Resizing Images with Photoshop

Why resize?

The biggest reason to resize is memory, whether it be the amount of disk space used to store the image or the amount of program memory the image takes up when loaded for display. Smaller images take less memory than large images. MS PowerPoint presentations will run more smoothly if images are resized and compressed. Web pages will load faster if smaller images are used.

Why shouldn't I resize in PowerPoint or Adobe PageMaker?

Resizing using the resize 'handles' or other methods in those applications does not change the amount of data in the image. It changes the amount of image data displayed but not the amount stored in memory.

A few words about resolution

Resolution, or the amount of information per unit of display, also plays a role in image size. Resolution is often measured in dots-per-inch (dpi). Today's image scanners can easily produce images that are 300-600 dpi. This is great if you are intending only to print the image on a printer. If the image is intended for a web page or a PowerPoint presentation, resizing is recommended. The reason is that a computer screen will only display 72-96 dpi, depending on screen size. LCD projectors behave similarly, though their pixels are generally larger on a projector screen. Anything more than 72-96 dpi on a projector or monitor is wasted. It will not improve the quality of the displayed image.

Resizing vs. Resampling

It is possible to change the resolution (dpi) without changing the amount of information contained in the image. An image that is 1 inch by 2 inches with a resolution of 300 dpi contains the same amount of information as an image that is 3 inches by 6 inches with a resolution of 100 dpi. The first is a common result after scanning an image into the computer, while the second is more appropriate for a PowerPoint presentation. The process of changing the resolution without resizing the image is called resampling.

Resizing actually changes the amount of information contained in the image. This is usually done without changing the resolution of the image. Resizing is normally making an image smaller, but also applies to making it larger. Enlarging images may not work well since the computer uses math formulas to create additional image data out of what already exists.

How to resample in PhotoShop

Open the image to be resampled in PhotoShop.

Select Image Size... from the Image menu.

Uncheck the box next to Resample Image. This will cause the pixel dimensions at the top of the resize dialog window to be grayed out.

Change the resolution as desired. 72-100 dpi is good for web and PowerPoint images. If the dimensions of the image become too large or too small at the desired resolution, then resizing is necessary.

Click OK to resample the image.

How to resize in PhotoShop.

Open the image to be resized in PhotoShop.

Select Image Size... from the Image menu.

Make sure both Constrain Proportions and Resample Image are checked. Constraining proportions ensures the image doesn't become distorted when resized:

Enter the desired size either in pixels or as a percentage (pixels or percent can be selected from the popup selector menu next to the number boxes. As a guide, a typical 14" (diagonal) computer screen displays 640 by 480 pixels. The uncompressed image size is given in K:

Click OK to resample the image.

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