Tutorial: Resizing Digital Images Using Photoshop (or other editor)

Resizing Digital Images

When you get your scanned images back from Pixmonix, you'll notice that the images are very large, sometimes too large to print using an online printing service (because it would take a long time to upload or they exceed the printing company file size limits), and are too large to use on a website. There is hope though, because you can easily resize your images. Photo editors like Photoshop give you many options to resize your images.

In Photoshop, resizing an image is simple. All sizing options are under the "Image Size" menu item. (You might also notice "Canvas Size..." item in the Image menu; we won't use this tool here as this can crop some of your image, as it only adjusts the image borders. It will not scale the image.)

• Open a copy of the image you want to resize in Photoshop
• Image -> Image Size...

Now that you have the "Image Size..." dialog up, you can resize the image. Let's look at a few specific examples:

• Resizing for the web: Suppose that you want to post an image to a website. Many of the images seen on the web are around 300 – 600 pixels wide. Based on the design of the webpage on which you are putting the image, you would like the image to be 400 pixels wide. Make sure that the "Resample Image" checkbox is checked. Simply type "400" into the width field, verify that the pull down next to the width field says "pixels" and hit OK. Photoshop will resize the image to be 400 pixels wide and scale the height to keep the same aspect ratio. You should then use the "Save for Web..." option (under "File") to save a copy of the image for the web. The "Save for Web..." dialog allows you to optimize the image quality while reducing the file size (but keeping the same pixel dimensions).
• Resizing for printing: Before using the Image Size tool, you should crop the image to the aspect ratio of your intended print size. Standard print sizes are 4"x6", 5"x7" and 8"x10". Please see our tutorial on cropping for printing for details. With the image properly cropped, we can now adjust the file size appropriately for the printing being done. Suppose that your printer (or the printing service that you are using) indicates that prints are made at 300 dots-per-inch (DPI). In the "Image Size..." dialog, change the Document Size widtha nd height to match your desired print size. Note that Photoshop will automatically calculate the resulting Resolution (shown in pixels per inch in the figure above). At this point, Photoshop has not changed the actual pixels in your image - rather only changing the way that they are interpreted. Now, change the "Resolution:" field to "300". Note that Photoshop automatically calculates a new pixel dimensions for the image (and changes the Dimensions width and height appropriately. Click "OK". Photoshop will adjust the pixels in the image to give you 300 DPI. Alternatively you can reduce the size of the resulting file to make it easier for uploading (if the original file was too large) either using this same method or by changing the pixel dimensions of the image as described above for producing web-ready images.

Whenever you change the size of your image you will alter the resolution. You can see the resolution of the image on the "Image Size" dialogue, in the "Document Size" entry. By default, Photoshop shows resolution in pixels per inch. This is very similar to DPI, or dots per inch, which is the amount of dots of ink in one square inch. You can read more about resolution here. When you make an image smaller, you raise the resolution, because you compress more pixels into one inch. If you make the image larger, you spread the pixels out thinner, reducing the resolution of the image. In general, if you shrink an image, you will not lose any quality. However, if you expand an image too much, you may make the image grainy or pixilated when it is printed.

There is one more option to explore: that of resampling an image. In some cases, even reducing the size of an image will lose you detail. If, for example, you have fine lines in your image, reducing the size of the image will cause some pixels to become indistinguishable, so a line with a thickness of one pixel may disappear. If you resample the image, however, the computer may be able to rescue these fine details. This is especially useful when dealing with images that have text that you don't want to lose. Unfortunately this technique can also cause your image to lose data, because it permanently changes the content of the image. If you expand a resampled image, you will lose resolution, even if the image was originally that size.

After you finish resizing your image, you can upload it to your website, print it, send it via email, etc. As always, please remember to back up your images or open a copy so you don't lose valuable photos by making an error while editing.