|Adobe Photoshop Basics|
Go to the Goodies\Stockart folder on your Photoshop 6.x CD and open the image 0006811.jpg. If you don't have access to this folder, open one of your own images. Let's start by duplicating the image so we're not working on the original. You do remember how to do that, don't you? (Continued below...)
The contents of the Graphics Software site are copyright © Sue Chastain and About.com. These pages may be printed for personal reference, but they may not be distributed or republished for any purpose without prior written permission. Please see the About.com User Agreement for more information.
Make the document window as large as possible and choose View > Print Size. The image will display quite large and probably fill the entire document window. Practice panning with the spacebar shortcut we learned earlier.
Choose the Image Size command. Note the pixel dimensions of this image are 880 by 1100 pixels. The print size is 12.222 by 15.278 inches with a resolution of 72 ppi.
Let's assume we want to print this image on a high quality printer, so we want a resolution of 300 ppi. Make sure that constrain proportions is checked and resample is unchecked, then type 300 in the resolution box. Notice what happens to the print size... it changes to 2.933 by 3.667. This is the maximum size you can print the image in order to get a high quality print.
That's a bit small, so let's change the width to 4 inches. The resolution field changes to 220. This resolution will still give us a fairly good quality print and allow us to print at 4 by 5 inches. It seems like a happy medium, so we're going to accept those numbers. Before you click OK, however, take a look at the pixel dimensions... as you can see they have not changed from the original size.
Now click OK. Your image onscreen should look exactly the same. So, how do you know it has changed?
Choose View > Print Size. The image will be resized to the approximate print size that we chose: 4 by 5 inches. If you were to actually print the image it would print at this size, and if you save the image, the print size is generally retained with it (unless you use the Save for Web command to save the image). By changing the resolution of the image we have not changed the pixel data at all. You've only given Photoshop information about how you want the image to be printed.
Now we are going to do another experiment that resamples the image.
Choose the Image Size command.
Type 8 in the Width field for print size and observe the following: Resolution remains at 220, but the width and height double to 1760 by 2200. Also notice the file size readout. It should say 11.1M (was 2.77M). The file size quadruples as we double the pixel dimensions!
Click OK now and let's see what happens to our image. By going to View > Print Size we can see it's approximately 8 by 10 inches on screen.
Now set the magnification to 100% and look at some of the areas where there was a lot of detail in the original, such as the ducks legs, or where the gravel meets the edge of the wheelbarrow. You'll notice these areas will be a bit blurrier than the original.
Next let's assume we want to post this image on the Web. The current size of 880 by 1100 is much too large to post on the Web because it will not even fit within the screen dimensions of many monitors. Most Web surfers have at least a resolution of 640 by 480 pixels, so we'll use that as a basis to cater to the lowest common denominator.
Close the duplicate we've been working with and make a new duplicate off the original. Choose the Image Size command. Set the resolution to 72, constrain proportions, and resample checked. Since we need the image no more than 480 pixels high, go ahead and enter 400 in the height (we'll go smaller since most browsers have title bars, menus and scroll bars that also take up screen space). Notice that the file size decreases considerably.
Click OK and set the magnification to 100% to see the image the actual size it would appear on the Web.
This covers the typical uses of the Image Size dialog box, but I am going to encourage you to go back and spend some time experimenting with all the options in Image Size, just to get a feel for what they do. For instance, try all three of the interpolation methods for resampling, change the pixel dimensions without the constrain proportions box checked, resample an image repeatedly and then compare it to an original to observe the effects of repeated resampling.
It's also worth mentioning that Photoshop has an Image Size wizard/assistant in the Help menu. Choose Help > Resize Image to access this wizard/assistant, and Photoshop will walk you through the process of resizing your image based on the output you choose. When you use the Resize Wizard/Assistant, Photoshop automatically makes a copy of the original image. So why didn't I tell you about this until after mucking around in the Image Size box? Because the Image Size box is much faster and it's important that you understand what effects these settings have on your image.
Next > Canvas Size