Photo Resizing 101

Anyone can click the crop tool in their photo-editing program to zoom in on an image, whether they’ve ever heard of a “pixel” or not. But understanding what you’re doing when you resize an image (and how it actually works behind the scenes) can help you achieve better results with photo cropping and resizing, and also keep you away from some common pitfalls. Finally, all in one place, here’s the complete story on pixels, ppi, and dpi.

Every digital photo is made up of tiny dots called pixels. Pixels contain the information necessary to render (i.e., “display” or “depict”) an image digitally. A photo’s virtual size is measured in both horizontal and vertical pixels—for example, a 10-megapixel image measures 3888 x 2592 pixels.* A photo with more pixels contains more data and has a larger file size. Why does this matter? Because more pixels also allow more flexibility for resizing and cropping.

* note: “Mega” means “million.” If you multiply 3,888 by 2,592, you get 10,077,696 pixels—or 10 megapixels. A 6-megapixel camera takes images that are 2048 x 3072, or 6,291,456 pixels.

	A Standard Photo This is an example of a photo that's 1800 pixels wide by 1200 pixels tall, which translates to 2.1 megapixels. It's also a standard-sized 4 x 6 photo print, at typical print resolution (300 ppi).
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PPI & DPI

Before you begin resizing photos, it’s helpful to understand the difference between ppi and dpi. These terms are sometimes used interchangeably, but they aren’t the same thing.

Ppi stands for “pixels per inch” and describes a digital photo’s resolution, or sharpness and clarity. To produce a high-quality print, a photo needs to be at least 300 ppi. You can print a photo with a lower resolution (e.g., 200 ppi), but the photo will look more pixilated and less sharp. Most computer monitors display images at 72 ppi, which means you can post lower-quality images online and they will still look decent on-screen. But that doesn’t guarantee they’ll look good when printed.

Dpi refers to the number of “dots per inch” that a printer lays down (typically 300 dpi). You can change the resolution (ppi) of a photo (which will affect the quality of the print), but you can’t change the number of dots laid down by the printer.

Lesson #1: Resizing Basics

When you resize a photo before printing, it’s best to preserve as many pixels as possible to ensure the highest quality print. You can make adjustments to either a photo’s physical dimensions (measured in inches) or its resolution (measured in ppi) without altering the number of pixels in the photo.

Use the method below when you want to change the photo's physical dimensions (say from 36 inches to 8.64 inches) without cropping, and you do not need to decrease the file size (measured in megabytes: 28.8M).

Here’s how to do it in Photoshop Elements:

	Example A This screenshot shows the original image size. Notice that Constrain Proportions is selected and Resample Image is deselected, as specified in step 2 above.
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	Example B When I typed in a new resolution (300 pixels/inch), notice how the other numbers updated automatically, as explained in step 3 above. In the Pixel Dimensions box, you'll see that the number of pixels hasn't changed from the original (28.8M).
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Note: Changing a photo’s physical dimensions will also change its resolution. As you make a photo smaller, the resolution will increase. As you make a photo larger, the resolution will decrease. For high-quality prints, make sure the resolution is at least 300 ppi (higher is fine).

Quick tip: After you change your Resolution from 72 to 300 ppi (as seen in Example B above), you may notice that the physical dimensions are quite large (8 x 12), as is the file size (28.8 megabytes). To reduce this size, just click Resample Image back on at this point, then specify a smaller height or width. You'll see both the file size and the physical dimensions decrease.

Word of caution from digital scrapbooker Aly Dosdall:
Even if you are just resizing, never save over an original photo. Always save an edited photo as a duplicate copy to avoid losing the high-resolution original. You never know when you might need a different version of the same photo.

Lesson #2: Resizing for the Web

Posting a full-size photo on the web can be very time-consuming, depending on the speed of your Internet connection. For a quicker upload, try decreasing the photo’s file size by following the steps below. Use this method when you want to e-mail a photo to someone who will not need to print it or when you want to post a photo on a website, in an online gallery, or on your blog.

Here’s how to do it in Photoshop Elements:

	Example C Do you see the file size information listed next to Pixel Dimensions? If you do not resample an image (as suggested in step 2 above), this information does not change. But if you resample an image, the new file size appears first and the old file size appears in parentheses. Notice how dramatic the change can be.
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Word of caution from article author Rachel Gainer:
When you resample an image, you change the original pixel dimensions. Decreasing the pixel dimensions is called “downsampling.” When you downsample, you essentially throw away some of the pixels in the image. This decreases the file size, but it also decreases the image quality. A monitor uses fewer pixels than a photo printer, so what looks good on screen won’t necessarily print well.

Lesson #3: Resizing & Cropping

Cropping a photo to a specific size is quite simple and very useful. Below are a few simple steps to walk you through the process. Use this method when you have plenty of pixels to work with and you want to zoom in on a smaller section of a photo.

Here’s how to do it in Photoshop Elements:

	Example D: This screenshot shows how my screen looks after I've specified a width and a height and selected the area of my photo to crop.
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	Example E Check the resolution of a cropped photo by looking at the Document Dimensions displayed in the lower-left corner of your workspace. (Where my mouse is pointing.) If your photo’s resolution is less than 300 ppi, try starting over and not cropping in so far.
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Quick Tip: After you've cropped your photo according to Lesson 3 above, it's possible that your file size may be too high to upload to an online photo printer. In order to reduce your file size, use what you learned in Lesson 1 above to resample the image. Just make sure Resample Image is selected, but leave the height and width alone, and change the ppi to 300.

Word of caution from Ella co-founder Angie Lucas:
If you want to crop in on a small detail in a larger photo, your original image needs to be very large. For example, if your original photo is 11 x 17 inches at 300 ppi, you can crop in really small and still end up with a clear 6 x 4 image at 300 ppi, which will look great when printed. However, if your original image starts out at only 6 x 4 inches at 300 ppi (as seen in the introductory image to this article) and you try to crop in and enlarge a small detail from that photo, you’ll pixilate or blur the image (as seen below). Note: since your computer monitor displays images at 72 ppi, you probably won’t notice there’s a problem until you print it.

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Note: Special thanks to fellow scrapbookers Aly Dosdall and Angie Lucas for their contributions to this article.

Credits: The model for this article is Ms. Shelby Gainer. The first photo is by Travis Lucas, while the photo used in the cropping/resizing examples is by Rachel Gainer.

Need More Photoshop Elements Tips?

Visit reneepearson.com to watch two exclusive FREE videos that will build on the lessons you just learned here. In the Study Hall section of the site, you can view a shortcut method of resizing a photo for the web and watch a tutorial that will show you the safest way to enlarge a photo! (You must be logged in to reneepearson.com to view the videos, but registration is free.)

Now that you feel comfortable resizing photos, why not add a few more digi tricks to your repertoire? Our hot-off-the-presses eBook, Special Effects for Digital Photos by Renee Pearson, offers ten step-by-step tutorials for adding flair to your favorite photos. Learn how to round corners, selectively colorize, and much more! Download it here for just $5.99.