JPG (JPEG Image)
The format that put millions of photos on the web by making large images small enough to share.
| Full name | JPEG Image |
| Extension | .jpg |
| MIME type | image/jpeg |
| Developer | Joint Photographic Experts Group (ISO/IEC JTC1) |
| Released | 1992 |
| Type | Raster image, lossy compression |
| Color depth | 24-bit (16.7 million colors) |
| Compression | Discrete Cosine Transform (DCT), lossy |
What is a JPG file?
JPG is the most widely used image format on the internet. It stores photographs and complex images with full color using lossy compression that shrinks file sizes dramatically. A typical photo that might be 20 MB as a raw file can become 2 MB or less as a JPG with little visible difference.
JPG uses a lossy compression algorithm based on Discrete Cosine Transform (DCT). When you save an image as JPG, the encoder discards some image data that the human eye is least likely to notice, especially in areas of complex texture and color variation. You can control how much data is discarded through a quality setting, usually from 1 to 100. Lower quality means smaller files but more visible artifacts, while higher quality keeps more detail at the cost of larger file size.
History
The Joint Photographic Experts Group, a collaborative committee under ISO and IEC, began developing the JPEG standard in 1986. The standard was formally submitted to the ITU-T in 1992 and approved by ISO/IEC in 1994 as ISO/IEC 10918-1. The companion JFIF container format, which defines how JPEG data is stored in actual files, was finalized in September 1992 by C-Cube Microsystems engineer Eric Hamilton after a meeting with around 40 industry representatives.
How it works
A JPG file is a container following the JFIF or Exif specification. It opens with a SOI (Start of Image) marker and ends with an EOI (End of Image) marker. Between those markers, the file holds metadata segments (including Exif data for camera settings, GPS, and timestamps), color space information, and the compressed image data in one or more scan segments. The image is divided into 8x8 pixel blocks, each transformed and quantized independently.
What it is used for
- Storing and sharing photographs from digital cameras and smartphones
- Publishing images on websites and social media where download speed matters
- Email attachments and messaging apps where file size is limited
- Scanned documents and printed photographs that need broad compatibility
How to open it
Every major operating system opens JPG files natively: Photos on Windows, Preview on macOS, and the default image viewer on any Linux desktop. Any web browser, photo editor, or image viewer released in the past 30 years supports the format.
Pros and cons
Strengths
- Excellent compression ratios that keep file sizes small for photos
- Universal support across every device, browser, and operating system
- Adjustable quality lets you trade file size against image fidelity
- Supports full 24-bit color, making it ideal for photographs
Trade-offs
- Lossy compression means some image data is permanently discarded on each save
- Repeated re-saving degrades quality each time, a problem called generation loss
- No support for transparency or alpha channels
- Poor fit for graphics with sharp edges, text, or flat colors, where PNG performs better
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JPG FAQ
What is the difference between JPG and JPEG?
There is no technical difference. JPEG is the full name of the format. JPG became common on Windows because early versions of Windows required three-letter file extensions, so the four-letter .jpeg extension was shortened to .jpg. Both refer to the same format and the same compression standard.
Does saving a JPG multiple times reduce quality?
Yes. Each time you open and re-save a JPG at a lossy quality setting, the compression runs again and discards more data. If you need to edit an image repeatedly, work in a lossless format like PNG or TIFF and export to JPG only at the final step.
Can JPG images have transparent backgrounds?
No. JPG does not support transparency. If you need a transparent background, such as a logo on a webpage, use PNG or WebP instead.
What quality setting should I use when saving a JPG?
For web images, a quality setting between 70 and 85 (out of 100) gives a good balance of small file size and acceptable detail. For print or archival use, 90 to 95 is safer. Going above 95 rarely improves visible quality but increases file size significantly.