Image Resolution: What You Need to Know

by Gloria Hansen

One of the issues that comes up repeatedly when I answer questions from artists working with digital images, concerns image resolution. There's a lot of confusion regarding image resolution because it can relate to so many things - printers, scanners, monitors, images, digital cameras, etc. Even more confusion can arise because of the different terms - such as dpi, ppi, lpi, pixel, megapixels - used to describe resolution.

To illustrate the confusion, try answering this question: Which of these digital images contains the optimal resolution for creating a 35mm slide?

1. A 300 dpi image measuring 6x4 inches;
2. An 1,800 pixel x 1,200 pixel image; or
3. A 2 megapixel image.

The answer is any of them. That's correct - they are all the same.

To clarify the confusion, first understand that resolution refers to the number of "pixels" - the smallest element of a digital image - that makes up an image in both height and width. It's generally expressed as a ratio; for example, 800x600 pixels. Next, let's look at the math in the above example: 300 x 6 = 1,800. 300 x 4 = 1,200. Thus, a 300 dpi (dots per inch) image measuring 6x4 inches is a 1,800 pixel x 1,200 pixel image. And, 1,800 x 1,200 = 2,160,000 pixels, which is just over 2 million pixels - in other words 2 megapixels (a megapixel is one million pixels).

Generally, a slide-making lab requests a "300 dpi image measuring 6x4 inches" for two reasons: (1) a 6x4 aspect ratio matches the aspect ratio of a 35mm slide; and (2) the dpi (or dots per inch) translates to the necessary resolution for quality (however, because of the differences in the imagers, always check with your processor first for their optimal settings).

Just because a 300 dpi image measuring 6x4 inches is optimum for creating a 35mm slide does not mean it is also good for a screen graphic. In fact, it's important to understand that dpi (dots per inch) has absolutely no relevance to a Web site or any other screen-displaying device. This is because digital images are always displayed at the image's full resolution; the size that the image will appear on a monitor always depends on the resolution of the monitor. For example, if you have a 600x400 resolution monitor, the image in the example, which is 1,800 pixels x 1,200 pixels, will not fit on the screen because it will be displayed at its maximum height and width. Such an image will require a lot of scrolling to view it. It will also not fit on an 800x600 resolution monitor, although more of the image will show. Thus, a 6x4 inch 300 dpi image - which is a 1,800x1,200 pixel image, is far more resolution than is needed. To get the image to properly fit, the image needs to be "resampled." Resampling is simply a way to change the pixel dimensions. "Downsampling" is commonly done to images for the Web, as it reduces the number of pixels.

One way to resample an image is to use image editing software such as Photoshop, Image Ready, or Photoshop Elements. In Photoshop, for example, go to Image>Image Size. The pixel dimension size is shown on the top, and the document size is underneath. Simply check "Constrain Proportions" and "Resample Image" (with "Bicubic" setting). Then change either the width or the height pixel dimension.

Returning to the example, what if we want to print the 300 dpi image measuring 6x4 inches? While it may seem the image will print at 6x4 inches, this is only true if the image is printed at 300 dots per inch. The same image printed at 600 dpi will print at 3x2 inches; printed at 150 dpi, the image will print at 12x8 inches. In other words, if the dots per inch are increased, the printed image will decrease in size (and the quality may increase); if the dots per inches are decreased, the printed image will increase in size (and the quality may decrease). The quality of the print depends on the resolution of the image.

For home inkjet printing, many people are surprised to find that today's print drivers can nicely cope with a wide assortment of resolutions. For example, printing a document from an Internet page, which is at a screen resolution, can look acceptable. How-ever, if you plan to print a photograph on good quality paper, a higher resolution will produce a better image. The rule of thumb is: look at your printer's resolution. The root resolution of many (but not all) inkjet printers is either 300 dpi or 360 dpi. For example, if the dots across are 1,200, then there are 300 dpi per color (four divided into 1,200 is 300 dpi; as four colors to create a print - black, magenta, cyan and yellow). Any print resolution on a subset of that number; e.g., 75, 150 or 300, will work. Some experimentation with your printer will give you an idea of what works best, and, for most print work, resolution at or under 300 or 360 dpi is quite acceptable and anything over is a waste. If your printer uses more than four colors (for example, the Epson 2200 uses 7 colors), the same "divide by four" rule applies because the extra colors generally "fill in" for one of the standard four colors to enhance highlight or shadow detail.

Another term that comes up when talking about resolution is "lpi" (lines per inch). Lines per inch (or lines per centimeter) is a commercial printing term. Before the advent of the digital print process, images had to go through a process known as "half-toning." This converted an image to a series of different sized dots. The bigger the dot, the darker the image appeared to be. Lines per inch related to the number of the smallest dots that could be printed. This varied a lot depending on the paper and ink used, the accuracy of the press - even the temperature. The higher the lpi, the better quality the image appeared to be. This process still happens today, but the images are converted digitally - in most cases by the print driver.

The rule of thumb for resolution for commercial printing is this: For monochrome images, the dpi should be at least 25 percent over the desired lpi. Thus an image that will be printed at 150 lpi should be 200 dpi. For color images, the dpi should be double that of the lpi. Most high-end magazines use 150 lpi. Thus, the image should be at 300 dpi. Keep in mind that using more resolution than is required will not improve the quality of the image. It will only make the image larger in size and take longer to print. When in doubt, ask your printer or service bureau employee.

By thinking of digital images in terms of resolution, you can eliminate a lot of confusion. By understanding how much resolution is needed for your particular needs, you can help create the best image possible.

Gloria Hansen is an award-winning quilt artist residing in East Windsor Township, New Jersey, is the co-author of "The Quilter's Computer Companion," and author or co-author of 13 other computer/craft-related books. She is co-owner of GloDerWorks, a web design, hosting, and programming company, with offices in the USA and UK. Visit her at http://www.gloriahansen.com/. Article provided by CraftsReport Magazine.

Edited by Robyn Kelly, Web Publications Manager: rkelly@iac.in.gov.
The IAC, a public catalyst, partner, and investor in the arts, serves the citizens of Indiana by funding, promoting, and expanding the arts. Funding is provided by the State of Indiana and the National Endowment for the Arts.