The image size (or pixel dimensions) of an image is a measure of the number of pixels along an image’s width and height. For example, your digital camera may take a photo that is 1500 pixels wide and 1000 pixels high. These two measurements indicate the amount of image data in a photo and determine the file size.
Resolution is the amount of image data in a given space. It is measured in pixels per inch (ppi). The more pixels per inch, the greater the resolution. Generally, the higher the resolution of your image, the better the printed image quality. Resolution determines the fineness of detail you can see in an image.
Although a digital image contains a specific amount of image data, it doesn’t have a specific physical output size or resolution. As you change the resolution of an image, its physical dimensions change, and as you change the width or height of an image, its resolution changes.
You can see the relationship between image size and resolution in the Image Size dialog box (choose Image > Resize > Image Size). As you change one value, the other two values change accordingly.
The Constrain Proportions option lets you maintain the aspect ratio (the ratio of image width to image height). If you select this option and change the image size and resolution, the image does not stretch or shrink.
The Resample Image option lets you change the size of an image without changing the resolution. If you need to print at a specific resolution, or at a smaller or larger resolution than the current image allows, resample the image. However, resampling can degrade image quality.
Your monitor’s resolution is described in pixel dimensions. For example, if your monitor resolution is set to 1600 x 1200 and your photo’s pixel dimensions are the same size, at 100%, the photo will fill the screen. The size an image appears onscreen depends on a combination of factors: the pixel dimensions of the image, the monitor size, and the monitor resolution setting. In Photoshop Elements, you can change the image magnification onscreen, so you can easily work with images of any pixel dimensions.
When preparing images for onscreen viewing, you should consider the lowest monitor resolution that your photo is likely to be viewed on.
You might need to change the print dimensions and resolution if you are sending the image to a print shop that requires files to be at a specific resolution.
If you are printing directly from Photoshop Elements, you don’t have to perform this procedure. Instead, you can choose a size in the Print dialog box and Photoshop Elements applies the appropriate image resolution.
To change only the print dimensions or the resolution, and adjust the total number of pixels in the image proportionately, you must resample the image.
Resample Image must be selected in order to use the Constrain Proportions and Scale Style functions.
Changing the pixel dimensions of an image is called resampling. Resampling affects not only the size of an image onscreen, but also its image quality and its printed output—either its printed dimensions or its image resolution. Resampling can degrade image quality. When you downsample, meaning that you decrease the number of pixels in your image, information is removed from the image. When you upsample, or increase the number of pixels in your image, new pixels are added based on the color values of existing pixels, and the image loses some detail and sharpness.
To avoid the need for upsampling, scan or create the image at the resolution required for your printer or output device. If you want to preview the effects of changing pixel dimensions onscreen or print proofs at different resolutions, resample a duplicate of your file.
If you’re preparing images for the web, it’s useful to specify image size in terms of the pixel dimensions.
Fast, but less precise. This method is recommended for use with illustrations containing edges that are not anti-aliased, to preserve hard edges and produce a smaller file. However, this method can create jagged edges, which become apparent when distorting or scaling an image or performing multiple manipulations on a selection.