Whether creating images for web or print, you’ll no doubt spend some time in the Image Size dialog box. But don’t turn your image resizing into a guessing game — educate yourself on the right way to handle all of your image resizing needs. Read on to discover the truth about image resolution and what really matters when it comes to pixels.
Myth #1: Web graphics should be 72 ppi
In the early days of the World Wide Web, the standard screen resolution was 800 x 600 pixels, which in turn, provided a resolution acceptable for viewing images at 72 or 96 ppi. Hence, 72 ppi images for the web became the norm, as well as part of the lingo. When we hear the term low-res image, a 72-ppi image is what comes to mind.
However, images don’t necessarily need to be 72 ppi to use online. Image resolution is of utmost importance for print graphics, but not so much for web graphics. For one thing, you can’t control what the end user has set for his monitor resolution. Another point is that changing the resolution alone doesn’t change the file size or the dimensions of the image. You need to resample the image to accomplish that.
To test this technique on your own:
- Launch Photoshop and open an image.
- Choose Image > Image Size to display the Image Size dialog box.
- Deselect the Resample Image check box.
- Change the resolution.
- Note that while the Document Size Width and Height values change, the Pixel Dimensions file size, Height, and Width remain the same.
When preparing images for the web, you need to pay more attention to your image’s actual pixel dimensions and file size. So if you need to create a banner graphic that is 400 pixels wide, you should focus on the Pixel Dimensions section of the Image Size dialog box, not the Document Size section.
Myth #2: Print graphics should all be 300ppi
If you’re preparing graphics for printed materials, you might assume that all images should be 300 ppi. That’s simply not true. You use a number of variables to calculate your image resolution for print jobs, including output device, paper stock and line screen.
Output device
Use Table A as a guideline to determine image resolution requirements for any graphics you may place in your documents. However, your printer’s manual or your service bureau’s specifications — which can supply you with tailored specifications — should trump any general recommendations. Table A
Note: For line art, you should set your resolution at 1200 ppi or the resolution of the final output device — whichever is lower.
Line screen and paper stock
Most output devices are capable of printing more than one screen frequency — for the most part, the paper that the piece is printed on determines the line screen. For instance, too high of a line screen may result in dot gain; too low of a line screen and not enough dot will be placed — it all depends on the paper’s characteristics. The typical LPI settings for various media and the paper stock they’re commonly printed on are shown in Table B. Your image’s resolution should be twice the screen frequency to produce enough halftone dot in print. If you need to resize an image, multiply its current resolution by the percentage of enlargement or reduction to get the final resolution it will need:
2 x LPI x resize % = PPI
Myth #3: Don’t resample your images
In the early desktop publishing and electronic image age, designers used great caution when resampling images. Much of the resampling hesitation stemmed from the fact that film scans were notoriously grainy. With today’s digital cameras, you’re often safe to upsample an image 200 percent! Now that’s progress!
Keep these tips in mind as you upsample (add pixels to) or downsample (remove pixels from) your images:
- Don’t resample incrementally. In Photoshop’s early years, pixel pushers had to resample in 10-percent increments to achieve the best possible quality. But that’s no longer the case. Photoshop’s Bicubic Smoother and Bicubic Sharper resampling algorithms work best in a single pass. In fact, incremental resampling in current Photoshop versions can have adverse effects on your images, such as accentuating the artifacts caused from resampling.
- Start with a quality image. You’ll get a much better resampled image if you start with a high-quality image. Photoshop isn’t a miracle worker, so don’t expect a lousy image to resample into a better one.
- Interpolation doesn’t add detail. When you resample images you’re relying on predetermined algorithms to either add or remove pixels from your image. You’ll never achieve more detail than what you start with.
- The more pixels, the merrier. If you have to resample images, you’re better off downsampling from high-resolution images with more pixel data, than upsampling from low-resolution images with less pixel data.
- Purchase a plug in. Since upsampling is generally more complicated than downsampling, consider purchasing a third-party plug-in designed specifically for this task.
Related Courses:
- InDesign CS3 - Level 1
- InDesign CS3 - Level 2
- Photoshop CS3 - Level 1
- Photoshop CS3 - Level 2
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