What is Resolution?
When you work with bitmap (BMP) images like digital photographs, you work with pixels. A pixel (short for "picture element") is the smallest unit in a computer image or display. Every image on your computer is made up of a colored grid of pixels.
Your digital camera records pixels, your scanner converts physical images into pixels, your photo editing software manipulates pixels, your computer monitor displays pixels, and your printer paints pixels onto paper. In the digital world, "inches" don't exist, only pixels do.
The key to successfully editing, scanning, and printing images lies in understanding how pixels transform into inches and vice versa. Resolution is the interpreter between the physical world of inches and the digital world of pixels. When you scan an image, the scanner translates inches into pixels using resolution. When you print an image, the printer translates pixels into inches using resolution. So what's resolution? Unfortunately, the word is used in different ways in different contexts. "Camera resolution" usually means something slightly different from "image resolution", and "printer resolution" is something else yet again.
We'll try to establish some consistent terminology with regards to resolution and make it all clear in the short tutorial below.
Resolution as Pixel Count
Before we go over specific types of resolution, let's cover the two basic ways the term resolution is used. In some contexts, resolution refers to the pixel count of an image. An image with lots of pixels is often called a "high resolution" image.
Resolution as Embedded Resolution
But in other contexts, resolution refers to the density of pixels in a given linear area such as an inch. This "density" is expressed as ppi (pixels per inch) or dpi (dots per inch) and this density number is embedded invisibly in a bitmap image, as an instruction to output devices, such as a printers.
What is the difference between pixel count resolution and embedded resolution?
Embedded resolution tells your printer how far apart to spread the pixels in a printed image. It determines how "fine grained" the printed image will look. It is completely independent of the pixel count of the image. A high-pixel-count image can have a low embedded resolution or vice versa. Embedded resolution is inversely proportional to the size of the printed image. Given the same pixel count, a high embedded resolution will result in a smaller printed image (the pixels are packed together more tightly), and a low embedded resolution will result in a larger image (the pixels are more spread out).
Embedded resolution, however, does not affect the size (in bytes) of your image or its appearance on a computer screen. Those properties are determined solely by the pixel count. The byte-size of the image file is directly proportional to the pixel count, as is its size on your computer screen, which simply displays all the pixels in the image in a fixed one-to-one grid.
What is my camera's resolution?
A camera's resolution is usually defined as the number of megapixels (or millions of pixels) that it can capture in a single photo. This is obviously a pixel-count resolution. Most digital cameras capture images on a CCD (charge coupled device) sensor. The camera's resolution is calculated by multiplying the number of pixels along the length and width of the sensor. Contemporary cameras typically capture between one million and six million pixels per image.
A two-megapixel camera, operating at maximum resolution, will create an image that has about two million pixels. However, most cameras offer at least three different pixel-count settings for taking pictures with varying degrees of quality. At lower settings, the camera reduces the number of pixels to create a smaller image that requires fewer bytes to store in memory.
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Printer Resolution and Image Quality
Printer resolution is the number of ink dots printed on an inch of paper measured in dots per inch (dpi). Most of today's printers have a resolution of 300 or 600 dpi. In most cases, the printer's resolution will not affect how you size and scale images. If you print your image at its actual size in inches, printer resolution will never effect the size of your printed image.
If you want to control the quality of your printed image for a professional result such as an image destined for publication in a newspaper or magazine, you can calculate the optimal resolution for your image based on a printer's LPI (lines per inch).
For most printers there is an optimum image resolution, beyond which increasing the embedded resolution of the image (ppi) makes no discernible effect on the output quality. Each printer can only print so many lines per inch (LPI). LPI measures the number of halftone dots a printer can create in an inch of paper. Halftone dots are how a printer simulates continuous shades of colors while only using four colors: cyan, magenta, yellow, and black (some photo-quality printers also add light cyan and light magenta). Every individual pixel in your image will be represented by a random pattern of these smaller, various-sized printer dots. This process is also called screening or halftoning.
To obtain a quality print, your image resolution should be slightly higher than the printer's given LPI. To calculate this ideal resolution, multiply the LPI by about 1.5 to obtain the ideal pixel per inch (ppi) figure for your image. For example, given a 360 dpi photo-quality inkjet printer with a 150 lpi, you can get a photo-quality print if your image has 150 x 1.5 or 225 ppi. If you don't know your printer's lpi, the best way to find your optimal print resolution is to experiment. Print a test image at various resolutions to find out what your minimum ppi is for a photo-quality print. A good test image is a close up picture of a newspaper or a photo that has some angled straight edges in it.
What is Actual Pixel Size and When would I Choose It?
In the foregoing discussion, a printer's resolution (both dpi and LPI) only affected your printed image quality not size. There is one exception. In programs that can print actual size, the printer will print one dot for every pixel in your image. The size of the printed image will be determined by the printer's resolution in dpi. For example, if your printer has a 600 dpi resolution a 1200 by 1600 image will be printed to a 2" (1200/600) by 2.66" (1600/600) area.
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