Feb 10, 2024

What is A JPEG File, Exactly?

Like, what does it do?

Hunter Scott
Hunter Scott
What is A JPEG File, Exactly?

The JPEG file format, an acronym for Joint Photographic Experts Group, is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality. But what does this all mean, and how does it affect your everyday interaction with digital images? Let's delve deeper into the world of JPEG.

Understanding JPEG

The JPEG format is one of the most widely used formats for storing and transmitting images on the web. This is primarily due to its ability to compress images into smaller file sizes without losing too much quality, making it ideal for web use where bandwidth and storage can be limited.

However, it's important to note that JPEG uses lossy compression, which means that some image data is lost during the compression process. This is in contrast to lossless compression formats, such as PNG or TIFF, where no data is lost. The lost data in JPEG compression is usually in the form of fine details or colors that the human eye is less likely to notice, especially at higher compression levels.

How JPEG Compression Works

When you save an image as a JPEG, the algorithm looks at the image in 8x8 pixel blocks. Within each block, it applies a discrete cosine transform (DCT) to convert the spatial domain data into the frequency domain. The DCT separates the image into parts of differing importance with respect to the image's visual quality.

Then, the algorithm quantizes the data, or reduces the precision of it, based on a quantization table. This table is designed to discard information that the human eye is less likely to notice. The result is a significant reduction in file size with a relatively small decrease in image quality.

Advantages and Disadvantages of JPEG

Like any file format, JPEG has its pros and cons. Understanding these can help you decide when it's best to use JPEG and when it might be better to opt for a different format.

One of the main advantages of JPEG is its ability to compress images into relatively small file sizes. This makes it ideal for use on the web, where smaller file sizes can lead to faster load times and less data usage. Additionally, because JPEG is so widely used, it's supported by virtually every image viewer and editor, as well as most digital cameras and other devices that capture or display images.

Disadvantages of JPEG

While JPEG's compression capabilities are a major advantage, they can also be a disadvantage. Because JPEG uses lossy compression, some image data is lost each time an image is saved in this format. This can lead to a decrease in image quality, especially if an image is saved multiple times as a JPEG.

Another disadvantage of JPEG is that it doesn't support transparency. This means that you can't have any parts of your image be see-through, as you can with formats like PNG. Additionally, JPEG doesn't support layers, which can be a drawback if you're doing complex image editing.

When to Use JPEG

Given its advantages and disadvantages, JPEG is best used in certain situations. One of the most common uses of JPEG is for photographs on the web. Because photographs typically have a lot of complex, detailed information, they can be compressed quite a bit without a noticeable loss in quality.

Additionally, because JPEG supports a wide range of colors, it's a good choice for colorful images. However, for images with sharp lines and contrasts, such as text or line art, a lossless format like PNG might be a better choice, as these types of images can sometimes appear fuzzy or pixelated when saved as a JPEG.

How to Save an Image as a JPEG

Saving an image as a JPEG is typically quite simple, regardless of what image editing software you're using. In most programs, you'll simply go to the "File" menu, select "Save As," and then choose JPEG from the list of available formats.

When you save an image as a JPEG, you'll usually be given the option to adjust the quality level. This is where you can choose how much compression to apply. A higher quality level will result in a larger file size but better image quality, while a lower quality level will result in a smaller file size but lower image quality.


In conclusion, the JPEG file format is a versatile and widely used format for compressing digital images. Its ability to reduce file sizes without a significant loss in quality makes it ideal for many uses, particularly on the web. However, it's important to understand the limitations of JPEG, such as its lossy compression and lack of support for transparency and layers.

By understanding what a JPEG file is and how it works, you can make more informed decisions about when to use this format and how to get the best results from it. Whether you're a professional photographer, a web designer, or just someone who likes to share photos online, a solid understanding of JPEG can be a valuable tool in your digital toolbox.