Resolving Resolution: Getting the Apples and Oranges to Line Up






Resolving Resolution: Getting the Apples and Oranges to Line Up

Sometimes when Photoshop users print an image that they’ve been working on, they’re upset that the printed image doesn’t look like the image on the monitor. The printed image looks jaggy or pixilated. When comparing an on-screen image to the image on the printed page, it’s like comparing apples and oranges: The screen image is composed of square pixels and the printed image is composed of tiny round dots, and (here’s the important part) both devices deliver high-quality images at different resolutions.

Pixels are the tiny squares that make up Photoshop images. Pixels are very small and when viewed from a distance, the colors in these tiny pixels combine to make up the image you see. Image resolution on the screen is discussed by using the term pixels per inch or ppi. Typical Web graphics are saved at 72 or 96 ppi. (A Mac monitor’s normal ppi is 72 and a PC monitor’s maximum ppi is 96.) All the images that you create in Photoshop look great at 100% magnification on the monitor at 96 ppi.

Printed images, on the other hand, are created with tiny round dots of ink or toner. Image resolu-tion for the printed page is discussed in dots per inch or dpi. A square inch removed from an image printed at 96 dpi contains 96 tiny dots across and 96 tiny dots down, making 9,216 dots in that one-inch area. A square inch removed from an image printed at 300 dpi contains 300 dots across and 300 dots down, for a total of 90,000 dots per square inch. As you can imagine, having more dots packed into an inch of printed area makes for a smoother-looking image. A typical LaserJet prints good-quality images at 300 dpi. Many home printers even go as high as 720 dpi. Commercial imagesetters can print images at 1200 dpi, 2400 dpi, and up.

Take a look at Figure. I saved the same image at three different resolutions. Check out how the different resolutions look on the printed page.

Click To expand
Figure: The same image saved at 72 ppi (top), 150 ppi (middle), and 300 ppi (bottom).

So, where does this leave you when you want to make great prints? First off, look at the documentation that came with your printer and find out what the maximum resolution output is for the printer. Next, choose Image>Image Size to open the Image Size dialog box shown in Figure.

Click To expand
Figure: Use the Image Size dialog box to find out the resolution of your image.

Take a look at the Resolution text box in the Document Size area. What does it say — 72 ppi, 96 ppi, 150 ppi, or 300 ppi? If you’re at the upper end (250 ppi or 300 ppi) your image will, most likely, print well and look pretty good. If your image is at the lower end, say 100 ppi, you’re going to have problems with dots and jaggies. You need to spend a little time experimenting with printing your image at different resolutions to see what kind of results you can get with your printer. (Also, some printers need special paper to create quality images. If you’re printing on standard office paper, chances are that you’re not going to get very good results.)

If you need to change the resolution of your image to make it look better in print, you can do it in one of two ways — by using the Image Size dialog box shown in Figure or by using Photoshop’s Print Preview feature. The Image Size dialog box gives you more control over the exact resolution, whereas the Print Preview dialog box lets Photoshop come up with the resolution based on the scaling size that you select.

To set a new resolution by using the Image Size dialog box:

  1. With the image open in Photoshop, choose Image>Image Size to open the Image Size dialog box shown in Figure.

  2. Enter a value in the Resolution text box in either pixels per inch or pixels per centimeter.

    Tip 

    Here’s another way to let Photoshop help you out. In the Image Size dialog box, click Auto to open the Auto Resolution dialog box shown in Figure. Enter a line screen setting of 133 lines per inch (or 52 lines per centimeter), and then select a Quality radio button to define the output quality that you want. Click OK to close the Auto Resolution dialog box and return to the Image Size dialog box. Based on your selections in the Auto Resolution dialog box, Photoshop calculates the resolution that it thinks will work best and enters it in the Resolution text box. Try printing with this resolution and see if the results work for you.


    Figure: Use the Auto Resolution dialog box to let Photoshop calculate a resolution for you.

  3. Optional: Uncheck the Resample Image check box.

    If you turn off this option, Photoshop doesn’t add or subtract pixels (depending on whether you are increasing or decreasing the resolution). Photoshop leaves the pixels intact and changes only how many pixels print per inch. (Test this and see if it works for your printer.)

  4. Click OK.

    The Image Size dialog box closes and Photoshop adjusts the resolution of your image. Now you’re ready to make a test print and see if the image prints to your satisfaction.

    Tip 

    If you find that the resolution you set doesn’t work, you can always return to the original resolution by using the History palette to select a history state before the resolution change. To find out more about the History palette, turn to Technique 7.

To let Photoshop set the resolution by using Print with Preview:

  1. With the image open in Photoshop, choose File>Print with Preview.

    In the Print dialog box that opens (see Figure), use the Scaled Print Size area of the dialog box to set the size of the image on the printed page.

    Click To expand
    Figure: Use the Scaled Print Size area to let Photoshop figure out the resolution.

  2. Enter a percentage in the Scale text box, select the Scale to Fit Media check box, or enter actual printed dimensions in the Height and Width text boxes.

    Percentages in the Scale text box work just like percentages when zooming in and out in Photoshop. 100% scale prints the image at actual size, 200% scale prints the image at twice its actual size, and 50% scale prints the image at half its actual size.

    Remember 

    By changing the scale of the image in the Print Preview dialog box, you’re only changing the size of the printed image, not the size of the actual image.

    Tip 

    Scaling the printed image greater than 100% lowers image resolution, and scaling the image below 100% increases resolution.

  3. Click Print to continue with the printing process.



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