Among Photoshop's numerous color adjustment tools, the Curves tool stands as the most fundamental and frequently used core instrument. Other adjustment tools like Brightness/Contrast are largely derived from it. Understanding the principles and operation of the Curves tool enables you to master more color adjustment commands through analogy, forming a key foundation for enhancing your PS image editing skills.

1. Preliminary Observation of Image Brightness Distribution

Before using the Curves tool, it's recommended to first observe the brightness distribution of the image, identifying the locations of shadow, midtone, and highlight areas to provide a basis for subsequent adjustments:

1. Open the target photo (refer to the tutorial's accompanying sample image).
2. Temporarily convert the image to grayscale mode (after viewing, press CTRL + ALT + Z to undo the conversion to avoid damaging the original).
3. Identify brightness areas:
   • Shadow areas: Such as nearby mountains with lower brightness values;
   • Highlight areas: Such as the sky with higher brightness values;
   • Midtone areas: Such as distant mountains with brightness between shadows and highlights.

2. Invoking the Curves Command and Interface Analysis

2.1 Methods to Access the Curves Command

Two methods can be used to open the Curves adjustment window:

1. Menu path: Click the top menu bar "Image" → "Adjustments" → "Curves";
2. Shortcut key: Directly press CTRL + M (Windows system) for efficient command invocation.

2.2 Core Elements of the Curves Interface

After opening the Curves window, the core interface elements and their meanings are as follows:

• Channel Options: The "Channel (C)" at the top of the window defaults to "RGB", which can be switched later according to single-color channel adjustment needs (such as R, G, B channels);
• Baseline Curve: The 45° inclined line segment displayed by default in the window is the "curve", representing the brightness mapping relationship of the unadjusted image;
• Gradient Bar Meaning:
  • Bottom gradient bar: Represents the "absolute brightness range", where all pixel brightness values are distributed between 0 (pure black) and 255 (pure white). The default is "black on left, white on right", which can be reversed for highlights and shadows using the bidirectional arrow in the middle (it's recommended to maintain the default for consistency);
  • Left gradient bar: Represents "brightness change direction", where moving any point on the curve upward "brightens" and downward "darkens", with the brightening limit at 255 and darkening limit at 0;
• Function Buttons:
  • Preview (P): When checked, you can view the adjustment effects in real-time. Press the ALT key to quickly enable/disable preview (only valid when the Curves window is open);
  • Load (L) / Save (S): Can load preset curve parameters or save the current adjusted curve configuration for future reuse;
  • Smooth (M): Optimizes curve transitions to avoid image banding after adjustment;
  • Auto / Options: Provides automatic adjustment functions and more advanced settings (such as black/white point definition).

2.3 Curve Analogy Understanding: Connection with Audio Equalizer Adjustment

To quickly understand the logic of how curves work, we can draw an analogy with the "tone adjustment" function of daily-used music playback software or stereo systems:

• Audio adjustment: The left end controls bass (corresponding to the shadow area of the curve), the right end controls treble (corresponding to the highlight area of the curve), moving upward enhances the corresponding frequency band, and moving downward weakens it;
• Curve adjustment: The left end corresponds to image shadows, the right end corresponds to image highlights, moving the curve upward enhances corresponding brightness, and moving downward weakens corresponding brightness - the logic is completely the same.

3. Practical Curve Adjustment and Brightness Change Principles

3.1 Basic Adjustment: Moving the Curve Upward to Brighten the Image

1. Ensure the "Preview" option in the Curves window is checked;
2. Click on the 45° baseline curve to generate a "control point" (it's recommended to click in the middle of the curve, prioritizing midtone adjustment);
3. Drag the control point upward to the target position, at which point the image will significantly brighten;
4. Compare effects: By checking/unchecking "Preview", you can intuitively view the differences before and after adjustment.

3.2 The Deeper Principles of Brightness Change

Why does moving the curve upward brighten the image? Analyzing the key points on the curve (assuming points a, b, c):

• Point a (shadows): Located on the left side of the curve, corresponding to darker pixels in the image;
• Point b (midtones): Located in the middle of the curve, corresponding to pixels with medium brightness in the image;
• Point c (highlights): Located on the right side of the curve, corresponding to brighter pixels in the image.

After adjustment, points a, b, and c all move upward along the Y-axis (brightness axis), but with different movement distances:

• The movement distance of midtone point b is the largest, with the most obvious brightening;
• The movement distances of shadow point a and highlight point c are smaller, with weaker brightening;
• The endpoints of the curve (pure black 0, pure white 255) do not move. If the original image has no pure black/pure white pixels, they still won't appear after adjustment (pure white/pure black can appear by increasing the adjustment amplitude).

4. Application of the Color Sampler Tool and Info Panel

To accurately verify brightness adjustment effects, the "Color Sampler Tool" and "Info Panel" need to be combined for data comparison:

4.1 Tool and Panel Invocation

• Info Panel: Press F8 to open, switch to "HSB" mode (in HSB, "B" represents brightness, making it convenient for intuitive viewing);
• Color Sampler Tool: Press the I key (or SHIFT + I to switch), icon is "☆", needs to be distinguished from the Eyedropper tool (icon is "□"):
  • Tool hotspot: Located at the eyedropper tail (lower left corner of cursor), if not comfortable, press CapsLock to switch to crosshair cursor;
  • Sampling limit: A maximum of 4 sampling points can be set at one time, existing sampling points can be dragged to adjust positions.

4.2 Practical Verification: Brightness Data Comparison

1. Mark 4 sampling positions on the original image: 1, 2 for highlight areas, 3, 4 for midtone areas (if shadows need to be measured, move points 3, 4 to shadow areas 5, 6);
2. Use the Color Sampler Tool to click points 1, 2, 3, 4, the Info Panel will display the brightness values (B values) of each point;
3. Compare brightness data before and after adjustment (example):
   • Highlight areas (1→2): Brightness increased from 87% to 95%, an increase of about 8%;
   • Midtone areas (3→4): Brightness increased from 55% to 75%, an increase of about 20%;
   • Shadow areas (5→6): Brightness increased by about 8%, similar to highlight areas.

The verification results are consistent with the curve principle: Midtone brightness increase is the largest, while shadow and highlight increases are smaller.

5. Dynamic Comparison of the Info Panel and Tone Level Conversion

5.1 Dynamically Viewing Pre- and Post-Adjustment Values

During curve adjustment, move the mouse to any position on the image, and the Info Panel will display two sets of values (format "Before Adjustment / After Adjustment"), for example:

• At coordinates (489,27): Brightness increased from 87% (before adjustment) to 100% (after adjustment), making this position a pure white pixel. This dynamic comparison function applies to all color adjustment tools. It's recommended to place the Info Panel at the corner of the interface for convenient real-time viewing.

5.2 Brightness Percentage and Tone Level Conversion

The "B value" in HSB mode of the Info Panel is displayed as a percentage, but actually corresponds to Photoshop's "Levels" (based on 256 levels of brightness, with a maximum value of 255). The conversion formula is: Level Value = 256 × Brightness Percentage

Example conversion:

• Brightness 50% → Level Value = 256 × 50% = 128;
• Brightness 71% → Level Value ≈ 256 × 71% = 182 (approximate value).

5.3 Input/Output Values of the Curves Window

After adding a control point to the curve, the "Input" and "Output" values will be displayed at the bottom of the window:

• Input: Represents the pre-adjustment level value;
• Output: Represents the post-adjustment level value;
• Dynamic value restoration: To re-view dynamic values when moving the mouse, click the blank area outside the curve region (avoid clicking near the curve to prevent accidentally adding control points).

6. Effect of Curve Dropping: Image Darkening Principle

After understanding "curve upward brightening", the reverse logic is equally applicable: If you drag the control point on the curve downward, the image will darken overall.

• Darkening patterns are consistent with brightening: Midtones have the largest darkening amplitude, while shadows and highlights have smaller darkening amplitudes;
• The endpoints of the curve (pure black 0, pure white 255) still don't move. If the original image has no pure black pixels, large downward adjustments can make shadow areas become pure black.

Summary

The Curves tool is the "core cornerstone" of Photoshop color adjustment. Its essence is adjusting image brightness and darkness by changing the brightness mapping relationship (0-255 levels). Combined with the "Color Sampler Tool" and "Info Panel", adjustment effects can be precisely controlled, avoiding over-brightening/darkening issues caused by operating based on feeling. After mastering the content of this article, you can further explore curve adjustments of single-color channels (R, G, B) to achieve more precise color correction.