Digital Imaging

Week 2: Adjusting Color

Lesson Objectives

After this lesson, students will be able to

  • Determine how the Histogram reflects the brightness and color in an image
  • Use Levels and Curves adjustments to manipulate brightness and color in an image
  • Use Adjustment Layers to modify or enhance an image


No matter what professional digital image editing software you use, you will encounter histograms, thus understanding how to read a histogram and why to use manual color and brightness adjustment methods such as the “Levels” and “Curves” tools in Photoshop is essential. This week we will investigate these topics in-depth.

Note: there are “Auto’ enhancements available in Photoshop and also many cell phone apps, but they don’t always give you desirable results and are often not part of the professional toolbox because you don’t have fine control over the output.

This lesson shows how to adjust photos manually with the two techniques and will also focus on using more Adjustment Layers.

How to Read Histograms

A histogram is the graphical representation of the tonal values of your image from black to white.Expressed numerically, the values range from 0 to 255 this is because 8-bit images (a standard jpg) contains 256 possible brightness levels but computers like to start counting from zero. You may see them in your camera and in all professional image editing software.

There is no incorrect histogram pattern in an image but it can tell you a lot about your image. It can be used to discover whether you have captured all the detail in the dark or light areas of an image.

The dark tones are represented on the left side of the histogram and light tones on the right.  The middle represents the mid-tones of the image.  The vertical axis of the graph represents how many tones are present in the image for that value.

In the cat image below, most of the photo is very dark. The histogram shows that more pixels are represented in the dark areas while very few pixels are present in the lighter area and none are present at the far right, white. Even the white areas of this photos are darker than 100% white. No doubt this is what the photographer intended to do in this case.

A photographer might use the histogram feature on their camera to quickly determine whether the shot they are taking is over or under exposed. This also helps us with image editing because sometimes it’s hard to tell by the eye. The under exposed image does not have good detail in the shadows and the histogram is piled up on the left. The over exposed image has blown out highlight areas and the histogram is piled up on the right.

Drag the slider left and right on the image below.

If the value area touches either the right or the left edges, it is called either shadow or highlight clipping and there is image detail that may have been lost in these areas of the image.   The dark cat image above shows shadow clipping (on the left side of the histogram), for example and the image below shows more shadow clipping and some highlight clipping (on the right side of the histogram).    This can usually be fixed by adjusting the exposure, making dark images a bit lighter for example but it may be the look you are intending.

Generally, a well-exposed image will display a histogram showing a full, continuous range of brightness levels from black to white without much clipping. So you can use it as a way to spot potential problems that your eye may not immediately see.  Again, this also may be part of the aesthetic value of the image so it depends on what you want.

Histograms display information for each color as well and every color in your image has it’s own level of brightness or luminosity. Yellows are brighter than Reds, for example.  In this case, the RGB histogram shows red green blue.   You can see that in the image below, the Blue areas are brighter than the Red areas because they are further to the right on the histogram.

The Histogram Viewer is found under the Windows menu in Photoshop

The Histogram can also give us information about the quality of an image, sometimes hinting at whether it has been adjusted properly.

Photoshop Levels vs Curves - What is the difference?

Essentially, the Levels and Curves tools perform very similar functions such as:

  • Adjustments on a RGB histogram
  • Set black, white, and gray point for the image
  • Adjust the zero point for color intensity
  • Adjust the max point for color intensity
  • Adjust contrast, brightness and color intensity
  • Set blackest black and whitest white values for image
  • Use presets for quick adjustments.

So, that’s confusing, if they function the same, what’s the difference and why use each one? Watch this video about the difference between Levels and Curves in Photoshop. Skip to about 1:09 if you like to get to his demonstration.

Adjusting Color

Before we get started, it might also be helpful to understand a little bit about how the numbers work for the additive color model RGB. Watch this 30 second video.

Watch this tool demo on using Levels and Curves in Photoshop.

Additional Resources & Activities

If after watching the lesson videos, you still have questions about histograms and Photoshop Adjustment Layers, please have a look at these resources:

Further Reference




Download the following images by clicking on them to open in a new tab and right-clicking to save image onto your computer.

Image 1


Image 2

Image 1 Instructions

On the first image, you will use a Levels Adjustment   to correct the brightness and color.

Save your image as a Photoshop file (.psd), with the title of   YourName-Week2-1.psd and upload it to your Google Drive folder for this class. Do not flatten layers.

Image 2 Instructions

On the second image, you will use a Curves Adjustment   to correct the brightness and color.

Save your image as a Photoshop file (.psd), with the title of   YourName-Week2-2.psd and upload it to your Google Drive folder for this class. Do not flatten layers.

Goals for Image 1 and Image 2 Adjustments

Your goals for both images are to:

  1. make the whites look white without going too bright (should see details and shadows still)
  2. make the colors look natural on the objects – not too bright or dark, good saturation and variation
  3. retain the details on the objects in the image
  4. if details are not present in some areas, bring up the detail (clouds in Image 2)
  5. if areas are too dark, make them lighter to show more detail (flowers in Image 2)
  6. correct any color cast (warm or cold)

Note: Do not use any other tools or techniques (no paint brush, lasso, burn or dodge – use of these will reduce your grade).


Tip #1 – The “Info” Panel

If you want to make sure an area is “white’ or “gray’ with no color cast, use the “Info’ window and check the RGB values of the pixel colors. (Window menu -> Info). This takes out guess work and also influence from your display’s color shift and environment, like colors of your room light and computer wallpaper colors.

Move the RGB 3 values closer to each other, like 209, 211, 203. A closer pixel value combination represents more neutral colors like white or gray, while values like “200, 151, 230’ indicate stronger color cast. So aim to get more neutral colors on areas that should be white or grey.

The info window works for both “Levels’ and “Curves’ so keep it open for both Images. Make sure to move your cursor around to measure values of multiple pixels, not only one pixel in one spot. Because the photo is grainy and has color noise, some pixels may show unexpected colors.

However, don’t obsess with the RGB values. It’s impossible to get perfect neutral values, like “214, 214, 214’.

Tip #2 – Actual Value

If you have trouble adjusting the Levels sliders, try clicking on the actual value and use the up and down arrow keys on your keyboard. This will let you change the value in small increments.

Journal - Blog Post

  1. Create a new post  on this course website
  2. Upload both JPGs of the Exercise into your post
    (remember, to create a JPG, use the Top Menu -> Export ->Save for Web (Legacy) option with JPG, 100% quality)
  3. Give your post the  category of “Week 2’
  4. Reflection   — Write a short reflection on the process you used to adjust your images.   Was it easy? Hard? Why?   Did you try any other adjustments to get the look you want?

Note: Put your Reflection and Exercise Week 2 JPEG on the same post.

See also  Grading Criteria  for Journal Posts.


View other student’s posts from last week (Week 1 Part II). Comment freely.

See also  Grading Criteria  for comments.

Oral Presentation #1

Your Oral Presentation #1 is also due this week. Please follow the instructions on the Oral Presentation #1 page.

Further Reference

Why create an introduction video other than for this class? What’s the point to this practice?