Making Over the Width and Height of your Digital Image

Changing the width and height of your images is one of the most common adjustments to make — especially when you're dealing with images that come from your digital camera. But adjusting width and height can be fraught with image-damaging interpolation — literally a change in the number and values of pixels in your image — if you are not careful.

But before you leap into the makeover adjustments found in this chapter, please read the Photoshop Confidential (on image sizes versus dimensions) so we can be sure we're all talking the same language.

Photoshop Confidential

Coming to Terms with Image vs. Dimension

We Photoshop users often frustrate ourselves, and each other, by confusing terms like Image size, dimension, and linear resolution. So let's clear the air and squelch the confusion before we go any farther. Photoshop's Image Size dialog box can act as a visual aid in our quest to sort out these characteristics. (Choose ImageoImage Size from the main menu to call it up. Go ahead; I'll wait for you.)

Okay, first we need to clearly separate image dimension from image or file size. In the world of Photoshop, image size (also known as file size) always refers to the combined amount of digitized stuff, measured in bits and bytes, in an image — essentially, how much space it takes up in your computer. In the Image Size dialog box pictured here, the image size is 14.1MB (megabytes — that is millions of bytes). Image dimension, on the other hand, always refers to the how much actual area an image covers — most commonly measured in either pixels or inches. The Photoshop Image Size dialog box shown here gives us those dimensions both ways: as 2560 pixels x 1920 pixels (the on-screen size), or as 35.556 inches x 26.667 inches (the size it would be if you printed it out). These two dimensional values are related to each other by the linear resolution number, which here is 72 ppi (pixels per inch). (Note:If you divide 2560 pixels by 72 pixels per inch, you get 35.556 inches, and 1920 pixels -f 72 pixels per inch gets you 26.667 inches, which is all the math you'll need to figure out linear resolution.) You'll be using this dialog box many times — and after a while, you get very comfortable with it. Having a low linear resolution is fine for display on low resolution devices such as monitors, but having higher linear resolutions is pretty much essential if you want to reproduce your image with high quality on higher resolution output devices such as printers. (It's also important to know the resolution requirements of your output devices prior to creating and/or adjusting your images.)

Keep in mind that digital-camera images usually have relatively large physical dimensions (say, 2 inches by 3 inches) and low linear resolution, usually 72 ppi (but sometimes as low as 1 ppi — as in, "Yikes, instant eyestrain"). Of course, low resolution won't work with large size — for instance, a 2-x-3-inch, 72ppi image is a nearly useless combination of resolution and dimension. The linear resolution (72 ppi) is too low for printing (which typically requires 200-300 ppi) and the image dimension (2 in. x 3 in.) is way too large for use on the Web.

Here's how to make over your digital camera image's width and height while minimizing the damage to your image.

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