Lesa's Graphic Secrets for MUGs

How to resize images from digital cameras

I bought my husband a Nikon D50 for Christmas a few years back and needless to say, he's totally obsessed with it. He's snapping pictures constantly and has outfitted it a couple of new lenses, a speed flash, and several multi-gig memory cards. He's all set, that is, until he transfers the images onto his Mac.

He doesn't know squat about resizing an image for print or an email. He's heard of resolution but doesn't have a clue what it means or when it matters. Bless his heart :)

Therefore, today I'm going to show you how to resize images with ease using Photoshop Elements. It's an amazingly powerful piece of software that's under $80 (click here to read the Macworld review). I'll start by covering basic graphic terminology and exploring the relationship between resolution and pixels. Read on!

What is resolution?

Image resolution is the number of pixels displayed per unit of printed length in an image. It’s a measurement used for printing (hence the phrase “of printed length”) and it’s usually stated in pixels per inch (ppi—used when referencing stuff onscreen) or dots per inch (dpi—used when referencing something that’s about to be printed because printers print dots).

What’s a pixel

The smallest picture element of any digital image is called a pixel. Images are displayed in tiny blocks (also referred to as dots) of color that together form an entire image. If you zoom into an image around 300% or so, you'll start to see them. TIP: In Elements, just press Command + several times to zoom in, then Command - to zoom back out.

How do resolution and pixels work together?

The way resolution affects an image is thus: For a given number of pixels in the width or height of an image, any image can be measured at any resolution. The higher the resolution, the closer the printer packs those pixels together in order to make a beautifully smooth and sharp print. The closer the pixels are packed together, the smaller the image will be physically when it's printed. Conversely, the lower the resolution, the more loosely those pixels are packed and the larger the image can be physically--though it will print terribly. It'll look pixelated and chunky and people will giggle behind your back. Seriously :)

It's like brown sugar... really

Think of all this as the pixels in an image were granules of brown sugar (stay with me, it'll make sense in a minute). If you pour brown sugar into a measuring cup without packing it down, it might take up about a cup in space. If you were to pack it down with your fist, the same number of granules now takes up a 1/2 cup of space. The same amount of granules are in both a cup and 1/2 cup, it's just the density that's changed thereby changing how much physical space the granules comprise. Loosely packed granules are the equivalent of low resolution, while densely packed granules are the equivalent of high resolution.

Because our monitors can only display a certain amount of information that our eyeballs/brains can process, resolution changes cannot be seen onscreen. Resolution only matters to printing devices (and technically scanners but that's fodder for another column). That’s right, resolution doesn’t mean a hill of beans until that image is headed for a printer, because only a printer can do anything with the resolution measurement. Your monitor just doesn’t care: A 72 ppi image will look exactly the same as a 3000 ppi image. That's why the whole resolution issue is so confusing.

How much do you need?

Now that you understand how resolution works, how much resolution do you need to make a gorgeous print? It depends on the printer. While the current crop of consumer-level inkjet printers do a fine job at 225-250 ppi, a professional service bureau may require 300+. If it’s your own printer, run a few tests. If someone else is printing it, ask them what resolution they need--and if they don't know, find another printer... fast :)

Resizing a photo for print

Let’s take a photo from a high-quality digital camera. I popped open the photo in Elements and took a peek at its Image Size dialog to see what the resolution currently is and how many pixels we have to work with. You can open this dialog by choosing File > Resize > Image Size. As you can see below, my photo is 2599 x 1949 pixels, but only 72 ppi. Does this mean my photo is low-quality and unsuitable for print? Not at all. It merely means that the resolution needs to be changed *before* I try to print it.

Important things to note in the screenshot above:

1. At the bottom of the document window, you can see that the file is big—it’s 14.4 MB (circled in red above).

2. Note the pixel dimensions shown in the upper portion of the Image Size dialog: 2599 x 1949 (also circled in red). Remember these numbers.

3. Note the honking big physical dimensions of this image. If I were to try printing at its current resolution, it’d be over 36 inches by 27 inches... INCHES! I don't know about you but I don't have a large format printer!

Now, watch carefully as I increase the resolution of the image without altering the number of pixels.

Step 1: In Photoshop Elements, choose Image > Resize > Image Size. (In Photoshop, choose Image > Image Size.) TIP: Keyboard shortcut for either software: Command + Option + I.

Step 2: At the bottom of the dialog, uncheck Resample Image. This all-powerful, magic option locks the number of pixels contained in the image, thereby locking quality. Enter 300 into the resolution box and watch how the document size changes:

The image now has a resolution of 300 ppi and the physical size has decreased to about 8.5 x 6 inches. See how the physical dimensions of the image changed but the pixel information didn’t? I have exactly the same number of pixels I started out with: 2599 x 1949; they’re just packed more tightly together. The file size didn’t change at all—it’s still 14.4 MB, as evidenced in the document window and in the Image Size dialog (another clue that the pixel data didn’t change). I haven’t changed the quality (pixels), I’ve just changed the measurement (ppi) which only affects the printed image.

Again, this is confusing because the onscreen image won't change a bit (see earlier paragraph on monitors, eyeballs, and brains). The truth is only revealed by the Image Size dialog. The thing to remember is that as long as you uncheck the Resample Image box, you can tweak the resolution ‘til the cows come home and you won’t alter the image quality at all. Ever. If you leave the Resample Image box checked, though, you’re messing with photo quality which you don't want to do when printing. If you're resizing an image for email or posting on a Web site, then leave the Resample Image box check because in those cases you really do want to lower pixel dimensions, thus lowering file size.

Making Elements do it for you

That being said, if you want to print a photo at a common size like 8 x 10 or 5 x 7, you can have Elements change the resolution for you. Just choose File > Print and pick a size from the Print Size pop-up menu towards the top of the dialog.

Resizing and changing resolution at the same time

You can also resize the image and change the resolution at the same time by using the Crop tool.

Step 1: Press C to select the Crop tool in either Photoshop or Elements.

Step 2: In the Options bar at the top of your screen, enter the dimensions of the desired image and the resulting resolution. TIP: Be sure to include the unit of measurement in the width and height boxes, as shown below. Draw a box around the important bits of your image and when you get it just right, press Return to accept the crop. TIP: Press the Escape key to exit a crop.

WARNING: If the image appears to enlarge on your screen, then you've surpassed the number of available pixels. In other words, you're trying to print it larger than it can really go at the resolution you've entered. If that happens, press Command + Z to undo and then enter smaller dimensions or lower resolution. It's okay if the image appears to get smaller on your screen.

Step 3: Choose File > Print to print it now, or save the photo to print later. If it’s being sent to someone else to print, choose File > Save As and pick TIF from the file format pop-up menu. TIFs can be opened in most image editors and are a lossless graphic format (high quality). You can also print directly from a native Photoshop or Elements document.

Resizing for an email

With the new Elements 6, resizing an image for email has never been easier. Just click the green Share tab at the top right of the interface and press the Email Attachments button (circled below in red). If the image is too big, Elements will alert you with a dialog and offer to resize it for you.

Next, it'll pop open your email client, create a new message, attach the photo, and allow you to specify just how big the photo should be via a handy pop-up menu at the bottom right.

That's all there is to it! So get out there and start printing and emailing those images. And if you haven't upgraded your copy of Elements yet, get with it. In this column series we're going to be covering all kinds of wonderful new features that you are sure to enjoy. At the very least, download the 30 day trial and give it a spin.

Until next month, happy shooting!