File Sizes

This page explains the size and resolution of pictures you need to supply for top-quality printed results.

When you take a photo with a digital camera, or scan text or images, you create an image that has specific number of pixels on each side. For example, your digital camera may make a photo that is 1024 pixels wide and 800 pixels high. These two measurements, referred to as the pixel dimensions, have a direct correlation to the image's file size, and both are an indication of the amount of information in a photo.

Resolution is the number of pixels per linear unit of measure (e.g. the number of pixels per inch). While a digital photo contains a specific amount of image data, it doesn't have a specific physical output size or resolution. As you change the resolution of a file, its physical dimensions change, or as you change the width and height of an image the resolution changes.

You can see the relationship between image size and resolution in the image size dialog box (choose Image > Resize > Image Size). Deselect Resample Image, because you don't want to change the amount of data in your image. Then change the width or the height or the resolution. Notice that as you change one value the other two values change.

The image size dialog box with Resample Image deselected. Generally, the higher the resolution of your image, the better the printed image quality. Some printing devices may require a specific image resolution, but for a photo inkjet printer, the resolution can range from 240 to 360 ppi.

Same image printed at 72-ppi and 300-ppi; inset zoom 200%. If you need to print using a specific resolution, or you want to print an image significantly smaller or larger than the image's pixel dimensions allow, you can resample the image. Resampling involves either throwing away or adding pixels to the image. For more information, see Resampling images.

Colour Modes
100%	400%

An image at 100%

There is some excellent, in-depth information on file sizes and resolution available at www.tasi.ac.uk.

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