Re-sizing Images

Re-sizing ImagesGreg McMillan

When it comes to re-sizing photos on a computer, I always rely on two simple numbers, 72 and 300, and a little bit of math. Once you try this out for a while and are familiar with it, re-sizing photos will be a breeze. But first, the MOST IMPORTANT PART!!! MAKE A COPY OF THE IMAGE TO WORK WITH! DO NOT RE-SIZE THE ORIGINAL FILE!

The two numbers, 72 and 300 pertain to resolution, or pixels per inch, PPI for short. Another way of referring to resolution is DPI which is dots per inch. The two are often confused and actually do serve their own purpose. PPI is the number of pixels per inch in a digital photo as it’s displayed on a computer screen, and computer monitors and screens are manufactured at a resolution of 72 PPI. This is also known as low resolution, or “low rez”. DPI is the number of dots of ink that are sprayed onto an inch of paper when printing with your inkjet printer, thus the term dots per inch. You’ll need a “high rez” image file to make a good print.

Now that we have the resolution part of the equation sorted out, the rest of the math is simply the Document Size and Pixel Dimensions. And for the most part, you’ll really only need to worry about two out of the three parts of the equation. The third part will “lie where it falls”. It’s a lot easier than it sounds.

Document size can be either the size you want your image to display on screen or the size of your print. Screen size is best referred to in pixels while print size should be expressed in inches.

Let’s start by sizing a photo for screen viewing. You would apply this to a photo that you want to send in an email or post to a web site such as Facebook or Flickr. The reason you want to make a low rez file is to allow for fast and easy transfer of your photo as well as fast loading times for the recipient, be it a web server or dear old Aunt Lynda, who might just have a dial-up internet connection. Plus, it makes it harder to make a quality print from a low rez file, which is what you want when posting to the web.

Okay, so here’s the math. I prefer to make a screen image a thousand pixels on the longest side. We’ll stick with a horizontal image for this example. My camera has a 10 MP sensor and takes a picture at 3888 pixels wide by 2592 pixels high. I always shoot at the largest size in case I get a nice photo I may want to blow up, but more on that later. If I want to send a photo to my Aunt Lynda in an email the first setting I make is to set the resolution to 72 pixels per inch. Then I make sure that whatever program I’m using to work on my photos (in my case Photoshop Elements) is set to keep the proportions locked. That way if I change the width of the image, the height changes with it to keep the aspect ratio intact. Next, I change the width to whatever it says to 1000 and make sure “pixels” is selected right beside that number. You can see how all that will look in the image below. The two parts of concern here are Resolution and Pixel Dimensions and the Document Size really doesn’t matter. Click on “OK” when you’re done and the image will change to the size you entered. Now when you save the re-sized photo, you can add “web” or something to the file name to let you know that it is sized for screen viewing.

Re-sizing for Screen

As I mentioned earlier you’ll need a high rez file to make a good print. I find that setting the resolution to 300 PPI is more than adequate for a good quality print. In fact, a setting like this will work on any inkjet printer as well as the big machines used in your favourite camera shop or photo lab. This time the two parts of the equation to use are Resolution and Document Size. Let’s say I want to make a 4 x 6 print.  Again, set the resolution first. Then make sure the proportions are locked, the units set to “inches” and the width set to 6. The height should automatically go to 4. (See example below)

Re-sizing for Print

Just for kicks, if we completed the equation in this example using the Resolution as 300 DPI and the Document Size as 4” x 6”, the Pixel Dimensions would be 1800 pixels wide and 1200 pixels high. A typical 19” monitor has a resolution of 1280 x 1024 and if you looked at a file like the one we prepped here at full scale, it would be larger than the screen itself. Now imagine if it was for an 8 x 10, or larger?

Back to Tutorials          Home