What Is Image Compression?

Q: What is the difference between lossy and lossless compression?

Lossy compression permanently removes some image data to achieve much smaller files. JPEG and HEIC are lossy. Lossless compression reduces file size without removing any data, so the image can be perfectly reconstructed. PNG and lossless WebP are lossless.

Q: What is the most efficient image compression format?

Among widely supported formats, AVIF and HEIC produce the smallest files at high quality, typically 40-50% smaller than JPEG. WebP is also very efficient. JPEG remains the most universally supported but is less efficient than newer formats.

Image compression is the process of reducing the size of an image file by encoding the image data more efficiently. Compression can be lossy (some data is permanently discarded for smaller files) or lossless (the original image can be perfectly reconstructed). Without compression, a single 12-megapixel photo would be roughly 36 MB instead of 1–3 MB.

Why Compression Exists

An uncompressed 12-megapixel photo at 8 bits per channel takes about 36 MB on disk. Multiplied across thousands of photos, that's hundreds of gigabytes — far too much for the average phone. Image compression takes advantage of patterns in image data and the limits of human vision to shrink files dramatically while preserving quality.

Lossy Compression

Lossy compression permanently discards image information that the human eye is unlikely to notice. JPEG, HEIC, and lossy WebP are all lossy formats. They achieve dramatic size reductions (often 10:1 or higher) by removing high-frequency detail, simplifying color information, and discarding subtle gradations. Each save further degrades the image, so editing the same JPEG repeatedly causes visible quality loss over time.

Lossless Compression

Lossless compression reduces file size without removing any data. Algorithms find patterns in the image (such as long runs of identical pixels) and encode them more efficiently. The original image can be reconstructed exactly, byte for byte. PNG and lossless WebP are common lossless formats. The trade-off is much larger file sizes than lossy — typically only 2:1 or 3:1 compression ratios.

Common Formats Compared

JPEG: Lossy. Universal compatibility, ~10:1 ratio. Best for photos.
PNG: Lossless. Best for screenshots, logos, transparency.
HEIC/HEIF: Lossy. ~50% smaller than JPEG at equivalent quality. iPhone default.
WebP: Both. Modern, ~30% smaller than JPEG.
AVIF: Both. Newest, even more efficient than HEIC.
RAW: Minimally compressed. Largest files, full editing flexibility.

How to Reduce Photo File Size on iPhone

Make sure Settings > Camera > Formats is set to "High Efficiency" so iPhone uses HEIC instead of JPEG. Use Settings > Photos > Optimize iPhone Storage to keep smaller versions on device while full-resolution stays in iCloud. For sharing, you can email a "Small" or "Medium" version of a photo from the share sheet.

Related Terms

Frequently Asked Questions

What is the difference between lossy and lossless compression?

Lossy permanently removes data for much smaller files (JPEG, HEIC). Lossless reduces size without removing data (PNG).

Does compression reduce photo quality?

Lossy compression always reduces quality by some amount, though modern formats make the loss usually invisible. Lossless compression does not reduce quality.

What is the most efficient image compression format?

AVIF and HEIC produce the smallest files at high quality, 40-50% smaller than JPEG.

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