Changing an Image’s Resolution and Size

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:

Photoshop tips by : http://www.graphics.com