Posted by Claire Wade

During our last ‘Ask the Panel’ night, one question posed was “In editing software, what’s the difference between Canvas size and Image Size?’

A good question that the panel managed to answer without having any images to demonstrate the result. So for those who missed the night, here’s my attempt at explaining!

Image size and canvas size sound similar, but they modify your photos in two different ways.

Canvas Size

When you increase the canvas size of an image, you are adding extra space around the photo. So say your original photo is 10 cms wide by 8 cms high and you increased the canvas size to 12cms x 10cms, the actual size of the image does not change, but it would have an extra 2cm border around it.

If you decrease the canvas size, the edges of the photo will be clipped.

Why use Canvas size?

It is a handy way to add exact border sizes to your image.

lion cub

This is the original image

This image has had the Canvas Size increased. Note that the actual size of the lion cub is the same but now it has a border around it

This image has had the Canvas Size increased. Note that the actual size of the lion cub is the same but now it has a border around it

Image Size

Now this one is a bit more difficult to explain.

Increasing the image size of an image will change how large the image is. If enlarged too much, the image will appear bigger, and may become blurry and the quality will be reduced.

Decreasing the image size will shrink the image, and may cause some detail to be lost.

Why use Image Size?

If you want to print your image at a particular size, you can use Image size to resize your photo to the exact dimension required. Be careful when you increase the size as doing it too much will lose quality.


The same lion cub image as before

This time, the image size was increased and so the lion cub is bigger


Image Size on the Internet

If you are changing your image size to use online, you need to work in pixels. This is because every screen is divided into pixels. The size in centimetres or inches is not relevant.

If you have an image that is 10cms wide for example, but is 300dpi (dots per inch), the actual pixel width would be 1181 pixels. But if an image that was still 10 cms wide but 72dpi, then it would be just 283 pixels wide. Both images would print at the same physical size, but were you to look at them online, the 300dpi image would be a lot larger. The 300 dpi image however, will print at a far better quality even if it is the same size as the 72 dpi image.

In Adobe Photoshop, under View, you can click on Actual Pixels or Print Size, this will show you the difference. I’m guessing that other editing software has a similar option, let me know!

What’s the point of DPI?

Well, higher DPI means more dots per square inch. That’s more information, so more dots in an area, gives a better quality printed photo. Less dots per inch is better for the internet, as it make the file size smaller and therefore quicker to load. The poorer quality in DPI is far less visible on a screen than on a printed image.

So why can’t I just increase the DPI to make an image a better print quality?

Well you can… a little. But when you increase the DPI, your editing software guesses the colours of the new dots and it’s not always that good at guessing and your increased image will look blocky and blurry. So always keep the original large size of your files and save new small copies for the internet.

This image shows what can happen if you increase the size of an image too much.