Updated August 2016
Resizing images is one of those things that a lot of people get wrong, not just photographers. I've seen many questions in online forums on this topic, and I found myself answering them in a similar way each time. So I decided to write up a small article to address all those questions in one.
There's two factors that go into a print's size. There's Print Size (6x4, 8x10 etc.) and Aspect Ratio (2:3, 5:4 etc.). Don't get them confused, because they're both very important to understand.
Print Size = Physical size of a print Aspect Ratio = Physical shape of a print
The aspect ratio of a straight-out-of-camera image, is 3:2 (landscape) or 2:3 (portrait). This means, if you want to resize an image to a size that isn’t within that same aspect ratio, you’ll actually stretch your image out of shape. Below you can see two aspect ratios, 3:2 and 5:4.
Here's a Scenario: I want to make a 10x8 print. I need to resize this image, because a 10x8 is a different shape. If I simply resize the width to 10 inches and height to 8 inches, the image is going to stretch and warp into the new aspect ratio as you can see below. We don't want that.
To fix this, we can link the Width and Height so changing the width to 10 inches automatically changes the height. However, this method keeps the image to the original 3:2 aspect ratio and doesn’t actually create a 10x8 print. It's more like a 10x6.
So how do we get around this? Well, we actually crop the image to the 10x8 shape first. This will cut off some of the image, but it will keep it from being stretched. Now that my image is in the correct aspect ratio, I can then resize the image to 10x8 and it’ll resize correctly without distorting the image.
Have a look below. The first image is the original photo as it comes out of the camera. The second image is a simple diagram to show you how much of the image gets chopped off the sides when cropping to a 10x8. The third image is the correct 10x8 achieved through cropping to the shape first, then resizing after. And the fourth image is the warped 10x8 caused by just resizing.
Make sure that if you're going to change the resolution, that you uncheck Resample Image first (I explain why below), then change the resolution and hit Okay. You can then go back into the Resize Image dialog, recheck Resample Image and make your resizing adjustments from there. This brings me to something else that so many photographers get wrong, and that's PPI and DPI.
PPI Vs DPI
Alright, let's get this straight once and for all. DPI has almost nothing to do with modern digital photography. There's three factors that go into an image file's size. That's X Pixels wide by Y Pixels high and Z bits of colour. For the purpose of this article, we don't have to worry about the colour data. The PPI of an image doesn't affect the file's size at all. DPI is a printing term and PPI is a digital imaging term.
DPI = Dots Per Inch PPI = Pixels Per Inch
DPI refers to the amount of individual dots of ink a printer can lay down in its dither pattern to create one inch of a print. Most printers print in the thousands of DPI. The most common number however, is 1440 DPI - AKA "Best Photo". I'm sure you've seen that term on your home printer. When you print an image, the printer converts the Pixels Per Inch into the equivalent Dots Per Inch. They nearly go hand-in-hand, however 99% of the time, you never have to worry about DPI at all. That's the printer's job.
PPI refers to the number of pixels available in your image that are used to tell the printer to print one inch. So long as your image is sharp, and is printed with proper technique, you'll get better results the higher the PPI. Even passed the old standard 300ppi. Changing the PPI doesn't actually affect the file at all. You can choose any number you like, from one all the way to infinity if you really want to. But this is where the Resample Image checkbox is detrimental to your image if you don't know how to use it. So in short... Don't.
Resampling your image will actually change the number of pixels, either removing or adding, based off your changes to PPI. So unless you are making a well informed decision as to what changes you're making, turn Resample Image off. Because this does change your image quality.
If you want to know the optimal PPI for your desired print size, simply divide the largest pixel dimension by the largest side of your print. For the 10x8 image above: 3585 pixels wide divided by 10 inches = 360ppi. This is what you'd set your PPI to when you save your image. The printer would then convert those 360 pixels per inch into 1440 dots per inch when printing your image (or whatever DPI setting the printer is using).
I hope this has shed some light on resizing images and hopefully you understand PPI and DPI a lot better too. If you have any questions, as always, just ask them in the comments below and I'll answer them.