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 3000 pixels wide and 2000 pixels high. These two measurements have a direct correlation to the image’s file size, and both are an indication of the amount of image data in a photo.

Resolution is the fineness of detail you can see in an image. 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.

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 a file, 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). Deselect Resample Image, because you don’t want to change the amount of image data in your photo. Then change the width, height, or resolution. As you change one value, the other two values change accordingly.

The same image printed at 72 ppi and 300 ppi; inset zoom at 200%

If you need to print at a specific resolution, or if 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 removing or adding pixels to the image to achieve the desired dimensions or resolution.

Printer resolution is measured in ink dots per inch (dpi). Generally, the more dots per inch, the finer the printed output you’ll get. Most inkjet printers have a resolution of 720 to 5760 dpi, the latter being the smallest dot size a printer can use.

Printer resolution is different from, but related to, image resolution. To print a high-quality photo on an inkjet printer, an image resolution of about 220 ppi should provide good results. Using a lower resolution, you can print a slightly larger photo—if you are willing to accept some image degradation. You’ll see a warning if you chose a print size that causes the photo to print at less than 220 ppi from the Editor or 150 ppi from the Organizer.

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 on‑screen 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 on‑screen, so you can easily work with images of any pixel dimensions.

A 620 x 400‑pixel image displayed on monitors of different sizes and resolutions

When preparing images for on-screen viewing, you should consider the lowest monitor resolution that your photo is likely to be viewed on.

1. In the Editor, choose Image > Resize > Image Size.
2. Make sure that Resample Image is deselected. If deselected, you can change the print dimensions and resolution without changing the total number of pixels in the image, but the image may not keep its current proportions.
   Note: Resample Image must be selected in order to use the Constrain Proportions and Scale Style functions.
3. To maintain the current aspect ratio, select Constrain Proportions. This option automatically updates the width as you change the height, and vice versa.
4. Under Document Size, enter new values for the height and width. If desired, choose a new unit of measurement. For Width, the Columns option uses the width and gutter sizes specified in the Units & Rulers preferences.
5. For Resolution, enter a new value. If desired, choose a new unit of measurement, and then click OK.
   To return to the original values displayed in the Image Size dialog box, use Alt + click Reset.

Changing the pixel dimensions of an image is called resampling. Resampling affects not only the size of an image on‑screen, 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.

Resampling an image

A.

Image downsampled

B.

Original image

C.

Image upsampled

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 on‑screen 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.

1. In the Editor, choose Image > Resize > Image Size.
2. Select Resample Image, and choose an interpolation method:

   Nearest Neighbor

   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.

   Bilinear

   Medium-quality.

   Bicubic

   Slow, but more precise, resulting in the smoothest tonal gradations.

   Bicubic Smoother

   Use when you're enlarging images.

   Bicubic Sharper

   Use when you’re reducing the size of an image. This method maintains the detail in a resampled image. It may, however, oversharpen some areas of an image. In this case, try using Bicubic.

3. To maintain the current aspect ratio, select Constrain Proportions. This option automatically updates the width as you change the height, and vice versa.
4. In Pixel Dimensions, enter values for Width and Height. To enter values as percentages of the current dimensions, choose Percent as the unit of measurement.

   The new file size of the image appears next to Pixel Dimensions, with the old file size in parentheses.

5. Click OK to change the pixel dimensions and resample the image.
   For best results in producing a smaller image, downsample and apply the Unsharp Mask filter. To produce a larger image, rescan the image at a higher resolution.