      5m 58s
    10. Brightening teeth
      4m 0s
    11. Intensifying eyes
      4m 43s
  11. 51s
    1. Goodbye
      51s

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