画質を落とさずにJPEG画像を圧縮する方法

Need smaller JPEG files that still look great? Here’s a practical guide to smart compression.

Compression vs Resizing vs Format Change

People confuse these three distinct operations:

• Compression: Reduce quality setting to shrink file size. Same dimensions, smaller file. Most noticeable at extremes (< 70%).
• Resizing: Reduce pixel dimensions (e.g., 4000px → 1920px). Fewer pixels = much smaller file.
• Format change: HEIC → JPEG or JPEG → WebP. Different compression algorithm entirely.

The Sweet Spot: 85% Quality

For most photos, 85% JPEG quality is the point where:

• File size drops ~60% vs 100% quality
• Visual differences are undetectable to the human eye
• Compression artifacts are invisible at normal viewing distances

Quality	12 MP File Size	Best Use
100%	~8 MB	Printing, archiving
92%	~4 MB	Default sharing
85%	~2.5 MB	Sweet spot — web, email
70%	~1.5 MB	Social media, thumbnails
50%	~800 KB	Aggressive compression

Resize Before You Compress

A 4032×3024 (12 MP) photo displayed at 800px wide on a blog is wasting 95% of its pixels. Resize first:

Original: 4032×3024 @ 92% = 4.8 MB
Resized:  1920×1440 @ 92% = 1.2 MB (75% smaller)
Resized:  1920×1440 @ 85% = 650 KB (86% smaller)

The combined approach — resize + compress — produces files ~86% smaller with no visible quality loss on screen.

Tools for JPEG Compression

Tool	Platform	Best For
heicgo.com	Web	HEIC → JPEG conversion + quality control
Squoosh	Web	Interactive quality comparison
ImageOptim	Mac	Lossless JPEG optimization
TinyPNG	Web	Smart compression for PNG/JPEG

The Batch Workflow

For compressing hundreds of photos:

1. Identify your output needs (social media = 1920px wide, archive = original size)
2. Choose quality: 85% for sharing, 92% for keeping
3. Batch convert using heicgo (HEIC input) or export from your photo app (JPEG input)
4. Download as ZIP

What NOT to Do

• Don’t re-compress JPEGs repeatedly — each re-compression adds artifacts (generational loss). Always compress from the original.
• Don’t use PNG for photos — a PNG of a photo is 5-10× larger than JPEG with no quality benefit.
• Don’t set quality below 60% — visible blocking and color banding appear.

Further Reading

• Does Converting HEIC to JPG Lose Quality? — quality considerations
• Reduce Photo File Size for Email — practical compression use case
• How to Convert HEIC to JPG — convert + compress in one step