Image Quality


In addition to giving you a choice of resolution settings, most digital cameras let you choose what they call “image quality.” Why provide two settings that seem to amount to the same thing? Because they are really two independent factors, each of which contributes to overall image quality. Resolution is one of these factors. The degree of “lossy” compression used in writing the file to JPEG (for Joint Photographic Experts Group) format is the other.

JPEG is the image file format that is most often used by digital cameras, because it saves space and allows you to record more images on the memory card. It accomplishes this by “reinterpreting” the picture so that it doesn’t have to record exact data for every pixel in the image. Instead, you decide how much compression you want to use. The lower the image-quality setting you use, the more compression the camera’s processor uses in saving the image to memory— and the more information is irretrievably lost (and replaced with little color anomalies that seem to resemble film grain). 

The first time a high-quality JPEG image is saved and then opened for viewing, it is difficult to tell that any information has been discarded at all. However, each subsequent file save recalculates the amount of data to be thrown away, and the same amount of data that was discarded in the first instance is discarded again. So the first thing you need to know about JPEG images is that when you open them, you must subsequently save them to a “lossless” format, such as TIFF (for Tagged Image File Format), or to your image editor’s proprietary file format (such as Photoshop’s PSD format) if you want to prevent continued image degradation.

As far as lower quality settings, the most important thing for you to remember about them is that you can never recover the image information that was lost. Each time you lower the quality setting, you raise the level of image compression—which is nothing more than further data loss. So you have to remember the same lesson you learned about reducing resolution. Once you’ve discarded image quality, you can never get it back, and you can almost never replace the picture you took in the first instance. Again, the only reason you should be willing to lose image quality is if your camera or computer doesn’t have the capacity to store many more pictures (and you haven’t had the foresight to buy extra memory cards).

Some of the more advanced digital cameras will also let you save files in either TIFF or RAW image file format. TIFF records every pixel in the exact shade of the 16 million possible colors that the image sensor could capture. Some of the best sensors can actually record a lot more than 16 million colors, thus allowing them to capture a range of brightness values closer to those found in bright, natural sunlight. That is why some cameras even let you record your images to a format called RAW (referring to untouched data as it comes directly off the sensors). RAW image files can contain as much data as your camera is designed to capture (higher cost digital cameras typically record 12 to 16 bits of information per

pixel instead of the usual 8 bits per pixel). RAW image files produce extremely high-quality images, but they also create data files that are eight to sixteen times as large as those recorded for a Super High Quality (SHQ) JPEG image. You must also have (or be willing to wait for) an image editor that will allow you to process image files that contain more than 8 bits of information per pixel.