Photoshop tip #10: Image Size vs. Canvas Size

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One of the first things that new users of Photoshop get confused by is the distinction between the Image Size and Canvas Size features. Apparently, they mean the same thing, but as you will see in this blog post, the result is different depending on which one you use. 

Image Size in Photoshop
First of all, let's explain what Image Size means. As you can imagine, Image Size really means the size of an image, be it expressed in pixels or as a percentage. With that in mind let's look at a brief example to illustrate this concept. 
Let's open a picture and let's enlarge it by 20% on both the horizontal axis and the vertical axis. For this purpose I have used a picture that comes with the Windows 7 operating system, but you can use your own. 

Open Photoshop and then go to File and click Open to open an image that you want to enlarge (browse to the image location of the file on your hard drive, select the image and click open). Let's enlarge the photo by 20%. To do this, go to the Image menu and select Image Size. You should see this window: 

Photoshop10ImageSize.jpg
Figure 1 - Photoshop: Image Size window

To enlarge the picture by 20%, in the Pixel Dimensions subgroup, click on the Widths drop down menu that currently displays pixels and select percent. You should see something like this: 
Photoshop10ImageSize2.jpg
Figure 2 - Photoshop: Image Size window

You should notice that the original width and height of the picture in pixels is now expressed as a percentage denoted by 100% for both the width and the height, meaning the picture in its original size has a 100% width and 100% height. 
Simply type in 120 for the width and 120 for the height to enlarge the picture by 20%. Press OK once you are done. Now you should see that your original photo gets larger. 

In this example we chose to enlarge the picture based on a percentage. If you would like to change the picture to a given size in pixels simple select pixels from the drop down menu in the Pixel Dimensions like in Figure 1. 

Canvas Size in Photoshop
The Canvas Size option in Photoshop provides, on the other hand, different results than Image Size. Canvas Size refers to the width and the height of the layer on which your picture is sitting. So, if you were to change the Canvas Size you would basically enlarge the working space around the picture without enlarging the picture itself. Let's go through a simple example to see how canvas size works. 

Suppose we want to enlarge the canvas size by 20%. To do this, first open Photoshop, click File and then Open to open a picture (browse to the image location of the file on your hard drive, select the image and click open). Go to the Image menu and click Canvas Size. You should see something similar to this window: 

Photoshop10CanvasSize1.jpg
Figure 3 - Photoshop: Canvas Size Window

In the New Size subgroup, for the width and the height of the canvas, select percent from the drop down menu. See Figure 4: 

Photoshop10CanvasSize2.jpg
Figure 4 - Photoshop: Canvas Size Window

The Canvas Size height and width should now change from their initial values expressed in inches to 100 percent. To enlarge the canvas type in 120 for both the height and the width to enlarge the canvas by 20%. See Figure 5:

Photoshop10CanvasSize3.jpg
Figure 5 - Photoshop: Canvas Size Window

Additionally you could also change the Canvas Extension Color by clicking on the colored box next to the drop down menu corresponding to the Canvas extension color submenu. 

Click OK once you are done to close the Canvas Size window and to see the result. If everything went well, you should now see the original picture having the same size surrounded by an additional 20% of canvas space. See Figure 5: 

Photoshop10CanvasSize4.jpg
Figure 5: Before and After using the canvas size feature

As you can now see, there is a big difference between using Image Size and Canvas Size in Photoshop in that Image Size determines the actual size of a picture while the canvas size changes the size of the layer beneath the picture without affecting the picture itself. See Figure 6:

ImageSize-CanvasSize.jpg
Figure 6 - Photoshop: Image Size vs. Canvas Size (Before & After)

If you have any questions, feel free to contact the STA Office, by e-mail at sta-help@trincoll.edu or by telephone at x2589.

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