Adobe Photoshop Basics

Go to the Stockart folder on your Photoshop 5.x CD and open the image IND_007.jpg. 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?

Choose the Image Size command. Note the pixel dimensions of this image are 2100 by 1400 pixels. The print size is 7 by 4.667 inches with a resolution of 300 ppi.

Uncheck the resample box and change the resolution to 72. Notice the pixel dimensions remain the same but the Document size is now 29.167 by 19.444.

Click OK. 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. Go to File > Print Options and you'll see in the Print Preview that the image is larger than the page.

Let's assume we want to print this image on a high quality printer, so we want a resolution of 300 ppi. Go back to Image Size and make sure that constrain proportions is checked and resample is unchecked, then type 300 in the resolution box. Notice what happens to the document size... it changes to 7 by 4.667. This is the maximum size you can print the image in order to get a high quality print.

Now change the width to 10 inches. The resolution field changes to 210. This resolution will still give us a fairly good quality print and allow us to print at 10 by 6.67 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: 10 by 6.67 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. You can go to Print Preview again to verify this.

Now we are going to do another experiment that resamples the image.
Choose the Image Size command.
Check Resample.
Type 20 in the Width field for document size and observe the following: Resolution remains at 210, but the width and height double to 4200 by 2800. Also notice the file size readout. It should say 33.7M (was 8.42M). The file size quadruples as we double the pixel dimensions!

Click OK now and let's see what happens to our image. Set the magnification to 100% and look at some of the areas where there was a lot of detail in the original, such as where the two gears come together. 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 original size of 2100 by 1400 is much too large to post on the Web because it will not even fit within the screen dimensions of most monitors. Most Web surfers have at least a resolution of 640 by 480 pixels, so we'll use that 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 propertions 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