1. Core Concepts of Image Dimensions

Understanding image dimensions requires distinguishing between "virtual screen dimensions" and "real-world print dimensions," which are connected through a specific bridge. Here are the fundamental definitions:

• Nature of Pixels: A pixel is a "picture element" that exists only in digital devices like computers, serving as a virtual unit. Similar to the RGB color mode, pixels are not a measurement standard in real life (for example, you can't describe a person's height in "pixels," but rather in meters, centimeters, or other traditional length units).
• Traditional Length Units: These include millimeters, centimeters, decimeters, meters, kilometers, light-years, etc., used to describe the physical dimensions of real-world objects and serving as the basis for measuring the actual size of printed images.
• Key Contradiction: The pixel dimensions on computers (such as 500×300 pixels) cannot directly correspond to real-world print dimensions (such as 17.64×10.58 centimeters). A conversion relationship through "resolution" must be established, which is the key to understanding image dimensions.

2. Pixel Dimensions: Display Size of Images on Digital Devices

Pixel dimensions determine how images appear on devices like computer screens and mobile phone screens, serving as the core reference in digital scenarios (such as web design):

• Definition and Viewing: Pixel dimensions are determined by the width and height pixel values of an image. In Photoshop, you can directly view this through the menu [Image] - [Image Size], corresponding to the "Pixel Dimensions" option in the interface (for example, an image might show pixel dimensions of 439.5K, with a width of 500 pixels and height of 300 pixels).
• Application Scenarios: This applies only to screen display scenarios such as web banner design, mobile wallpaper creation, and social media graphics. In these scenarios, traditional length units need not be considered, and dimensions can be defined directly according to the pixel requirements of the target device (for example, a common web banner size is 1920×1080 pixels).
• Key Characteristics: Pixel dimensions directly affect screen display clarity. Under the same screen resolution, larger pixel dimensions result in more detailed image display (for example, on a 1920×1080 pixel monitor, a 1920×1080 pixel image can be displayed clearly in full screen, while a 960×540 pixel image becomes blurry when enlarged).

3. Print Dimensions: Physical Output Specifications of Images

Print dimensions refer to the actual physical size when images are output to paper, fabric, or other media through printers or large-format printers, serving as a core indicator in physical designs (such as posters and flyers):

• Definition and Viewing: Print dimensions correspond to the "Document Size" option in Photoshop's [Image Size], with units in traditional lengths (such as centimeters and inches). Specific values can only be determined in combination with resolution (for example, an image of 500×300 pixels has default print dimensions of 17.64×10.58 centimeters, suitable for A4 paper).
• Common Misconceptions: Pixel dimensions should not be directly equated with print dimensions (such as assuming 500 pixels = 17.64 centimeters). There is no fixed correspondence between the two. Images with the same pixel dimensions can output different print sizes by adjusting resolution.
• Media Adaptation: Print dimensions must be determined according to the output media, such as A4 paper (21×29.7 centimeters), A3 paper (29.7×42 centimeters), and large-format advertising (common sizes of 1×2 meters, 2×3 meters, etc.). Media specifications should be clarified during design.

4. Print Resolution: The Bridge Connecting Pixels and Traditional Lengths

Print resolution is the core of converting between "pixel dimensions" and "print dimensions," fundamentally different from "display resolution" (screen pixel density) in Lesson #01:

4.1 Nature of Print Resolution: Pixel Density per Unit Length

• Definition: Print resolution refers to "the number of pixels printed per unit length," determining the clarity of printed images - higher resolution means more pixels per unit length, resulting in finer images; conversely, lower resolution leads to blurrier images.
• Illustrative Analogy (Tree Planting Case): Understanding resolution through "planting trees on a 200-meter street":
  • Street length (200 meters) = Print dimensions (real-world physical length);
  • Total number of trees = Total pixels (image width/height pixel values);
  • Distance between trees (40 meters/50 meters) = Print resolution (number of pixels per unit length).
  • Conclusion: With the same "street length" (print dimensions), smaller "spacing" (resolution) means more "total trees" (pixels); with the same "total trees" (pixels), smaller "spacing" (resolution) results in shorter "street length" (print dimensions).

4.2 Units of Print Resolution: dpi and pixels/cm

• Standard Unit: dpi (pixels/inch): dpi is an abbreviation for "Dots Per Inch," meaning "number of pixels printed per inch," and is the internationally recognized unit for print resolution. This is because imperial units are widely used in displays (such as TV and monitor "inch" measurements) and the publishing industry, facilitating global unified calculations.
• Common Unit: pixels/cm: This aligns with domestic users' length habits and can be manually modified in Photoshop's [Image Size] (for example, 72dpi can be converted to 28.346 pixels/cm). Modifying this does not affect the image's essence, only changing the unit display format.

4.3 Unit Conversion Example

Given a print width of 10 centimeters with a resolution of 80 pixels/cm, calculate the image width in pixels:

1. Conversion logic: Width in pixels = Resolution × Print width;
2. Specific calculation: 80 pixels/cm × 10 cm = 800 pixels;
3. Similarly: If print height is 6 cm, height in pixels = 80 pixels/cm × 6 cm = 480 pixels, resulting in a final image pixel dimension of 800×480 pixels.

5. Conversion Formulas and Practical Examples for Pixels, Resolution, and Print Dimensions

Mastering conversion relationships is a core skill in image design. Below are specific formulas and practical examples, noting the "unit consistency" principle (resolution units and print dimension units must match):

Core Conversion Formulas

1. Print dimensions = Pixel dimensions ÷ Resolution (given pixel dimensions and resolution, find print dimensions)
2. Pixel dimensions = Resolution × Print dimensions (given resolution and print dimensions, find pixel dimensions)

6. Standard Print Resolution for Different Design Scenarios

Resolution selection must match design purposes, as different scenarios have varying clarity requirements. Below are industry-standard guidelines:

• Printing Scenarios (High Clarity Requirements):
  • Standard Resolution: 300dpi;
  • Application Scenarios: Newspaper ads, magazine layouts, book illustrations, photo albums, flyers (requiring close viewing with high detail);
  • Considerations: Resolution below 300dpi will cause printed images to appear jagged or blurry, failing to meet commercial printing standards.
• Printing/Large-Format Printing Scenarios (Medium to Low Clarity Requirements):
  • Standard Resolution: 72dpi;
  • Application Scenarios:
    • Printing: Home/office inkjet printer output (such as document covers, small posters, photo prints, viewed at close distance but not requiring print-level clarity);
    • Large-format Printing: Large-scale advertisements (such as bus stop lightbox ads, street banners, exhibition backdrops, viewed from a distance where 72dpi meets visual needs);
  • Cost Differences: Large-format printing has no minimum cost, suitable for small batches; printing has fixed minimum costs (for example, printing 1000 copies costs 500 yuan, printing 3000 copies costs approximately 1000 yuan), suitable for large orders.
• Screen Display Scenarios (No Need to Consider Print Resolution):
  • Core Reference: Only pixel dimensions need to be considered;
  • Application Scenarios: Web design, mobile wallpapers, social media graphics, UI design;
  • Examples: Web banners commonly use 1920×1080 pixels, mobile wallpapers commonly use 1080×2340 pixels (suitable for most Android phones).