Explainer

Image Compression: Lossy vs Lossless Explained

March 22, 2025·6 min read

Every digital image uses some form of compression to reduce file size. The two fundamental approaches, lossy and lossless, make very different trade-offs between file size and quality. Understanding how each works helps you make informed decisions about which format and settings to use for any given situation.

What is lossless compression?

Lossless compression reduces file size without discarding any image data. When you decompress the file, you get back the exact original pixels, bit for bit. PNG and TIFF use lossless compression. The algorithm finds patterns and redundancies in the data and represents them more efficiently, similar to how ZIP compression works for files. Typical compression ratios are 2:1 to 3:1.

What is lossy compression?

Lossy compression achieves much smaller files by permanently discarding some image data that the human eye is less likely to notice. JPG is the most common lossy format. It exploits the fact that human vision is more sensitive to brightness changes than color changes, and more sensitive to gradual changes than sharp ones. Compression ratios of 10:1 to 20:1 are common.

Visible artifacts of lossy compression

Over-compressed JPG images show characteristic artifacts: blocky patterns around sharp edges, color banding in gradients, and a general softening of fine details. These artifacts are called quantization noise. They become more pronounced each time you re-save a JPG, which is why you should always keep your original and only export to JPG as a final step.

Choosing the right approach

Use lossless formats (PNG, TIFF, WebP lossless) when you need pixel-perfect reproduction: screenshots, text overlays, medical imaging, or source files for further editing. Use lossy formats (JPG, WebP lossy, AVIF) when file size matters more than pixel accuracy: web images, social media, email attachments. Most real-world use cases benefit from lossy compression at quality 80-90.

Modern hybrid approaches

WebP and AVIF offer both lossy and lossless modes in the same format, letting you choose based on each image. Some tools use smart compression that applies lossless encoding to regions with sharp edges and text while using lossy encoding for photographic regions. This hybrid approach gives the best of both worlds for mixed-content images.