One of the original reasons digital cameras offered more than one file format in the first place is to limit the size of the file stored on your memory card. Early dSLRs used tethered hard disk drives slung over a shoulder, enabling the photographer to store 200 megabytes worth of 1.3 megapixel images. PC Card memory that fit the slots still found in notebook computers (but generally used for WiFi plug-ins today) worked for a while, until SanDisk invented the CompactFlash card in 1994. Still, all these memory options were limited in size, so more efficient file formats had to be used. If a digital camera had unlimited memory capacity and file transfers from the camera to your computer were instantaneous, all images would probably be stored in RAW or TIFF format. TIFF might even have gained the nod for convenience and ease of use, and because not all applications can interpret the unprocessed information in RAW files. Both RAW and TIFF provide no noticeable loss in quality.
JPEG was invented as a more compact file format that can store most of the information in a digital image, but in a much smaller size. JPEG predates most digital SLRs, and was initially used to squeeze down files for transmission over slow dialup connections. Even if you were using an early dSLR with 1.3 megapixel files for news photography, you didn’t want to send them back to the office over a modem at 1200 bps.
Alas, JPEG provides smaller files by compressing the information in a way that loses some information. JPEG remains a viable alternative because it offers several different quality levels. At the highest quality level you might not be able to tell the difference between the original TIFF file and the JPEG version, even though the TIFF occupies, say, 14MB on your memory card, while the high-quality JPEG takes up only 4MB of space. You’ve squeezed the image 3.5X without losing much visual information at all. If you don’t mind losing some quality, you can use more aggressive compression with JPEG to store 14 times as many images in the same space as one TIFF file.
RAW exists because sometimes we want to have access to all the information captured by the camera, before the camera’s internal logic has processed it and converted the image to a standard file format. RAW doesn’t save as much space as JPEG (although RAW files can be a lot smaller than TIFF files). What it does do is preserve all the information captured by your camera. Think of your camera’s RAW format as a photographic negative, ready to be converted by your camera or, at your option, by your RAW-compatible image editing/processing software.
You can adjust the image size, file size, and image quality of your digital camera images. The guidebooks and manuals don’t always make it clear that these adjustments are three entirely different things. However, image size affects file size and image quality, and image quality affects Working RAW 89 Whither DNG? To complicate things, in late 2004, Adobe Systems introduced its new DNG (Digital Negative) specification, which purports to translate RAW files from any camera type to a common RAW format compatible with any camera or software. Of course, the specification allows adding what Adobe calls “private metadata” to DNG files, “enabling differentiation,” which is another word for non-standard/noncompatible. If DNG is adopted, most of the data from your camera’s RAW images might be convertible to the new standard, but there’s no guarantee that all of it will be. Because Photoshop can already handle the most common types of RAW files we we work with, most dSLR owners are likely to be underwhelmed by this initiation for the foreseeable future. file size. File size, while it’s dependent on the other two, has no direct effect on image size or quality. No wonder it’s confusing! It’s a good idea to get these three terms sorted out before we move on, so that we’re all talking about exactly the same thing. Here’s a quick summary:
■ Image size. This is the dimension, in pixels, of the image you capture. For example, if
you have a 6MP camera, it may offer a choice of 3008 × 2000, 2560 × 1920, 1600 ×
1200, 1280 × 960, and 640 × 480 resolutions. Each reduction in resolution reduces the
size of the file stored on your memory card. A TIFF file at 2560 × 1920 pixels might
occupy 4MB; a 1600 × 1200 pixel image, 1.7MB; a 1280 × 960 pixel image, 1.3MB, and
a 640 × 480 pixel image less than 1MB.
■ File size. This is the actual space occupied on your memory card, hard disk, or other storage
medium, measured in megabytes. The size of the file depends on both the image size
(resolution) and quality/compression level. You can reduce the file size by reducing the
image size or using a lower-quality/higher-compression setting.
■ Image quality. This is the apparent resolution of the image after it’s been compressed and
then restored in your image editor. The TIFF format can compress the image, somewhat,
with no loss of image quality, but JPEG compression does reduce the image quality, for
reasons that will become clear shortly.
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