JPEG Quality Factor
Also known as: jpeg quality setting, jpeg q factor, jpeg quality factor
The JPEG quality factor is a 1-100 setting that controls how aggressively a photo's color and detail data is quantized during compression. Higher values keep more detail and a larger file; lower values shrink the file by discarding visual information.
- Quality factor scales the JPEG quantization table: higher = more detail and a bigger file, lower = smaller file with more artifacts.
- JPEG is lossy, so re-saving at high quality cannot recover detail discarded by an earlier low-quality save.
- Chroma subsampling (commonly 4:2:0 on phones) shrinks files by storing color at lower resolution than brightness.
What the quality factor actually changes
JPEG splits an image into 8x8 pixel blocks, runs each block through a discrete cosine transform (DCT), then divides the resulting frequencies by a quantization table. The quality factor scales that table: a high factor (say 90-100) uses small divisors that preserve fine detail, while a low factor (50 or below) uses large divisors that round many high-frequency coefficients to zero. Those zeroed coefficients are what make the file small and what produce blocky, smeared artifacts when overdone.
Because JPEG is lossy, the quality factor is a one-way trade. Re-saving a JPEG at a high quality cannot restore detail that an earlier low-quality save already threw away, and every additional save compounds the loss. This is why repeatedly editing and re-exporting the same photo slowly degrades it ("generation loss").
Chroma subsampling and file size
Most JPEG encoders pair the quality factor with chroma subsampling, which stores color (chroma) at a lower resolution than brightness (luma) because the eye is far more sensitive to luminance. Common modes are 4:4:4 (no color downsampling), 4:2:2, and 4:2:0 (color stored at quarter resolution). Phone cameras typically encode at 4:2:0, which cuts size with little visible loss on most scenes but can soften sharp colored edges and red text.
In practice the quality slider you see in editors, social apps, and export dialogs is the single biggest lever on a photo's size. Dropping from quality 100 to around 80-85 often shrinks a file by half with almost no perceptible difference, which is exactly the headroom a cleaner exploits when it offers to recompress large images.
Why it matters for storage
A camera roll full of maximum-quality JPEGs, or images that were edited and re-exported many times, carries far more bytes than the visible quality requires. Identifying oversized photos and offering a lossy recompression to a sensible quality factor is a standard way to reclaim space without deleting anything. Cleanor surfaces the largest images and lets you compress or remove redundant copies so your library stays lean.