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Sensors, Pixels and Image Sizes

Pixels Per Inch

Normally you don't have to change the number of pixel's in an image to change the size of a printout. That task is handled by the software program you use to print the image. For example, if you place an image in a program such as QuarkXpress or PageMaker, it's automatically printed at the size you specify in those programs. In Photoshop there are two ways you can change an image's size—by changing the number of pixels in the image; or by changing the size of the area in which the available pixels are printed or displayed the document size. These two procedures are separate but related.

In Photoshop, you can display the image size dialog box, turn off "Resample Image" and then specify any print size. The resolution of the image is calculated and displayed in pixels per inch. If it's between 200-300, the results should be good on most inkjet printers.
  • Pixel dimensions specifies the number of pixels an image contains. Initially determined by the number of pixels captured by the camera there are times you may want to change this size by deleting or adding pixels. For example, you may want to e-mail or post an image on a Web site. For this purpose it's best if an image is no larger than the lowest common denominator screen, usually 640 x 480, or 800 x 600. Reducing an image's size also makes the file size smaller so the image can be sent or displayed more quickly. The main reason you would increase the number of pixels in an image is to make large prints. Since most images print best when they are printed at 200-300 ppi you may get better results by enlarging the image rather than letting thepixels per inch fall below 200.

    To change the number of pixels in an image, you resample it to make it smaller by removing pixels, or larger by adding them. Reducing an image usually has less affect on its appearance than does enlarging one. This is because enlarging requires the program to add pixels—a process called interpolation. The computer analyzes adjoining pixels to determine the color of the new ones it inserts. Normally, you can double the size of an image without effects showing. However, trial and error is the only way to be sure because images vary so much. Look for the image becoming soft, as if it's not sharply focused. If you are making other changes to the image, resampling it should be done after all other changes other than sharpening . This is because most adjustments work best where there are the maximum number of original pixels to work with.
  • Document size specifies how large an image will be printed or displayed, especially in other applications. Normally you change the document size with resampling turned off. As a result, as the size increases, the pixels per inch decrease because the same number of pixels are spread over a wider area. If the resolution falls below 200 or so pixels per inch, you may want to consider resampling the image. There are problems printing with less than 200 pixels per inch and with resampling to increase the number of pixels so you'll have to experiment to see which works best for a particular image. Just be sure your image is not too large to fit on the page. Many printers can't print to the edge of the sheet so there is always a border. To print the full image, it must fit inside this border area.

    To change either the pixel dimensions or document size, select Image> Resize>Image Size in Photoshop Elements or Image> Image Size in Photoshop to display the Image Size dialog box having the following settings:
  • Pixel Dimensions shows the image's Width and Height in pixels and next to the heading is the size of the image file. You can click the drop-down arrow to specify changes as a percentage.
  • Document size shows you the current Width and Height of the image in inches, centimeters, or any other unit of measurement you select with the drop-down arrow. Resolution displays the image's pixels per inch at the current document size. This number changes as you change image width and height. If you make the image larger, the existing pixels are spread over a larger area so the pixels per inch decreases. The only way to change this relationship is to add more pixels to the image by resampling it.
     
    . The document size that you specify for an image determines its size if you copy or place it into a document created with another application.

    . Some people swear that when you enlarge an image by resampling it, you get better results if you enlarge it in 10% steps until it reaches the size you want.

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  • Constrain Proportions check box determines if one of the photo's dimensions will adjust automatically when you change the other. If you turn this off, the image's proportions or aspect ratio changes and the image is stretched in one direction. Unless you are after a special effect, you normally leave this check box on. Chain link icons connecting the width and height settings indicate when this setting is on.
  • Resample Image check box determines if the number of pixels in the image will change when you change the size. When specifying a size for printing you usually turn this off. When you resample an image to add or subtract pixels, you can choose a process that trades off quality versus speed. Nearest Neighbor is fast but doesn't give the best results, Bilinear is faster and gives better results, and Bicubic, the default, is slowest but best. One thing to keep in mind is that if you enlarge a print too much, it won't be as sharp as you may desire. That's because a certain minimal number of dots per inch, usually between 200 and 300 are needed to get a good print. Pixels begin to show when the print is enlarged to a point where the dots per inch (dpi) fall too low. If your printer prints the sharpest images at 300 dpi, you need to determine if the size of the image you plan on printing will fall below this level. To do so, you divide the chosen dpi by the width of the image in inches. For example, if you print an image that's 1600 pixels wide so the print is 10" wide, there are only 160 dots per inch (1600 pixels 10 inches = 160 pixels per inch). However, if you print the same image so it's 5 inches wide, the dots per inch climbs to over 300.
Imagine an image made up of dots printed on a rubber sheet. As you stretch the rubber to make the picture larger, the dots spread out lowering the pixels per inch.
 
If you make any mistakes in the dialog box, hold down Alt (Option on Macs) to change the Cancel button to Reset and click it to start over.


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