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1.8 IMAGE FILE SIZES | ||
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When you take a photograph, the size of the image file is huge compared to many other types of computer files. For example, a low-resolution 640 x 480 image has 307,200 pixels. If each pixel uses 24 bits (3 bytes) for true color, a single image takes up about a megabyte of storage space. As the resolution increases, so does the file size. At a resolution of 1024 x 768, each 24-bit picture takes up 2.5 megabytes. Image file size is somewhat of an indicator of image quality. Very high end camera users often refer to file sizes instead of pixel counts. For example, someone may say a camera creates 30-Megabyte files. This is just a form of shorthand.
To make image files smaller and more manageable, almost every digital camera uses some form of compression. Compressing images not only let's you save more images on the camera's storage device, it also allows you to download and display them more quickly. In addition to compression, most cameras also allow you to control file sizes by choosing between two or three image sizes.
During compression, data that is duplicated or which has no value is eliminated or saved in a shorter form, greatly reducing a file’s size. When the image is then edited or displayed, the compression process is reversed. There are two forms of compression—lossless and lossy—and digital photography uses both forms.
Lossless compression (also called reversible compression) uncompresses an image so its quality matches the original source. Although lossless compression sounds ideal, it doesn’t provide much compression. Generally, compressed files are still a third the size of the original file, not small enough to make much difference in most situations. For this reason, lossless compression is used mainly where detail is extremely important as in x-rays and satellite imagery. A leading lossless compression scheme is LZW (Lempel-Ziv-Welch). This is used in GIF and TIFF files and achieves compression ratios of 50 to 90%
Although it's possible to compress images without loosing some quality, it's not practical in many cases. Therefore, all popular digital cameras use a lossy compression (rhymes with bossy) that degrades images to some degree and the more they're compressed, the more degraded they become. In many situations, such as posting images on the Web, the image degradation isn't obvious. However, enlarged prints show it off.
Although lossy compression does not uncompress images to the same quality as the original source, the image remains visually lossless and can appear normal. The trick is to remove data that isn’t obvious to the viewer. For example, if large areas of the sky are the same shade of blue, only the value for one pixel needs to be saved along with the locations of where the other identical pixels appear in the image. The leading lossy compression scheme is JPEG (Joint Photographic Experts Group) used in JFIF files (JPEG File Interchange Format). This scheme allows you to select the degree of compression. Compression Ratios between 10:1 and 40:1 are common.
Because lossy compression affects the image, most cameras allow you to choose between different levels of compression. This allows you to choose between lower compression and higher image quality or greater compression and poorer quality. The only reason to choose higher compression is because it creates smaller file size so you can store more images, send them by e-mail, or post them on the Web. Most cameras give you two or three choices equivalent to Good, Better, Best.
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| Nikon 950 image resaved with lowest compression and highest quality. | Same image resaved with highest compression and lowest quality. |
A few cameras allow you to select a mode that doesn't compress the image at all. This mode will give you the highest quality but stores the fewest images because the files are so large. Some cameras also ofrer a RAW mode that stores data off the image sensor without processing it. This keeps the file size smaller and speeds up the time between photos. The RAW file is processed into a full-color image only after its transferred to the computer.
Instead of using compression, some cameras allow you to change resolution as a way of controlling file sizes. Because you can squeeze more 640 x 480 images into memory than you can 1800 x 1600 images, there may be times when you'll want to sacrifice quality for quantity.