This article will provide a detailed explanation of how to use the "Auto" levels/curves feature in Adobe Photoshop, along with important precautions, and the operation techniques of the black/gray/white point eyedropper tools. It will help users quickly master core skills in image contrast adjustment and color cast correction, suitable for photography post-processing and image optimization scenarios.

1. The "Auto" Button Function in the Curves Adjustment Panel

In Photoshop's curves adjustment panel (accessible through "Image > Adjustments > Curves"), there is an "Auto" button on the right side. Its core function is to expand the contrast of image channels, but attention should be paid to usage scenarios to avoid color casts.

1.1 Basic Functions of the Auto Button

• When an image is in RGB mode: The "Auto" button expands the levels of the R, G, and B channels across the full range, optimizing overall contrast.
• When an image is in CMYK mode: The "Auto" button expands the levels of the C, M, and Y channels across the full range, meeting printing color requirements.
• Important note: If the image channel distribution itself is uneven (such as missing pixels in a single channel), using the "Auto" function may cause serious color casts, requiring histogram analysis before operation.

1.2 Histogram Changes with the Auto Function

Taking an RGB mode image as an example, the histogram differences before and after using the "Auto" button are as follows:

1. Before adjustment: The "peaks" (pixel dense areas) and "valleys" (pixel sparse areas) of the RGB channel histograms are in similar positions, with overall low contrast.
2. After adjustment: The pixels of the RGB channels are stretched to both ends of the levels (0-255), the histogram coverage expands, and contrast significantly improves.
3. Viewing channel changes: In the curves settings panel's "Channel" dropdown menu, you can switch to individual R, G, B channels to directly observe how the "Auto" function modifies each channel's curve.

2. Auto Color Correction Options Settings

If the adjustment results of the "Auto" function don't meet expectations, you can customize the algorithm through the "Options" button to ensure precise effects.

2.1 Opening Auto Color Correction Options

1. After entering the curves adjustment panel, click the "Options" button on the right to open the "Auto Color Correction Options" dialog box.
2. The dialog box contains two core setting modules: "Algorithms" and "Target Colors and Clipping".

2.2 Algorithm Selection and Default Settings

The "Algorithms" module provides 4 adjustment logics, with "Enhance Per Channel Contrast" recommended for beginners:

• Enhance Per Channel Contrast: Stretches the level range of each channel individually. This is the basic algorithm of the "Auto" function, suitable for most regular images.
• Enhance Monochromatic Contrast: Adjusts based on image brightness rather than individual channels, avoiding color casts but with weaker contrast enhancement.
• Find Dark and Light Colors: Automatically identifies the darkest and brightest pixels in the image, using them as benchmarks to stretch levels, suitable for images with unclear light-dark boundaries.
• Align Neutral Midtones: Corrects color casts by identifying neutral gray pixels, suitable for scenes with slight color shifts.

Recommended setting: After selecting "Enhance Per Channel Contrast", check "Save As Defaults" at the bottom of the dialog box. This will apply the algorithm by default in future use of the "Auto" function without repeated settings.

2.3 Target Colors and Clipping Settings

"Target Colors and Clipping" controls the pixel retention range of shadows and highlights. Default parameters are suitable for most scenarios:

• Shadow clip: Default 0.10%, meaning 0.10% of shadow pixels are allowed to become pure black to avoid excessive loss of shadow details.
• Highlight clip: Default 0.10%, meaning 0.10% of highlight pixels are allowed to become pure white to balance highlight brightness and details.
• Midtones: Default neutral gray (equal RGB values), requiring no frequent modifications unless intentionally adjusting the image's light-dark tendency.

3. Use Cases and Common Issues with the Auto Function

Through practical case analysis of the "Auto" function's applicable scenarios and potential issues, this helps users avoid operational mistakes.

3.1 Color Cast Case in Sky Image Auto Adjustment

Using a "sky + ground" image as an example to demonstrate issues with the "Auto" function:

1. Operation steps: After opening the image, execute the levels command (shortcut: CTRL+L), then click the "Auto" button.
2. Problem phenomenon: After adjustment, the image shows serious color casts - shadow areas (such as ground shadows) become darker in color, highlight areas (such as clouds) lose blue, and the overall tone becomes grayish.
3. Cause analysis:
   • Before adjustment: Checking the image channel histogram reveals that shadow areas only contain red channel pixels, with no green or blue channel pixel distribution (meaning shadows lack green and blue light components).
   • After adjustment: The "Auto" function forcibly stretches green and blue channel pixels to the shadow areas, causing abnormal shadow colors; simultaneously compressing blue channel highlight pixels, making clouds lose their blue luster.

3.2 Key Differences Between Luminosity Levels and RGB Levels

The core misunderstanding in the above case is confusing "luminosity levels" with "RGB levels", with their differences directly affecting image adjustment effects:

• RGB Levels: Only reflect the pixel distribution of individual R, G, B channels, unable to represent the final light-dark perception of the human eye (such as red channel shadow pixels being dense but with extremely low brightness, invisible to the human eye).
• Luminosity Levels: Based on the overall brightness calculation after RGB tricolor light mixing, accurately reflecting the actual light-dark distribution of the image (in the case, the pre-adjustment luminosity histogram showing no shadow pixels proves the image has no real shadows).
• Important conclusion: When judging the actual light-dark degree of an image, use the "luminosity level histogram" as the standard; when adjusting individual channel colors, refer to the "RGB level histogram". Both should be used together.

3.3 Further Adjustment Space After Using the Auto Function

The "Auto" function only modifies the curves of individual R, G, B channels without affecting the RGB composite channel curve. Therefore, further manual adjustments can be made after using "Auto":

• For slight color casts: Switch to the corresponding channel (adjust the green channel if it's greenish) and fine-tune the curve's shadow/highlight sections.
• For insufficient contrast: In the RGB composite channel, slightly raise the curve's highlight end and lower the shadow end to enhance overall layering.

4. Black/Gray/White Point Eyedropper Tools in the Curves Panel

Three eyedropper tools located at the bottom right of the curves adjustment panel are used for "Set Black Point," "Set Gray Point," and "Set White Point." These are core tools for correcting color casts and defining light-dark benchmarks. The principle is to define the clicked pixel as pure black, pure gray, or pure white, automatically adjusting each channel's curve.

4.1 Basic Functions and Operation Logic of the Tools

4.2 Practical Application of the White Point Eyedropper Tool (Color Cast Correction Case)

Using the "Island Road Leading Sheep" image as an example to demonstrate how the white point tool corrects greenish tones:

1. Image problem: The original image is generally greenish, with roads and wool showing green tendencies that need to be restored to natural colors.
2. Operational premise: Based on common sense, "the sheep's head wool should be pure white," using this as the white point benchmark.
3. Specific steps:
   • Open the curves adjustment panel and select the "Set White Point Eyedropper".
   • Click the eyedropper on the sheep's head wool area. Photoshop will automatically adjust the R, G, B channel curves.
   • Effect verification: After adjustment, the wool returns to pure white, the greenish road disappears, and color accuracy significantly improves (observable through the RGB histogram showing the red channel's stretching effect).
4. Important note: Avoid "highlight clipping"
   • Highlight clipping definition: If the "white area" clicked has insufficient brightness, the tool will forcibly stretch that area and surrounding pixels to pure white, causing detail loss (such as wool texture disappearing).
   • Prevention method: Before clicking, zoom in on the image and select areas with higher brightness and clear details (such as the bright parts of wool rather than dark parts), observing whether there are "blown-out" areas after clicking.

4.3 Usage Risks and Principles of the Gray Point Eyedropper Tool

The uniqueness of the gray point eyedropper tool lies in "removing color cast," and improper use can cause serious color shifts, requiring careful operation:

1. Negative examples:
   • Clicking "green leaves": The tool will define green as pure gray. To neutralize green, it will automatically add "green's complementary color (magenta)," causing the entire image to lean toward magenta.
   • Clicking "blue mountains": The tool will define blue as pure gray. It will automatically add "blue's complementary color (yellow)," causing the entire image to lean toward yellow.
2. Core principle: Pure gray has equal RGB values (such as 128,128,128). When clicking non-gray areas, Photoshop will "add complementary colors and reduce target colors" to convert that area to pure gray, thus affecting overall color balance.
3. Usage recommendations:
   • Only use in scenarios where "neutral gray position is clearly known" (such as gray card areas in photography).
   • When uncertain, prioritize using white point/black point tools to avoid disrupting image colors due to gray point settings.

4.4 Using the Black Point Eyedropper Tool

The operational logic of the black point eyedropper tool is consistent with the white point tool, with the core being "defining the image's shadow benchmark":

1. Operation steps:
   • Open the curves adjustment panel and select the "Set Black Point Eyedropper".
   • Click on areas in the image that "should be pure black" (such as dark objects in shadows or areas without light).
   • Effect: The tool stretches the target area to pure black (RGB: 0,0,0) while stretching each channel's curve to enhance shadow contrast.
2. Important notes: Avoid losing shadow details. Don't click on "blown-out black areas" (pure black pixels without details), as this will further compress shadows and lose layering.

5. Curve Operation Reset Techniques

If black/gray/white point settings or "Auto" function adjustments are incorrect, there's no need to exit the curves adjustment panel. Quick resets can be performed through keyboard shortcuts to improve operational efficiency.

5.1 Reset Operation Steps

1. Keep the curves adjustment panel open.
2. Hold down the "ALT" key (Windows system). The "Cancel" button in the dialog box will automatically change to a "Reset" button.
3. Click the "Reset" button to undo all operations since entering the curves settings panel (including "Auto" adjustments, black/gray/white point settings, etc.), restoring to the pre-adjustment state.

5.2 Extended Applicable Scenarios

This reset technique applies not only to the curves adjustment panel but also to all setting dialog boxes with a "Cancel" button in Photoshop, such as:

• Image Size dialog box ("Image > Image Size")
• Hue/Saturation dialog box (shortcut: CTRL+U)
• Levels dialog box (shortcut: CTRL+L)
• Practical value: Avoid repeatedly opening dialog boxes due to operational mistakes, reducing repetitive work.

6. Photoshop Default Black/Gray/White Point Values and Customization Considerations

Photoshop sets default values for black/gray/white points to ensure color adjustment consistency. When customizing, follow color logic to avoid disrupting subsequent operational effects.

6.1 Default Value Standards

Photoshop's default black/gray/white points are all "grayscale" (equal RGB values), with specific values as follows:

• Default black point: RGB (0, 0, 0) - Pure black, no light reflection.
• Default gray point: RGB (128, 128, 128) - Neutral gray, medium brightness, no color tendency.
• Default white point: RGB (255, 255, 255) - Pure white, maximum light reflection.

6.2 Methods for Customizing Black/Gray/White Points

If custom values are needed based on special requirements (such as adapting to specific output devices), follow these steps:

1. In the curves adjustment panel, double-click the eyedropper icon for "Set Black Point," "Set Gray Point," or "Set White Point."
2. The "Color Picker" dialog box will appear. Enter target values in the "RGB" value boxes (in RGB mode, ensure all three values are equal, otherwise it will cause color shifts in subsequent adjustments).
3. Click "OK" to complete the custom settings.

6.3 Customization Considerations

• Consistency principle: If the white point is set to "reddish" (such as RGB: 255,240,240), all subsequent images using the white point tool will lean toward red. It's recommended to modify temporarily only for specific projects, not recommended for long-term use of non-default values.
• Save prompt: After customizing black/gray/white points, when confirming curve operations, Photoshop will prompt "Save as default values." If you choose "No," the values will automatically revert to original defaults after this operation. If you choose "Yes," note that all subsequent adjustments will use the new values.
• Recommended approach: Beginners should retain default values to avoid color system confusion due to customization. Professional scenarios can temporarily adjust according to output requirements (such as printing, screen display).