Image Resolution Explained: Finally Understanding What Those Numbers Mean

Resolution confuses absolutely everyone.

Pixels, PPI, DPI, megapixels...

Let me explain it with a story.

---

The Short Version

Screen display: Only pixels matter, DPI is irrelevant.

Print: DPI is everything.

记住了（记住这句话）.

---

Basic Concepts

Pixel

The smallest unit of an image. Each pixel is a tiny colored square.

1920×1080 pixels = 2.07 million pixels

---

Resolution

Width × height.

Common Resolutions	Pixel Count	Description
720p	1280×720	HD ready
1080p	1920×1080	Full HD
2K	2560×1440	2K display
4K	3840×2160	Ultra HD

---

DPI / PPI

• DPI = Dots Per Inch (for printing)
• PPI = Pixels Per Inch (for screens)

Screens only care about PPI. DPI on screens is basically just marketing talk.

---

The 72 DPI Myth

You might have heard "screens are 72 DPI."

That's outdated information.

Modern screens:

• iPhone: 460 PPI
• MacBook Pro: 227 PPI
• 27" 4K monitor: 163 PPI

"72 DPI is for screens" was a rule from the 1984 Mac. It's been obsolete for decades.

---

Screen Image Sizes

By Use Case

Use Case	Max Width	Typical Size
Large banner	1920px	1920×1080
Blog header	1200px	1200×630
Content image	800px	800×600
Thumbnail	400px	400×300
Avatar	150px	150×150

---

Print Image Sizes

By Print Size (300 DPI)

Print Size	Minimum Pixels	Recommended Pixels
4×6"	1200×1800	2400×3600
5×7"	1500×2100	3000×4200
8×10"	2400×3000	3600×4500
A4	2480×3508	3508×4961

300 DPI is the print standard.

---

Common Mistakes

Mistake 1: Thinking Enlarging Improves Quality

Problem: Taking a small image and stretching it, thinking it'll get clearer.

Truth: Enlarging just makes each pixel bigger. No new information is added.

Solution: Accept reality. Start with a large enough image in the first place.

---

Mistake 2: Calculating Print Size Wrong

Problem: Want to print 8×10" but image is only 1000×1500 pixels.

Calculate:

8×10" at 300 DPI needs: 2400×3000 pixels
Your image: 1000×1500 pixels
Result: Will be blurry when printed

Solution: Calculate required pixels before printing.

---

Mistake 3: Not Understanding Aspect Ratios

Problem: Image ratio doesn't match print size, gets cropped.

Example:

• Original: 1920×1080 (16:9)
• Print: 8×10" (4:5)
• Result: Either crops or distorts

Solution: Calculate ratio first, then crop.

---

Quick Calculations

Print Pixel Calculator

def calc_print_pixels(width_inch, height_inch, dpi=300):
    """Calculate pixels needed for printing"""

    width = int(width_inch * dpi)
    height = int(height_inch * dpi)
    mp = (width * height) / 1_000_000

    return {
        'width': width,
        'height': height,
        'megapixels': mp
    }

# 8×10" at 300 DPI
result = calc_print_pixels(8, 10, dpi=300)
print(f"Needed: {result['width']}×{result['height']} ({result['megapixels']:.1f} megapixels)")

---

Common Aspect Ratios

Ratio	Common Name	Use Case
1:1	Square	Avatars, Instagram
4:3	Standard	Old TV, screenshots
3:2	DSLR ratio	Photos
16:9	Widescreen	Video, banners
9:16	Vertical	Mobile stories, video

---

Web vs Print

Web Images

from PIL import Image

def prepare_web(input_path, output_path, max_width=1920, quality=85):
    img = Image.open(input_path)

    # Only resize if over max width
    if img.width > max_width:
        ratio = max_width / img.width
        new_height = int(img.height * ratio)
        img = img.resize((max_width, new_height), Image.Resampling.LANCZOS)

    # Save
    img.save(output_path, 'WEBP', quality=quality)
    print(f"Done: {img.width}×{img.height}")

prepare_web('photo.jpg', 'photo.webp')

Print Images

from PIL import Image

def prepare_print(input_path, output_path, print_w, print_h, dpi=300):
    img = Image.open(input_path)

    # Calculate needed size
    need_w = int(print_w * dpi)
    need_h = int(print_h * dpi)

    # Check if large enough
    if img.width < need_w or img.height < need_h:
        print(f"Image too small!")
        print(f"Needed: {need_w}×{need_h}")
        print(f"Have: {img.width}×{img.height}")
        return False

    # Scale while maintaining ratio
    target_ratio = need_w / need_h
    current_ratio = img.width / img.height

    if current_ratio > target_ratio:
        new_h = need_h
        new_w = int(new_h * current_ratio)
    else:
        new_w = need_w
        new_h = int(new_w / current_ratio)

    img = img.resize((new_w, new_h), Image.Resampling.LANCZOS)

    # Save
    img.save(output_path, 'JPEG', quality=95, dpi=(dpi, dpi))
    print(f"Done: {img.width}×{img.height} @ {dpi}DPI")

    return True

prepare_print('photo.jpg', 'print.jpg', 8, 10, dpi=300)

---

Summary

1. Screens only care about pixels - DPI doesn't matter on screens
2. Print needs 300 DPI - Calculate by actual print size
3. Start large enough - Enlarging is a last resort
4. 72 DPI is ancient history - Don't believe that
5. Calculate before printing - Don't guess

---

Explained this countless times. Questions? Leave a comment.