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Main / How to: Understanding Resolution

When you work with bitmap 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.

The Two Faces of Resolution
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. 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. For clarity, we will refer to the first type of resolution as pixel count resolution and the second as embedded resolution.

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 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 the difference between ppi and dpi?
The term ppi (pixels per inch) originated in the world of computers, and dpi (dots per inch) in the world or printing, but today they are often used interchangeably.

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.

Which resolution setting should I choose when I take a picture?
It depends on what you want to do with the picture. Do you want to e-mail it to friends, post it on a Web site, make it your computer's wallpaper, print it as a 4" x 6" photograph, or create a poster-sized print? For images that will be viewed on a computer monitor (such as those you send by e-mail or post to the Web), a low pixel-count setting is perfectly adequate. Since most people view images on monitors that display only 800 x 600 pixels, a low pixel-count image, such as a 600 x 400 photograph, will fill up most of their screen without running off the edges. A low pixel-count setting will also reduce the file size of the image and reduce time it takes others to download or display your image.

Printers, however, can print at much higher resolution than a typical computer screen. Images that you intend to print should be captured at a higher pixel-count setting.
For a 2" x 3" print, the image dimensions should be 400 x 600 pixels minimum

For a 4" x 6" print, the image dimensions should be 800 x 600 pixels minimum

For a 5" x 7" print, the image dimensions should be 1000 x 1400 pixels minimum

For an 8" x 10" print, the image dimensions should be 1600 x 2000 pixels minimum

You can learn more about printing and resolution by reading this section.
If you don't know what you want to do with your image the moment you take a picture, to be safe, it's a good idea to set your camera to the highest resolution setting. You can always reduce the pixel-count of your image later for e-mailing or web publishing.

What resolution should I use to scan an image?
As with taking a digital photo, your intended use for the image determines your best scanning resolution. For example, a 4" by 6" photograph scanned at 300 ppi will have large pixel dimensions and will appear large on a computer screen, while the same photograph scanned at 72 ppi will have fewer pixels and will appear much smaller on a screen.

Scanning resolution is expressed in terms of dpi or ppi, which you will recognize from our earlier discussion of embedded resolution. The scanning resolution not only determines how many pixels will be captured, it also gets embedded into the image as an invisible piece of information for future output devices. To calculate the pixel dimensions of a scanned photograph, multiply the scanning resolution by the dimension in inches: Resolution x Inches = Pixels. Using this formula you can calculate the pixel dimensions, for example, of a 4" x 6" photograph scanned in using 300 ppi:

300 pixels/inch x 4 inches = 1200 pixels
300 pixels/inch x 6 inches = 1800 pixels

So your 4" x 6" photograph will be displayed as 1200 x 1800 pixels on your monitor. Unless you have very large monitor, the image will spill over the edges.

Of course, you could later resize the scanned photograph in FotoFinish, or you could rescan it using a lower resolution (ppi) setting. To calculate the resolution you should use to scan a photo, first decide how big you want your image to appear on your monitor (Pixels / Inches = Resolution). For example, if you want your 4" by 6" photograph to appear as 400 x 600 pixels on the monitor, then you would scan it in at 100 ppi. If you want to create a 100 x 150 pixel thumbnail, you would scan in your 4 inch x 6 inch photograph at 25 ppi.

Keep in mind that as you increase your scanning resolution you create larger files that might be inappropriately large for e-mailing or web publishing.

What embedded resolution should I use to create a new image?
The appropriate embedded resolution depends on how you want to use the image. If you want to print your image, you should create it with a high embedded resolution (200 to 300 ppi).

When you create graphics for the screen, embedded resolution does not matter. Simply pick the pixel dimensions of your image and don't worry about embedded resolution. Most people use 72 ppi for creating web graphics but this is just an arbitrary embedded resolution that has become a standard. You don't need to use 72 ppi to create a web graphic. Since monitors display images based on pixel dimension, embedded image resolution will not affect how large or small an image looks on the screen. A 300 by 199 pixel image set to 72 ppi and a 300 by 199 pixel image set to 300 ppi will look the same on the same screen. As discussed above, the embedded resolution (ppi or dpi) of an image only affects the image when printed.

What embedded resolution should I use to print an image?
To understand how embedded resolution affects the printed dimensions of an image, take a sample photograph and choose the Scale command in the Image menu in FotoFinish. As you increase the resolution, the printed dimensions of the image go down. As you decrease resolution, the printed dimensions go up. The printed dimensions change because FotoFinish spreads or condenses the fixed number of pixels in your image to achieve the specified resolution. To create a higher resolution, the pixels are condensed to a smaller area, and vice versa.

How do I calculate what resolution I need to print a 4"x6" image?
With FotoFinish you can easily print a 4"x6" photo using the standard photo size templates. If you use the Photo Wizard, you don't need to know anything about resolution to print your image. Simply pick a template and print. However, if you want more control over print quality and size, you can use the simple resolution formula to calculate the embedded resolution you need for any given output.

For example, let's imagine a 2 megapixel camera that creates a 1600 by 1200 pixel image. You can print this image at any number of different sizes by specifying different embedded resolutions. Take the length of the image in pixels and divide it by your target length in inches. The resulting number is the embedded resolution in ppi (or dpi):

1600 pixels / 11 inches = 145 ppi
1600 pixels / 10 inches = 160 ppi
1600 pixels / 9 inches = 177 ppi
1600 pixels / 8 inches = 200 ppi
1600 pixels / 6 inches = 266 ppi
1600 pixels / 4 inches = 400 ppi
1600 pixels/ 2 inches = 800 ppi

As your desired printing area decreases, your resolution increases. As a general rule, you can get a good, photo-quality print at 200 ppi or above. However, photo quality is in the eye of the beholder. You'll probably need to experiment to find the resolution that looks acceptable on your printer.

Why does my image look blocky and blurry when printed?
If you choose to print a small image at a large size, the pixels in your image are stretched to fill the requested print area. In effect, FotoFinish has to reduce the image's embedded resolution. Your eye perceives this as a blocky, blurry, or "pixelated" effect.

Why does my image look different on my monitor from when I print it?
One of the hardest things to understand about desktop publishing is that the size of the image on your screen does not accurately reflect the size of your image when you print it. An image that fills your entire screen might only be a small thumbnail when printed (although this would be an extreme case).

When your image's embedded resolution is higher than the monitor's display resolution, the image appears larger on the screen than when printed. Luckily, FotoFinish allows you to see a preview of how your printed image will look. Just set the View menu to View at Output Size. Your image will be displayed on-screen as it would be printed if your chose to print it at 100% of its actual size.

What is my monitor's resolution?
Monitor resolution refers to the number of pixels or dots displayed in a given unit length of the monitor. Your monitor's resolution will depend on the size of your monitor (15 inches, 17 inches, etc.) as well as its screen area setting (1024 x 768, 800 x 600, etc.).

What is printer resolution and do I need to worry about it?
Printer resolution is the number of ink dots printed on an inch of paper measured in dpi (dots per inch). 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 pick a template in FotoFinish or 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. If you choose Actual Pixel Size in FotoFinish's Print dialog then printing at 100% of pixel size means 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. Choose FotoFinish's Actual Pixel Size option if you want to print the best quality image your printer can create.

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