Image Size & Canvas Size - Photoshop Tutorial

Resolution Revisited

This Photoshop tutorial is related to the tutorial on resolution. Read it first to understand some underlying principles.

Resolution is a key to understanding image size and canvas size. This is because understanding resolution is key to understanding Photoshop as a whole.

Resolution, as a reminder, is how many pixels are in each inch of the photo. You can have a 4x6” photo with 100 pixels per inch (PPI) or a 4x6” photo with 300 ppi. One just has more “dots” in every inch, and therefore more detail.

Image Size

Image Size in Photoshop is measured in pixels and is expressed in Width x Height. Strictly speaking, the size of an image is in pixels, but resolution comes into play because Photoshop puts the image also into either inches, or centimeters (or points or picas for those of us who would ever use them).

So when you are talking about an image size, there are actually two possible ways to talk about the size of the image. In the end, they are basically the same, but the second has more context and is renamed “document size.”

These are:
Image Size: Pixel Dimensions
Document Size: Inch Dimension * Pixels Per Inch

Explanation: When talking of resolution I mentioned a 4x6” print that is 300 ppi. That is expressing the document size as the Inch Dimension and the Pixels Per Inch. If you take that expression and want to make it into just Pixel Dimensions, then do the following math:

(4 in * 300 pix/in) x (6 in * 300 pix/in) = 1200 pixels x 1800 pixels

That is the image size in Pixel Dimensions.

So when you say ‘A 4x6” image at 300 ppi,’ you are giving a mathematical expression. When you do the math, you get ‘A 1200 x 1800 pixel image.’ The two expressions mean the exact same thing in the end (in terms of actual number of pixels), but they have different contexts.

When you apply a resolution (let’s use 300 ppi) to the image that was 1200 x 1800 pixels, you get a 4x6” image at 300 ppi. Now it is more than just a bunch of pixels. You have a “goal” in mind, and that is a 4x6” image. You also have a “goal” of good detail, because you used an excellent resolution. This gives the image more context, and it is the expression of the document size in Photoshop.

This is because the same image (1200 x 1800 pixels) can be a 12x18” image if it is changed to 100 ppi. That is a very different image size in one respect (inches), but the exact same image size in another respect (pixels). Your “goal” now is a much larger print (document) with less detail in every square inch.

Image RE-Size

Resizing images is its own discussion. It is a very intense and even heated topic. So far I have alluded to changing the print size (in inches) of an image, and having the pixel image size stay constant (changing a 4x6” @ 300 ppi to a 12x18” @ 100 ppi, where the image stays at 1200 x 1800 pixels).

There are many more ways to change the image size. I can take away pixels, but keep the “aspect ratio” the same. I can try to add pixels and give it more detail. I can make one dimension larger while keeping the other dimension the same (make a person tall & skinny or short & fat like a fun house mirror).

The most hotly debated area of resizing is when you try to “add pixels” or “increase the resolution.” For example, you wanted the 12x18” @ 100 ppi to now have 300 ppi. If you do the math, you are trying to go from a 1200 x 1800 pixel image to a 3600 x 5400 image. That is a pretty tall order. Another way to express it is in mega pixels (or millions of pixels).

That would be going from a 2.2 MP image to a 19.4 MP image. To put that in context, as of today, camera phones can give you a (crappy) 2 MP image, but you have to spend almost $10,000 on a camera + lens to start getting images that start out at 20 MP.

Long story short: there are ways to get a bit more pixels out of an image. You can increase detail and pixel density. Going from 2 MP to 20 MP is basically impossible, though your CSI show will make you think otherwise. Moderate increases are possible with the right tools and techniques.

Those tools and techniques are the “hotly debated” topics.



Canvas Size

For most part, the canvas size and the image size are one in the same. But, we do have to distinguish between them.

I’m going to give you the punch-line first, and hopefully it will make sense as you read:

Image Resize: The image borders “stick” to the image and the image is pulled or squished with the borders.

Canvas Resize: The borders expand or contract without the image.


The image size has been detailed in the above sections. The canvas size is a little different. The canvas can be thought of as the border of the image.

When I resize an “image,” I make the entire file larger or smaller. When I resize the “canvas,” I enlarge or shrink the borders while the image stays the same size.

So, if I have an image of a flower, and the petals reach the edges of the image and almost touch the borders, then we can see the difference between canvas and image size.

Resizing the image will keep the image looking the same, it will just be a different size. Resizing the canvas will expand or contract the borders. If I make the canvas smaller, then the borders come in and the petals are clipped. If I make the canvas larger, then I am adding space around the image.

Now a confusing fact is that once the canvas is re-sized, everything resets with respect to image size. Your newly resized canvas is now also the size for the entire image. Yes, this is confusing. The video will help the most, but to explain with the example:

If the flower image was 2x3, that was the image size and canvas size (remember these two are almost always the same). I add 1” total (.5” to top, .5” to bottom, .5” to right, etc.) to both the height and width of the canvas. The image stays at 2x3”, but the canvas enlarges to 3x4” with 0.5” extra space around the original image.

A split second later, after the canvas resize is complete, you have a new image. Photoshop doesn’t keep tabs on the original image size. It is only concerned about the new image size that is a result of expanding the canvas.

In other words, the reason the canvas size and image size are mostly the same is because in the end, Photoshop only sees image size. It doesn’t see canvas size for the most part. So what is the difference?

The difference is only during the resizing effects. Both effects are going to affect image size. So you could say that both effects move the image borders (by either expanding or contracting them). The difference is this:

Image Resize: The image borders “stick” to the image and the image is pulled or squished with the borders.

Canvas Resize: The borders expand or contract without the image.

If that makes sense, you are on your way to “getting” Photoshop.

Flower image used in video: (c)2007 Derek Ramsey, taken at the Chanticleer Garden, cropped to fit the example.