File Formats are a standard means for saving digital images. There are a variety of common file formats and many more less common file formats. The primary difference between many formats is how an image is compressed. Compressing images is a method used to make the file smaller and speed up transmission of the file. Compressed images are the standard for web pages as users do not want to wait a long time simply to view an uncompressed image.
There are two types of compression used in digital image file formats: lossy compression and lossless compression.
Lossless Compression reduces the file size but preserves detail within the image itself. The file will tend to be larger than a lossy version of the same file.
Lossy Compression reduces the file size by discarding or averaging visual information. When high quality settings are used, the file will be larger but quality is generally maintained. When low quality settings are used, the file is much smaller but there is usually visible degradation of image quality.
Click to enlarge the image above for a comparison. In particular, notice the clarity of the eyelashes and the clarity of the skin .
Resolution can be defined in PPI or DPI. PPI refers to "pixels per inch" and DPI refers to "dots per inch." PPI is a measurement of how many pixels are present within a given inch of a digital device or image. DPI is a measurement of how many printed dots are present within a given inch of a printed image. The primary difference is that PPI refers to digital information and DPI refers to analog (i.e. printed) information. Since the concept is very similar, PPI and DPI are often used interchangeably.
An image that is 100 ppi means that there are one hundred pixels in each inch of the image while 300 ppi means that there are 300 pixels in each inch. Obviously, the pixels of a 300ppi image are much smaller than the pixels of a 100ppi image.
|Low resolution vs. High resolution|
See also: Pixel