So we need to learn the difference between resizing our image and resampling the image. To do this, we'll look at the Image Size dialog box. You can see in the top part, we just have a simple area for Width and Height, and the Width and Height are calculated in pixels. So these numbers just directly correspond to how many pixels they are wide versus how many pixels they are high in the image. In the Document Size below, this is where it gets a little bit trickier, because in order to calculate the document size, you need the width and height, plus the resolution.
So first let's turn off the option to resample your image. As soon as I uncheck that, you'll notice that I cannot actually change the total pixel dimensions. All I can do is I can change the way that those pixels are distributed. So right now, we can see that we could print this at 60 x 40 at 72 Pixels/Inch, but 72 Pixels/Inch is a very low Resolution. I want to go ahead and switch this up to 300 Pixels/Inch so that I can print this image to my inkjet printer.
Well, when I change it to 300 Pixels/ Inch, because I've told Photoshop that it can't add or subtract any other pixels, with this option here Resample Image turned off, then Photoshop had to use more pixels, 300 of them, per inch, so it couldn't print this image as large as it was at 72 pixels per inch. If I click OK right now, you'll notice that the size of the image in my preview area here, it didn't change because I'm not adding or subtracting any pixels.
Now let's go back to Image > Image Size, and this time I'm going to tell Photoshop to Resample the Image. Now I can either resample the image up or I can resample the image down. One of the nice new features in Photoshop CS6 is that Photoshop will automatically pick the correct resampling algorithm. It will automatically choose Bicubic Smoother if I make my image larger, and it will choose Bicubic Sharper if I reduce the size of my image. So I make sure that I leave that on Bicubic Automatic.
Now, if I wanted to print this image with a maximum width of 10 inches and I enter in 10, you'll notice that this time that we are in the Image Size dialog box, because I have the option to Resample Image checked on, I'm actually taking away pixels. My total size used to be 36M, now it's down to 17, and I had a lot more pixels in my Width and Height area. So by turning on Resample Image, Photoshop is allowed to take away or make up pixels.
If I wanted to print this 20 inches, now we can see that Photoshop is actually going to have to add pixels. The file that used to be 36M, now I've told that that I need file that's 68M. So Photoshop will automatically interpolate up, or add pixels, in order to give me this document size with 20 inches wide, 13.3 inches high, and a resolution of 300 Pixels/Inch. This time when I click OK, you can see that my document actually got larger, because Photoshop had to make up those additional pixels.
I am going to undo that by choosing Edit and then Undo Image Size, just so that I can show you the other direction as well. If I go to Image and then Image Size and we decide that I only need to print this maybe 6" x 4" at 300 Pixels/Inch, well, again, because I have Resample Image turned on, what used to be a 36M file, is now going to be just a little over 6M. When I click OK, we can see that my file actually got smaller.
So there you have it. With the Resample Image option on in the Image Size dialog box, you're allowing Photoshop to either make up pixels or throw them away. When you don't have that on, then all you are telling Photoshop that it can do is redistribute those pixels at the resolution that you specify, and the width and height in the document area of the Image Size dialog.
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