Click to see the effects of compression.
Click to explore the differences between JPEG and RAW formats.
Here, two versions of the same image have been enlarged. The image on the left is an uncompressed file. The one on the right is a compressed JPEG file.
On many cameras, one of the many important choices you'll make is what format and image quality to use when shooting photos.
- JPEG is the default format used by almost every digital camera ever made. Named after its developer, the Joint Photographic Experts Group (and pronounced "jay-peg") this format lets you specify both image size and compression. The smallest size is best for the Web and e-mail (although it will usually have to be reduced) and the largest for prints.
The JPEG format compresses images to make their files smaller, but many cameras let you specify how much they are compressed. This is a useful feature because there is a trade-off between compression and image quality. Less compression gives you better images so you can make larger prints, but
you can't store as many images. Because you can't add pixels as well or remove the effects of compression after the fact, it's usually best to use the largest size and least compression. If you have to reduce either, you can do so later in a photo-editing program.
- RAW format is available on many cameras. One of Ansel Adam's better known expressions, drawn from his early experiences as a concert pianist, was "The negative is the score, the print is the performance". In digital photography, the image file is your score and your photo-editing program is where you perform. The printer then just does what you've told it to do as you edited the image. To get the highest possible quality, you want to start with the best possible score— a RAW image file. These files contain all of the image data captured by the camera's image sensor without it being processed or adjusted. You can interpret this data any way you want instead of having the camera do
it for you. If you want total control over exposure, white balance, and other settings, this is a format you will learn to love. Only four camera settings permanently affect a RAW image— the aperture, shutter speed, ISO, and focus. Other settings may affect the appearance of the thumbnail or preview but not the image itself. Since each camera company has defined its own proprietary RAW format, many operating systems and even photo-editing programs are unable to recognize some or all of these files. If the camera supports the RAW format the camera manufacturer always supplies a program along with the camera.
With many cameras you can capture RAW images by themselves or with a companion JPEG image that gives you an identical high quality RAW file and a smaller, more easily distributable image file. Both the RAW and JPEG files have the same names but different extensions.
The number of new images you can store at the current settings is usually displayed on the camera's monitor or control panel.
Advantages of Using the Raw Format
There are a number of advantages to using the RAW format:
- RAW lets you decide on most settings after you've taken the picture, not before. For example, when you shoot a JPEG image under fluorescent lights, the camera adjusts the image to remove the yellow-green tint. Any changes you make later are on top of this initial change. If you shoot the image in RAW format, the camera just captures the images as is and you decide what white balance setting to use later. You can even create different versions of an image, each with its own white balance.
- RAW images aren't compressed using a lossy compression scheme that throws out data to make image files smaller. Although some cameras have a compressed RAW format, these images are compressed using lossless compression. When you open these images, they contain all of the original image data.
- RAW images aren't processed in the camera as JPEG images are. When you take JPEG photos, a processing chip with the power of a small computer manipulates them based on the camera settings you have used and then compresses them to reduce their size. The changes made to your images cannot
be undone later because it's the final, altered image that is saved in the image file. Some of the original image data is lost for good. With RAW images, all of the original data captured by the camera is saved in the RAW image files so you can process them later on your computer. The settings used to take RAW images are saved, but they are not permanently applied to your images until you save a version of them in another format such as JPEG or TIFF.
- RAW images have greater color depth and that gives you smoother gradations of tones and more colors. For example, JPEG images use only 8 bits per color (RGB) or 24 bits total. This means that JPEG images can have only 256 tones (28) and 16,777,216 colors (224). Meanwhile many RAW images are initially captured by the sensor in 48 or 36 bit RGB (16 or 12 bits per channel) and only reduced to 24 bit RGB (8 bits per channel) when converted into JPEG files. The full 48 or 36 bits are retained in the RAW file after the images are processed on your computer because the original file isn't overwritten with your changes. You can even retain all 16 or 12 bits per color by saving images in a format such as TIFF or Photoshop's PSD format.
- RAW images can be processed again at a later date when new and improved applications become available. Your final image isn't permanently altered by today's generation of photo-editing applications.
- You can use a RAW image to generate alternate versions of the same image. For example, many photographers will adjust highlight and shadow areas and save these versions separately. Using a photo-editing program, they then combine the two images and by selectively erasing parts of the top image let areas of the lower image show through so all areas have a perfect exposure.
Disadvantages of Using the Raw Format
Admittedly, there are drawbacks to using RAW images.
- RAW files in the camera are quite large. If you use this format a great deal you will need more storage space in the camera and computer and processing times will be longer.
- Since RAW images aren't processed in the camera, you have to process them on the computer. When you are done shooting for the day, there is still work to do. You need to convert them to another format when you want to e-mail them, post them on a Web site, print them, or import them into another program to create a slide show or publication. Many cameras help you get around this by simultaneously capturing JPEG versions at the same time they capture RAW images. You can use these more universally supported images for many of your applications and reserve the high quality RAW versions for when you need the highest possible quality.
- RAW images are not always noticeably better. Where they shine is when you have exposure or white balance problems. Because RAW images have 16 or 12 bits per color instead of the 8 bits used by JPEG's you have dramatically more information to work with when making adjustments.