When changing the size of an image in Photoshop, there's really two ways to go about it. You can either resize the image, or you can resample it. A lot of people use the terms resizing and resampling as if they mean the same thing, but they don't, and there's an important difference between the two. As we'll see in this Photoshop tutorial, the difference, as important as it is, is controlled by nothing more than a single checkbox option at the bottom of the Image Size dialog box.

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As I just mentioned, the choice of resizing or resampling your image is handled by the Image Size dialog box, which can be found under the Image menu at the top of the screen. The difference between resizing and resampling has to do with whether or not you're changing the number of pixels in the image, or as Photoshop calls it, changing the pixel dimensions of the image. If you're keeping the number of pixels in the image the same and simply changing the size at which the image will print, or in Photoshop terminology, changing the document size of the image, that's known as resizing. If, on the other hand, you are physically changing the number of pixels in the image, that's called resampling.

Again, just to make sure we're on the same page so far:

• Image Resizing: Changing the size the image will print without changing the number of pixels in the image.
• Image Resampling: Changing the number of pixels in the image.

See? You already know enough so that the next time someone refers to changing the number of pixels in the image as resizing the image, you can look them proudly in the eyes and say, "I think what you really meant to say there, Bob, is that you're going to be resampling the image, not resizing it." Assuming, of course, that the person's name is Bob. And assuming you don't want Bob to like you much anymore because you think you know it all.

Let's look more closely at the difference between resizing and resampling.

Resizing vs Resizing An Image

First, we need a photo. This one will work nicely:

Let’s look at what the Image Size dialog box is telling us about this image. To access it, I’ll go up to the Image menu at the top of the screen and choose Image Size:

As mentioned previously in the "Image Resolution" and "Image Resizing" sections, Photoshop's Image Size dialog box is divided up into two main sections - the Pixel Dimensions section on top, and the Document Size section below it. The Pixel Dimensions section tells us the width and height of our image in pixels, and also tells us the file size of our image. The Document Size section tells us how large or small our image is going to print based on the image resolution, which we also set in the Document Size section. You can think of the Pixel Dimensions section as the section you'd want to change if you were working on an image for the web or simply to display on your computer screen, while the Document Size section is used when you need to control how large your image will print.

So, generally speaking:

• Pixel Dimensions = web
• Document Size = print

Let's take a look at the Pixel Dimensions part of the Image Size dialog box to see exactly how large our image is, in pixels.

We can see here that the photo is quite large, with a width of 3456 pixels and a height of 2304 pixels. That's a combined total of nearly 8 million pixels (the camera used was an 8MP camera, hence the 8 million pixels in the photo), which gives us lots of image information to work with in Photoshop.

Now let's look at the Document Size section:

The Document Size section is showing us what the current print resolution of the image is, which in this case is 72 pixels per inch, and it's showing us how large the image will print at that resolution, which at 72 pixels per inch will give us an image that's 48 inches wide by 32 inches high. If you've read the section on how print resolution affects image quality, you'll know that even though we'd be getting a very large image when we print the photo, a resolution of only 72 pixels per inch is simply not high enough to give us anywhere near professional image quality with our prints. For that, we'd have to change the resolution to at least 240 pixels per inch, while 300 pixels per inch is considered the professional print standard.

Before we go any further, there's three more options in the Image Size dialog box below the Document Size section:

• Scale Styles
• Constrain Proportions
• Resample Image

The first one, Scale Styles, has to do with Layer Styles and how they're affected by resizing or resampling the image. We'll ignore that option since it has nothing to do with this topic. The second option, Constrain Proportions, which is enabled by default, links the width and height of the image together so that if you make a change to the width of the image, for example, Photoshop will automatically change the height, and vice versa, so that the proportions of the image remain the same and don't get distorted. This is normally what you want, but if for some reason you want to be able to change the width and height independently of each other, simply uncheck the Constrain Proportions checkbox.

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