I’ve met many photographers who don’t understand how to change an image’s size or resolution correctly. The problem usually stems from a lack of understanding about image resolution and how it affects image size.

Caution: Always save a master file with all layers before changing size or cropping. Otherwise it may be hard to back up and resize the file for a different output use later.

One of the things that can be the hardest to get your head around when you start down the digital path is resolution. This confusion is compounded by the fact that there are two different kinds of resolution in the digital world. One is dots per inch, and the other is pixels per inch.

1. Dots per inch (dpi). This refers to the number of dots per inch that an inkjet printer is capable of applying to a sheet of paper. It can range from 720 to 2800 and more. The closer these dots are to each other, the more they blend together forming continuous tones on the print. Naturally, this depends on the paper that’s being printed on. If the paper is porous watercolor paper, the dots soak in and blend just fine at lower dpi settings like 720. On glossy photo papers, a higher setting—such as 1440—is needed because the ink dries on the surface. About the only time people discuss dpi is when they are talking about a printer.
2. Pixels per inch (ppi). This is what is usually discussed when talking about resolution in digital photography. It refers to the distance between the pixels that make up digital images. Pixels per inch is an important setting because it determines what digital images look like when they’re displayed and printed. Sometimes a lower value is desirable, and other times a higher value is preferred.

If you have a file with a resolution of 120 ppi or lower, you run the risk of seeing the space between the pixels when you print. This causes edge detail, which should be smooth in the print, to look jagged. The goal is to get the pixels close enough together so that these single dots form continuous tones and lines. The illustration below gives you an idea of how this works. As the dots get closer to one another, they begin to form a line. When you zoom out, the individual dots disappear. (It’s similar to the idea of getting ink dots close together on a printer (dpi).

Tip: Keep in mind that some people mistakenly use the terms dpi and ppi interchangeably, saying dpi when they really mean ppi. If they’re not specifically talking about a printer’s output, then they probably mean ppi.

The thing to keep in mind when it’s time to change resolution is that if you have an image file that’s 150 ppi and you simply change its resolution to 300 ppi for printing at a lab, the new image’s dimensions are affected because the distance between the pixels is cut in half. An 8 × 10 becomes a 4 × 5 at 300 ppi.

I know I just said the dreaded M word, but please keep reading because in the next few paragraphs I show you how to use simple math to understand what Photoshop is doing when you resize your photo files. Begin by getting a better handle on how resolution works. Follow these steps:

• Choose File > New to create a new file. The New dialog box appears.
• Set the following attributes, as shown below, and then click OK to open the new file:
  1. Width = 8 inches
  2. Height = 10 inches
  3. Resolution = 150
  4. Color Mode = RGB Color, 8-bit
  
  

  

  
  
• Choose Image > Image Size (Alt+Command+I/ Alt+Ctrl+I). The Image Size dialog box appears. Notice that the starting dimensions are the same as what you specified with the New File command.
• Deselect the Resample Image option. Notice that the Pixel Dimensions area at the top of the Image Size window goes gray when Resample Image is turned off. That means that the number of pixels being used in your image is fixed at 1200 × 1500 pixels.
• Deselect the Resample Image option. Notice that the Pixel Dimensions area at the top of the Image Size window goes gray when Resample Image is turned off. That means that the number of pixels being used in your image is fi xed at 1200 × 1500 pixels.
• Change the resolution setting to 300. Leave the measurement at pixels/inch. Notice that the size of the image goes from 8" × 10" to 4" × 5", as shown below. That’s because you’re only changing the distance between pixels as you modify the resolution of the file. You have fixed overall pixel dimensions of 1200 × 1500. When you set your resolution to 300 ppi, the math dictates that the image is 4" × 5" (4" × 300 ppi = 1200 pixels, and 5" × 300 ppi = 1500 pixels). When the resolution is set to 150 ppi, the image must be 8" × 10" in size (8" × 150 ppi = 1200 pixels, and 10" × 150 ppi = 1500 pixels).
  
  

  

  
  Resampling the Image Size
  

  In the previous example, you were asked to turn off Resample Image because Resample Image has a special function that affects the math involved in resizing an image. When you turned off Resample Image, the Pixel Dimensions remain fixed when you changed the resolution value. Take a look at what happens when Resample is turned on:

  1. Begin with the file you created in the previous set of steps, and return to its original state using the History panel. If you closed it, then go back and complete Steps 1 and 2.
     
     Tip: If you still have the Image Size window open from the preceding example, you can reset it to the settings it had when you opened it by holding down Alt and clicking Reset—where the Cancel button used to be. This works with almost every dialog box where you see a Cancel button.
  2. Choose Image > Image Size (Alt+Command+I/ Alt+Ctrl+I). The Image Size dialog box appears.
  3. Leave the Resample Image option selected or select it if it isn’t already checked.
  4. Change the Resolution to 300. Leave the dimensions at pixels/inch. Notice that the Image Size remains at 8" × 10". What changed was the Pixel Dimensions at the top of the window. They went from 1200 × 1500 to 2400 × 3000.
  5. Change the Resolution to 600. Now the Pixel Dimensions changes to 4800 × 6000, but the Document Size remains fixed, as shown below.
     
     

     

     
     

  Something to notice when comparing the two previous illustrations is the little chain icon that appears to the right of the Document Size settings. This icon indicates that these values are linked. When one value is changed, the other linked value is affected. In the first illustration, where Resample is off, the Width, Height, and Resolution settings are all linked. In the second, where Resample is on, only the Width and Height values are linked. Resolution is not be affected by changes to these values. This is a great way to remember which Resample setting to choose.

  When Resample Image is turned on, modifi cations to Resolution or Document Size in the Image Size dialog box affect only the Pixel Dimensions. When you make an image’s width and height dimensions smaller, or reduce its resolution, resampling takes pixels from the image. This is called downsampling.

  If you make the Document Size larger or increase the Resolution value, resampling adds pixels to the image. This is called upsampling. You can verify that this is taking place by looking at the file size readout next to Pixel Dimensions in the previous illustration. The original file size was 5.15 megabytes (M). Now, with the addition of all the new pixels being added by Photoshop, the file size is 82.4M.

  Tip: There are two different ways to measure a photo file’s size in megabytes. One is the size of the photo when it’s open. This size is determined by the pixel dimensions — width × height × three color channels. This is the size shown next to Pixel Dimensions in the Image Size dialog box. The second measurement is for when the file is saved to disk. Naturally, this varies by the type of file that’s saved. For example, a saved TIFF file is always bigger than a JPEG saved from the same file because the JPEG is compressed. This size is the size shown when you look at the file in your Mac Finder/Windows Explorer.

  Keep in mind that adding lots of pixels to an image can affect the image’s quality. Photoshop is pretty good at upsampling, but only so much can be done. Lots of guesses need to be made on Photoshop’s part when deciding what color to make a new pixel. The quality of the outcome depends on the size and quality of the original file. When a quality file is used, it’s easy to double, and even triple, the size of the file. However, if you push it too far and try to upsample an image beyond Photoshop’s capabilities, you can hit a point of diminishing returns where quality begins to suffer. For projects that require massive upsampling beyond Photoshop’s abilities, look to a plug-in like Genuine Fractals by onOne Software. This plug-in uses fractal math to accomplish some amazing upsampling feats.

  So remember, if you only want to change the resolution of the file, uncheck Resample. If you need to make the image smaller or larger, then Resample must be checked. Use the table below as a recap of the relationships you covered in this section:

  
  

  

  
  Using the Correct Image Interpolation Method
  

  There’s one more wrinkle to throw at you before moving on. In the previous examples, you probably noticed a pop-up menu next to Resample Image in the Image Size dialog box, as shown below.

  

  This box allows you to change the image interpolation that’s used when you resample an image. Different interpolation settings affect the way new pixels are assigned color based on the pixels that surround them. There are five options in this menu:

  • Nearest Neighbor. Fast, but not very precise; best for illustrations with edges that are not anti-aliased rather than photos. It preserves hard edges.
  • Bilinear. Medium quality results with most types of images.
  • Bicubic. Slower but more precise; produces smoother graduations than the two previous methods.
  • Bicubic Smoother. Based on Bicubic Interpolation, but designed for enlarging images.
  • Bicubic Sharper. Based on Bicubic Interpolation, but designed for reducing image size because it maintains the detail of the original image.
  
  Changing Size and Resolution Together
  

  Sometimes it’s necessary to change the document size and resolution at the same time. For example, suppose you have a file that’s sized to 8" × 10" at 250 ppi, and you need to change it to a 4" × 5" at 300ppi. Making this change with the Image Size command requires two steps. You can carry out both in one use of the Image Size command:

  1. Choose File > New to open the New dialog box; make the file measure 8" × 10" at 250 ppi.
  2. Choose Image > Image Size (Alt+Ctrl+I). The Image Size dialog box appears.
  3. Deselect the Resample Image option, and change the resolution to 300. Notice that the Document Size changes to 6.67" × 8.33" because you’re moving the pixels closer together.
  4. Select Resample with the Bicubic Sharper option from the Resample Image menu, and change the Document Size Width to 4 inches.
  5. Click OK. Now you have a file that is 4" 4 × 5" at 300 ppi.