WHAT IS IMAGE SIZE?
This article is somewhat technical. You may want to read
What is a Pixel?IMAGE:
An image is a representation of a real object or scene. An image can be viewed on the monitor, on printed paper, etc.There are two properties of digital images that often get confused. The first
is the pixel size (visual) and the second is the file size (bytes of storage). Image quality is yet another property that is not often understood. I will separate and clarify these properties below.IMAGE SIZE – PIXELS:
Digital images are always rectangles (or are square). They are constructed of pixels (What is a Pixel? link above). There is a horizontal number and a vertical number of pixels. When you multiply these two numbers, you get the pixel size of the image. A million pixels is called a megapixel.For example, an image that is 2048 pixels wide and 1536 pixels high is 2048 x 1536 = 3,145,728 pixels, called a 3-megapixel image, a very common image size (see illustration below). This determination of image size is constant and true regardless of file size or quality!
NOTE:
Digital camera menus allow you to select image sizes other than the maximum rated megapixel value. For example, the common 3-megapixel camera would also offer 1600 x 1200 (2 megapixels) and 1024 x 768 pixels (e-mail and monitor size). Check your camera’s menu for the selectable image sizes.FILE SIZE – BYTES:
When an image is stored on the camera’s memory card or the computer’s hard drive, the file size in kilobytes/megabytes (amount of storage space used) is as follows:With a
TIFF image (file extension .TIF) the file size is determined only by the number of pixels. The above 3-megapixel image would be 3 megapixels x 3 colors = 9 megabytes (each pixel occupies 3 bytes because of 3 colors R G B). TIFF is the highest quality because it is not compressed. It also has a very large file size!If the image is a
JPEG (file extension .JPG), the file size is always smaller than the pixel size because JPEG is a compressed file format. ( What is a JPEG? )For JPEG images, the file size depends greatly on another property: the degree of compression. This also establishes the image quality. More compression = smaller file size (bytes) but less quality.
IMAGE QUALITY – COMPRESSION:
Digital cameras allow you to select the degree that the JPEG image will be compressed. Common selections of quality are: SQ (Standard Quality), HQ (High Quality) SHQ (Super High Quality). Other manufacturers have Basic, Normal and Fine Quality, etc.IMPORTANT - these quality selections are not for image size, which is determined only by the number of pixels (see Image Size - Pixels: above).
The purpose for the different quality selections is to obtain the desired file size (bytes) while also retaining acceptable quality (a result of compression) for the application. E-mail requires both a smaller image size (pixels) and smaller file size (bytes) - quality isn't very important on a monitor. However, for printing, select a large image size (more pixels) and select the highest quality (least compression).
I hope this clears the air a little bit
. J© 2004 Fred Hall