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Understanding the histogram

From: Inkjet Printing for Photographers

Video: Understanding the histogram

So this image here has a bunch of pixels in it, and each one is colored a shade of gray. If I zoom in here, we can actually see individual pixels. Now, some of these pixels might be the same color. There might be a black pixel here and a black pixel there. But because they are spread all over, it's difficult for us to see how many of each there might be. Well, why do we care? you might be thinking. It just looks like the image. We care because a good print has certain characteristics. Among those, it has blacks that are truly black, it has whites that are truly white, and if we don't have those things, we're going to get a weaker print.

Understanding the histogram

So this image here has a bunch of pixels in it, and each one is colored a shade of gray. If I zoom in here, we can actually see individual pixels. Now, some of these pixels might be the same color. There might be a black pixel here and a black pixel there. But because they are spread all over, it's difficult for us to see how many of each there might be. Well, why do we care? you might be thinking. It just looks like the image. We care because a good print has certain characteristics. Among those, it has blacks that are truly black, it has whites that are truly white, and if we don't have those things, we're going to get a weaker print.

But, you might be thinking, I can see those on screen. There is some, some black right there. The thing is, black and white are actually specific quantifiable phenomena. The tone might look black onscreen; it might not actually be true black. It might simply look black because of the way your monitor is adjusted, or it might look black to you because your eye is not yet trained enough to recognize the difference between true black and a really dark gray, and even a tiny difference can have a big impact on your print. So, since you can't trust your monitor and maybe you can't trust your eyes, it's critical that you turn some analysis tools on your image, and the one that's going to be the most useful is the histogram.

As I said before, I have a lot of different tones in the image, but I don't know what they are specifically or how many of each there are. The histogram shows me that in a very simple way. A histogram is simply a bar chart showing the distribution of tones in an image. I'm going to jump into Photoshop here and fire up its histogram display, which is right here. You should already be familiar with this if you've been using Photoshop a lot; if you're not, you can get it up here from the Window menu. There is a Histogram option.

So this is the histogram. Again, it's just a bar chart showing the distribution of tones in my image. Black is over here on the left, white is on the right, and everything in between is a shade of gray. So what I'm seeing from my histogram here is that I have a lot of black in my image and I've a lot of dark gray, so black is probably some of this stuff in here. This is also a lot of the dark gray all through here. I have some lower middle tones in here. That's going to be all this gray in the hand.

I have a big spike on this side. That means that I have actually clipped to the shadows or underexposed the shadows completely; some of them have gone out to complete black. That's okay. I like having this really dark stuff in the image. I also have over here a little spike on the right. This is white right here, and what this indicates is that there are some pixels that have blown out to complete white, and that's probably this highlight area right here or this-- this was actually a view out of the window or out of door. Probably some of these. And I have very little white tones in here.

Middle gray in the image is going to be about right here. The histogram is again a statistical analysis tool showing me the distribution of tones, and what I can see is that the image is trending in general to being lower than middle gray. So this is a dark image. It's got some overexposed white, but it has otherwise very few light tones in it. Now, I want to you give you a quick little thought experiment here. I can flip this image in Photoshop. There is a simple command that will give me a mirror image of this particular image.

Think for a moment. If I do that--and I am going to do it here in a second--if I do that, what's going to happen to the histogram? Is it going to stay the same, is it going to change? If it's going to change, how is it going to change? So I'm going to do that flip right now. I'm going to go up here to Image > Image Rotation > Flip Canvas Horizontal. And keeping an eye on my histogram here and my hands, notice that no time are they leaving my arms. And as I flip the image, my histogram doesn't change at all. If that doesn't make sense to you then you're not quite getting the histogram. The reason it doesn't change at all is because there's no actual correspondence between anything in the histogram and any specific geographic location in the image.

The histogram is simply a graph of the distribution of tones in the image, and flipping the image does not change that distribution. It doesn't add more black. It doesn't add more white. It doesn't lighten anything. So my histogram should stay the same. I'm going to Undo that and again, my histogram doesn't move. In Photoshop, you'll sometimes in the Histogram palette see this little exclamation mark here. That simply means that the histogram has not necessarily been updated to be completely accurate. If I click on it--there we go-- I've seen a few changes. It turns out there is a little more overexposure than I thought, and then the exclamation mark goes away.

There's no correct, desirable shape to a histogram. There are things in the histogram that may not be best for your particular image, but you don't shoot to get a bell curve or an image that looks like a duck or anything like that. The histogram is simply a reflection of what's in your image. Now I promise you, if you don't follow the histogram, you are simply not going to get good prints. If you're thinking, oh the histogram, that's one of those fancy high-end technical things, you know, I'll just do it by eye, that's easier, you cannot just do it by eye. It's that simple. You're not going to get prints as good if you don't follow the numbers.

Now you can do a print, see if it looks okay, and then make adjustments and do another, but you're going to go through a lot of expensive ink and paper that way. If you instead use the histogram, because the histogram is the key to getting predictable results, then you'll reduce and maybe eliminate a lot of those test prints that you need to do if you're simply winging it. Understanding the types of edits that you need to make will also get easier if you understand the histogram. Another thing you're going to need to know throughout this course is how to make localized adjustments.

If you look here, we've got some highlights in here that are probably showing up somewhere in here. We're very likely going to want to be able to adjust those independent of the rest of the image, and the mechanism we're going to use for that is an adjustment layer and its corresponding layer mask, and that's going to all show up over here in the Layers palette. If you are not comfortable with adjustment layers and layer masks, then go back to the lynda library. There is lots of Photoshop instruction that's going to teach you how to do that. One thing you may not quite know yet, even if you're familiar with adjustment layers and layer masks, is that when you make an adjustment layer and a corresponding mask, the histogram that you're seeing up here changes.

I'm going to quickly create a little mask here, just of the fingers, and this should be familiar to you. As I said, if you don't know how to do this, then you need to find some other instruction in the library. And now look at what's happening to my histogram display here. This histogram looks different than this histogram. This histogram is a histogram of the final image. This histogram that's showing up in my Adjustments palette here--I created a Levels adjustment layer-- this histogram is showing me only the histogram of the unmasked area.

So this is a histogram of just these bits on the fingers here. This becomes very, very useful as you're making selective adjustments for printing, because I can see precisely that this image that I've masked right here--or rather, this image that's unmasked right here, these bits of the fingers--don't have any true black. And my white adjustment there was a little extreme. So understanding that the histogram that you see here is a histogram of only the unmasked area is going to make it much easier for you to get the white and black correct for print in each little simple masked area that you create.

This image needs some work, and we're going to see the actual author of this image, the girl who shot this image, go through that work here in the next few movies.

Show transcript

This video is part of

Image for Inkjet Printing for Photographers
Inkjet Printing for Photographers

68 video lessons · 13540 viewers

Ben Long
Author

 
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  1. 9m 18s
    1. Welcome
      1m 50s
    2. Exploring why we print
      4m 3s
    3. Understanding what you need for this course
      3m 25s
  2. 13m 29s
    1. Why inkjet printing?
      4m 36s
    2. Understanding ink types: Dye vs. pigment
      4m 26s
    3. Discussing considerations for black and white
      1m 48s
    4. Reviewing the features
      2m 39s
  3. 1h 1m
    1. Printing and your workflow
      3m 3s
    2. Printing black-and-white photos
      6m 49s
    3. Understanding the histogram
      7m 37s
    4. Understanding what localized adjustments are used for
      2m 38s
    5. Explaining the histogram with a practical example
      6m 51s
    6. Making a localized adjustment in a practical example
      5m 30s
    7. Evaluating a localized adjustment in a practical example
      2m 29s
    8. Refining a localized adjustment for effect
      13m 36s
    9. Making a gradient adjustment
      6m 47s
    10. Paying attention to viewing conditions
      4m 49s
    11. Summing up
      1m 50s
  4. 54m 36s
    1. Understanding pixels, printer dots, and resolution
      2m 44s
    2. Understanding resolution
      2m 33s
    3. Defining resampling and interpolation
      3m 41s
    4. Understanding where resizing fits into your workflow
      2m 12s
    5. Defining native printer resolution
      2m 39s
    6. Understanding the relationship between viewing distance and print size
      2m 1s
    7. Reducing image size in Photoshop
      9m 11s
    8. Cropping to a specific size and resolution using Canvas Size
      4m 34s
    9. Cropping to a specific size and resolution using the Crop tool
      5m 15s
    10. Enlarging an image in Photoshop
      7m 7s
    11. Creating a triptych
      3m 55s
    12. Creating a triptych using Automator on a Mac
      4m 5s
    13. Exploring the aesthetics of print size
      4m 39s
  5. 1h 17m
    1. Understanding how sharpening works
      3m 18s
    2. Sharpening in JPEG mode
      1m 26s
    3. Exploring sharpening workflows
      3m 47s
    4. Sharpening in Camera Raw
      6m 17s
    5. Looking at noise reduction
      1m 46s
    6. Sharpening output with Smart Sharpen
      11m 52s
    7. Understanding selective sharpening
      4m 25s
    8. Sharpening through an edge mask
      7m 17s
    9. Reviewing high-pass sharpening
      4m 30s
    10. Applying aggressive sharpening
      8m 53s
    11. Exploring advanced sharpening techniques
      9m 7s
    12. Exploring the Print dialog
      11m 35s
    13. Proofing at smaller sizes
      3m 3s
  6. 53m 9s
    1. Exploring how color works
      2m 5s
    2. Reviewing color models
      2m 56s
    3. Defining gamut and color space
      9m 55s
    4. Reviewing when colors go out of gamut
      4m 54s
    5. Configuring Photoshop's color settings
      5m 47s
    6. Changing color space in Camera Raw
      4m 7s
    7. Working in an advanced color space
      6m 13s
    8. Assigning a color space in Photoshop
      2m 20s
    9. Correcting a color image
      9m 17s
    10. Printing a color image
      3m 30s
    11. Evaluating the print
      2m 5s
  7. 34m 46s
    1. What is color management?
      4m 16s
    2. Profiling a monitor
      8m 45s
    3. Evaluating a monitor profile
      4m 37s
    4. Exploring paper profiles
      5m 17s
    5. Understanding soft proofing
      11m 51s
  8. 24m 33s
    1. Understanding how paper quality affects the appearance of black in prints
      3m 26s
    2. Looking at third-party papers
      3m 46s
    3. Looking at paper finish
      3m 44s
    4. Understanding paper traits
      6m 31s
    5. Discussing paper choice and presentation
      7m 6s
  9. 23m 18s
    1. Printing a black-and-white image
      11m 45s
    2. Printing a color image
      11m 33s
  10. 1m 16s
    1. Goodbye
      1m 16s

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