Pixels and File Formats
These two items determine how clear and precise your photos will look and print—and how large the resulting files will be.
Resolution
The camera’s image sensor captures an image via millions of photodiodes; each photodiode corresponds to a single pixel in the final image.
Naturally, the more pixels in a photographic image, the more accurately that photo captures the original image.
All this talk of photodiodes and pixels begs an important question—how much resolution do you need? Camera resolution is measured in millions of pixels (megapixels); to a point, the more megapixels, the better. But only to a point.
Even the lowest-priced digital cameras sold today deliver 5 megapixel resolution; higher-end consumer cameras go up to 10 megapixels or more, and professional cameras even higher. Depending on what type of picture you’re taking—or more accurately, what type of final output you want—you may find that even 5 megapixels is overkill.
For example, if you’re taking a photo to upload to a website, that photo will display in a web browser on a computer screen. A typical 15.4-inch notebook PC screen is sized at 1280 pixels wide by 800 pixels tall, so you don’t want your photo to exceed those dimensions. (Otherwise, the entire photo won’t fit onscreen, and viewers will have to scroll to see the whole thing.) Because a photograph sized at 800 pixels tall comes out at 1,000 pixels wide, that translates into a total of 800,000 pixels—less than 1 megapixel.
On the other hand, if you want to print out an 8×10-inch landscape photo, you’ll need more pixels. For the best print quality, you want your photo to have a resolution of 300 pixels per inch (ppi). That translates into a height of 2,640 pixels by a width of 3,000 pixels, for a total of 7,920,000 pixels—approximately 8 megapixels.
Of course, you can print at a lower ppi, but you may notice fuzziness in the pictureespecially at large sizes. For smaller-sized prints made with a home inkjet printer, however, you can obtain satisfactory results at 200 ppi. For example, to print a 5×7-inch photo at 200 ppi, you only need a resolution of 1,000 pixels by 1,400 pixels—just 1.4 megapixels in total.
The table below details the resolution required for best reproduction of various size photos at different ppi.
It’s easy enough, using a photo editing program, to reduce the resolution of a photo for web and other low-resolution uses. It’s impossible, however, to increase the resolution of a photo beyond that at which it was originally shot. So if you want to create larger prints, you’re better off with a higher-resolution camera—even if you end up downsizing your photos for other uses.
Digital File Formats
When a digital camera takes a photo, it stores the information for that photo in digital format. Essentially, each pixel of the image translates into a series of digital instructions for how that pixel should look—its brightness, color saturation, tint and so forth.
How that digital information is stored depends on the file format used. As you might suspect, the more information stored, the larger the resulting file size, the larger the image, the larger the file.
JPG and TIF Formats
Most digital image file formats compress the original picture information to some degree. When an image file is compressed, redundant information is extracted from the image. For example, if an image has a large area of blue sky, many of those blue pixels can be extracted from the digital file, and the information for one blue pixel used to create similar areas of blue sky. That said, some file formats compress the image more than others do. A compressed image might look fine on a relatively low resolution computer screen, but produce unsatisfactory results when printed at large size or in a quality book or magazine; when a compressed image is blown up too large, you’ll see compression artifacts in the form of blockiness in the picture.
There are two different types of image compression.1. Losslesscompression discards none of the original picture information and instead looks for more efficient ways to describe the image. 2. Lossycompression, on the other hand, achieves smaller file sizes by removing less-important pieces of the image; lossy compression results in more picture degradation than does lossless compression.
It’s important, then, to understand the different image file formats available and choose those formats that work best for the digital photos you take. The most popular file formats used in digital cameras today are the JPG and TIF formats. The lossy JPG format is okay for web display and printing small prints on home printers, but the lossless TIF format is better if you want to create larger or higher-quality photo prints.
Raw Format Files
Better than either JPG or TIF is the so-called “raw” format. A raw file is the digital equivalent of a film negative; it contains all the information of the original image with no compression, no processing, and no application of filters or special effects. It’s truly a “raw” image that you can then edit and adjust as appropriate. Raw files, however, are never used as-is; they have to be converted to another format (JPG or TIF) to be printed or viewed online. In addition, there is no single “raw” format; every camera manufacturer uses a slightly different algorithm (and file extension) to create raw files.
Digital Image File Formats
Which file format should you use? Professional photographers and serious hobbyists save their photos in their camera’s native raw format, if their camera offers this feature, and then convert to TIF format for printing. Casual users tend to use the JPG format, because it’s the only file format available on most lower-end digital cameras. In addition, JPG files are better suited for web use and for emailing to friends and family. The TIF format is better if you intend to produce large prints (8×10 inches or larger) or if you’re shooting for publication.