Compression and File Types

Compression done by digital cameras

Digital cameras compress an image before saving it. Compression is discarding the surplus information to reduce the file size so that less memory space is utilized to store it in the memory card. The image quality is inversely proportional to the level of compression. The greater the compression, the lesser is the image quality. The lesser the compression, the greater is the image quality. A variety of compression level settings are provided by digital cameras, namely, Super Fine, Fine, Normal and Basic (the terms vary with different manufacturers). The Super Fine setting produces the best image quality with the least amount of compression. The selection of the compression level should be done according to the use of the image taken. If the image is meant for printing then the least amount of compression is recommended; however, if it is meant for sending via email then the maximum amount of compression should be done.

Compressed files are categorized as "lossy" or "lossless". Lossless compressed files preserve all image information, so that if the file is decompressed than it would perfectly match the original uncompressed file. Lossy compression selectively discards image data, which can never be recovered, and they are lost forever. The differences between the compressed and the uncompressed files are known as "compression artifacts".

File types used by digital cameras

Digital cameras use many types of image formats and new ones are regularly added. However, the three formats most relevant to digital photography are JPEG, TIFF, and RAW file formats. Most digital cameras support JPEG due to its smaller file size. High-end digital cameras support all three formats.

JPEG file format - JPEG (pronounced as Jaypeg) is an acronym for "Joint Photographic Expert Group" and was developed to store photographs in digitized form. It gives the .jpg extension after the filename. It has become the standard for displaying graphics on Internet web pages and for storing in memory cards of digital cameras. JPEG utilizes lossy compression by selectively retaining the details that matter most, and discarding the details that it considers as less important. Since the human eye can perceive slight changes in brightness better than changes in color, hence the JPEG algorithm preserves brightness more than color. The JPEG algorithm gives utmost priority to high-contrast edges and degrades subtle textures, as the compression level increases. Thereby, it succeeds in decreasing the file size. JPEGs store important image information and camera settings as EXIF (Exhangeable Image File) data. EXIF data stores information like date and time, focal length, depth-of-field, shutter speed, exposure compensation, metering pattern, and the usage of flash. The drivers of image editing software, in order to enhance images resulting in better prints, automatically read EXIF data. Browsers supporting JPEG file format can also read the EXIF data to display correct images.

TIFF file format - TIFF is an acronym for "Tagged Image File Format" and has become the standard in the printing and publishing industry. An image file in TIFF format has the .tif extension after the filename. They can be uncompressed or compressed using lossless compression reducing to about one-third the original size of the image. They are much larger in size than JPEG, can store multiple layers, and can have bit depth of 16-bits per channel over and above the standard 8-bits per channel. TIFF files are extremely useful as intermediate files, which may be edited later, since they preserve all layers and do not introduce any compression artifacts. If you compress a file multiple times, during your image editing, then the compression artifacts are amplified and subsequently the file size increases. Keeping the noise level as low as possible also reduces the file size. Digital cameras take longer time to process TIFF files. Like JPEGs, TIFF files also have EXIF information embedded within it.

RAW file format - This format contains "raw" pixel information straight from the digital camera's sensor, it is untouched. This format is considered as the digital negative. The RAW file format has not undergone demosaicing, so at each pixel location, it contains just one red, green, or blue value. The RAW file is "developed" by the digital camera by converting it into a full color JPEG or TIFF file, and then storing the RAW file in the memory card. The digital camera develops a RAW file to a final JPEG or TIFF image file in several steps, each of which may have numerous irreversible image adjustments. The RAW file format allows the photographer to postpone applying these adjustments, thereby giving the chance to the photographer to apply the adjustments themselves. The adjustments are typically demosaicing, white balance, tone curves, dynamic range and exposure compensation, contrast, color saturation, sharpening, conversion to 8-bit, and JPEG lossy compression. The photographers themselves can do these adjustments by using a computer to their satisfaction and it is much faster due to the high memory capacity of the computer than the digital camera. Foveon-type sensors capture all three colors at each pixel location and can be very nicely processed in a computer using RAW file format. Since the RAW format is not very standardized, each camera has its own proprietory RAW format, resulting in one program being unable to read all RAW formats. Adobe has announced the standardization of RAW format and has named it as Digital NeGative or DNG format.

Which format is best?

The best format depends on the type of photography being done and hence there can be no definitive answer to this question. Generally, for unfinished editing work and for printing, TIFF format is best; for sports and press photography JPEG format is best; and for landscape and fine art photography RAW format is best. However, which format to choose depends entirely upon the subject to be photographed and the editing work to be done on it. All the formats try to maximize the image quality potential of the digital camera and try to produce a superior digital photo as far as possible by digital camera technology.