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Resolution
Revisited
Originally
appeared in PCPhoto Magazine, September, 1999
www.pcphotomag.com
Answers to important, frequently asked questions By William Sawalich Whether you're using a digital camera or traditional film, if you're working with photos in the computer, you're dealing with different kinds of resolution every step of the way. And since that one term, "resolution," refers to a few different things, we're here to help you understand this sometimes complicated issue. Resolving the many different aspects of resolution will go a long way toward improving the quality of your digital images.
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Most printers are optimized for 240 to 300 dpi images. The lantern looks good at that resolution, but at 72 dpi (about three-quarters fewer pixels), it loses detail and the pixels becomes obvious, so we see "jaggies." Contax Aria, Contax 28-70mm, Kodak Royal Gold 100 |
How is resolution measured?
Resolution is two things: 1) the number of pixels per area (as in dpi--dots per inch); and 2) pixels measuring a fixed area (such as 1600 x 1200). These two types of resolution particularly affect three different working needs: input, image and output.
Input. You'll find input resolution measured in two numbers (such as 1260 x 1020 pixels) for area resolution, or one number (such as 300 dpi) for the number of pixels per inch. They each measure how many little elements, or pixels (short for picture elements), make up the actual image. A general rule: More pixels means better detail.
Image. Once the image has been entered into the computer, it has a fixed file size. That size is determined by the image's resolution at a specific "real-world" size. A 300 dpi, 8x10 image is about 24 MB, as are a 600 dpi, 4x6 image and a 2400 dpi, 1x1.5 image. In the same way, the original 300 dpi 8x10 is only a 4.6 MB file when its dimensions are reduced to 4x6 at 300 dpi, and only 400 KB when it's shrunk to 1x1.5 at 300 dpi. When we talk about big files, we mean more megabytes, which can come from more pixels per inch at a fixed size, a larger physical dimension at a fixed dpi, or more of both.
Output. Output resolution is often most important to photographers. It's a measure of the highest resolution your printer can handle, and you want your print to be good, right? Most printers are optimized for image resolutions of 240 to 300 dpi at the size printed. That's not the same as the number of ink dots a printer puts out, which is the printer resolution. If you have a 300 dpi, 4x6-inch image file, but you enlarge it to an 8x10 for printing, the computer has to interpolate (or make up) image information to fill in the gaps, and it will lose overall quality. It's the printer's resolution (usually measured in larger numbers, such as 720 dpi) that dictates how many physical ink dots into which the printer converts your image file. But dpi isn't the only measure of a printer's ability. The size of each dot, as well as how the dots are laid down, can dramatically affect how a printed image looks to the human eye, which is why some printers that output at 720 dpi with specialized ink patterns and dots look as good or better than other printers at 1400 dpi.
The other output resolution is the monitor. Its resolution affects your work in two ways. First, it dictates what you see on the screen. Most monitors display 72 to 100 dpi, which is why a full-sized, 72 dpi image looks the same onscreen as a full-sized, 300 dpi image (unless you zoom in to the picture until the image pixels are bigger than the screen pixels). Second, if your images are intended only to be viewed on other computer monitors (as with e-mail or the Web), they don't need to be bigger than 72 to 100 dpi because you won't get a better-looking image. That's why some online pictures from people who don't understand monitor resolution seem to take forever to load--they're way too big, even though they look just like smaller images.
What's the difference between a printer optimized for 300 dpi and a print resolution of 1200 dpi?
These are two different measurements--1200 dpi is a measure of the number of dots of ink the printer lays down, while 300 dpi is the picture resolution that the printer needs for a good print. Simply, printer ink dots are interpreted versions of the image pixels. Even with image resolution set below 300 dpi, at its highest quality, the printer will still lay down a line of 1200 dots of ink in an inch.
What's interpolation?
Imagine a 300 dpi, 4x6-inch image file as a grassy field with 300 different golf balls equally spaced over two square yards of turf. There would be space between each of the balls. When you spread out the 300 golf balls to cover four square yards of the field, there's more space between each one.
That's what would happen if you enlarged the 300 dpi, 4x6 image to 8x10. Now, if you didn't have enough golf balls (dpi) to fill in the gaps at 8x10, you'd have to guess at which balls to throw in. The computer also "guesses" what would be in between those spaces in a photo and fills them in with other pixels accordingly. That's interpolation. We recommend that you don't use interpolation with scanners at all, and sparingly for anything else.
How should the input resolution be set?
If you're making prints from a digital camera, keep it at its maximum. But for a scanner, you need to consider how it's going to be output. If you're going to make a printout on a desktop inkjet, most of those printers are optimized for 300 dpi resolution, so anything more than that for the size being printed is overkill. It just uses up disk space and processing power.
What's a megapixel?
The term "megapixel" refers to one million pixels. A megapixel digital camera has an image sensor that's made up of a million pixels (1024 x 1280 = 1.3 M). Typically, each sensor pixel determines each resulting image pixel. More pixels on the sensor means the camera has a better ability to resolve fine detail. As in Figure 2 of the golf balls, where there are more pixels to represent the red line, that line is defined more accurately. That's why you get more detail from higher resolution input.
Why is the resolution of my film scanner so much higher than my flatbed scanner?
The two resolution measurements define the maximum number of pixels into which the scanner can convert the image. And while it would seem that a scanner of 2400 dpi (like your film scanner) would make an image of much higher quality than one of only 300 dpi (like your flatbed), remember that they're each looking at drastically different image sizes. A flatbed scanner is meant for images usually up to about 8x10, but a film scanner is examining an area of only about 1x11/2-inches. Think about it--a one-inch, 2400 dpi scan gives you an image made up of 2400 pixels, but an eight-inch scan at 300 dpi also gives you a 2400-pixel image (8 x 300 = 2400), so they end up with the same image resolution.
How does a scanner's resolution dictate an image's resolution?
A scanner's resolution (like 600, 1200, etc.) is a measure of the maximum number of dots into which it can convert the image. Imagine the green field again as your original. It's 3888 golf balls long and 2592 wide. When you adjust the scan, you're just telling the scanner how much of the field to examine, as well as which balls to look at or ignore. The balls don't change in size or spacing, nor does the field--it's like cropping a print in the darkroom. If a scanner is set to 300 dpi, a resulting 8x10 is 2400 x 3000 dots (area resolution). At 600 dpi, it's 4800 x 6000 pixels--a big file. At 1200 dpi, it's 9600 x 12,000 pixels--a huge file. The number of dots per inch multiplied by the total area determines the image's area resolution.
Does file format alter resolution?
No. However, the compression can affect your image. Compression means that the computer disregards some image information so that it can shrink the file size. There are two types of compression: lossless and lossy. Lossless (such as LZW compression in the .tif format) means there's no degradation of image quality. But lossy compression (such as .jpg compression) means that you may notice a loss of quality between the original and compressed images, but resolution doesn't change.