Most online print services specify a resolution setting and won’t accept images that are configured differently. But what resolution should you choose if you’re printing the image yourself, and does it really matter? The answer to the second question is: Yes, it matters because resolution choice can impact image quality. The answer to the first question is a little more complicated.

Resolution Defined
Unfortunately, largely because of camera vendors, the term “resolution” is rarely used correctly. For instance, people speak of a camera having “a resolution of 12 megapixels.” “Twelve megapixels” is not a resolution, it’s a pixel count.

Resolution is the measure of how many pixels fit within a given area. In other words, resolution determines how closely pixels are spaced. “12 megapixels of resolution” is an incorrect statement because it doesn’t tell you if those 12 million pixels are arranged so that they cover an inch or a mile.

An image has no inherent resolution—it’s just a grid of pixels. Instead, you get to set the resolution by specifying how closely spaced the pixels are. Depending on the resolution you choose, your image will cover more or less area (Figure 1).

Figure 1. The first example below has not been resampled. The second example has been resampled. For more on what that means, keep reading.

If your image will only be viewed onscreen, then the file’s resolution is irrelevant. A monitor has a specific resolution. Its pixels are spaced a certain way, and there’s nothing you can do about it.

Resizing Revisited
When it comes to size, every image file has three properties:

1. Pixel dimensions
2. Print size
3. Resolution

Pixel dimensions are simply the number of horizontal and vertical pixels.

Print size is measured by how wide and high the outputted image is; for example, 3″ wide and 5″ high.

Resolution determines how closely those pixels are spaced, and a higher resolution will yield a smaller print size, because the pixels will be packed closer together.

Photoshop’s Image Size dialog box makes this relationship very clear. In Figure 2, you can se that I have an image with 3744 x 5616 pixels. My resolution is set to 300 pixels per inch, so when all those pixels are lined up that closely, my final print size works out to be 12.48 x 18.72 inches.

Figure 2. At 300 dpi, 3744 x 5616 pixels result in a final print size of 12.48 x 18.72 inches.

The Image Size dialog box gives you visual clues as to the relationship of the print size parameters In the Document Size Area, the black line thingy to the right of the Inches pop-up menus indicates that Width, Height, and Resolution are locked together. You can’t change one without changing another. For example, if I set the resolution to 600, the print size goes down, as reflected in Figure 3.

Figure 3. With an increase in resolution, the print size goes down because pixels are now more tightly packed.

Similarly, if I change print size to, say, 8″ x 12″, the resolution goes up:

Figure 4. Larger print size yields a smaller resolution.

At this point, I can’t change one of these properties without changing the other. So if I wanted to create a 4″ x 6″ print at 300 pixels per inch, I’d be out of luck.

Fortunately, there’s a simple switch to throw in the Image Size dialog box that allows this.

Resampling
The process of changing the number of pixels in an image is called “resampling”. To understand that term, think of it this way: If I have a 12-megapixel file, but I need an 8-megapixel file, then I need to take an 8-million pixel sample from those original 12 million pixels. I must keep some pixels and throw the rest away.

To do this in the Image Size dialog box, I simply check the Resample Image box. When I do, a few things change, as you can see in Figure 5.

Figure 5. With Resample Image checked, I have more options.

First, notice that the Width and Height fields under Pixel Dimensions are now editable. I can change the number of pixels in this image, which is great, because my original goal was to end up with fewer pixels.

Note too, that in the Document size area, the black line thingy is no longer showing a connection between Width/Height and Resolution. In other words, print size is now independent of resolution. Because I’m allowing Photoshop to resample, I’ve given it permission to discard (or create) pixels to get to an image with a specific print size and resolution.

Finally, The Constrain Proportions check box is now active. If I turn this off, I can change Width independently of Height, which lets me create a distorted image.

With Resample Image checked, I can now enter the exact size and resolution settings that I want.

Figure 6. By checking Resample Image, I allow Photoshop to discard pixels, so that I can shrink my file to a 4″ x 6″ image at 300 dpi.

Beneath the Resample Image check box is a pop-up menu that contains different interpolation algorithms. These determine how Photoshop goes about choosing which pixels to keep and which to discard. Each includes a parenthetical statement about its purpose. I’ve chosen Bicubic Sharper because Adobe says it’s “best for reduction.” And if Adobe said “go jump in the lake…”

What Resolution Should You Choose?
Now that you have the ability to choose resolution, what resolution should you choose? Your gut reaction might be to leave it as high as possible; more is always better, right? Wrong. If that were the case, there’d be no reason for this article.

Say you want to print your photo on a desktop inkjet printer. All inkjet printers have a native resolution. If the image you send to the printer is not in the printer’s native resolution, then the printer will resample the image to what it wants. If your printer has a native resolution of 300, and you send it an image with a resolution of 600, the printer will downsample it.

“In that case,” you might be thinking, “why should I worry about resolution if the printer takes care of it?”

While your printer can resample on its own, its resampling algorithms might not be as sophisticated as your image editor’s. In theory, it’s possible that you’ll get a better-quality resizing if you do it yourself. In practice, it’s hard to find a visible difference.

More important to control yourself is sharpening. The sharpening algorithms in an image editor work by creating light and dark halos around the edges in your image. These halos make edges more acute and thus make the image look sharper. But sharpening should be applied precisely. Too much, and your image will be visibly degraded.

Consequently, it’s always best to apply sharpening at your final print size and resolution. When you do this, you’re creating halos of a particular width to achieve the sharpening effect that you want.

But if you sharpen an image, and then it’s resampled by a printer—that is, the pixel count changes—those sharpening halos may be enlarged, and therefore more visible; or smaller, and therefore less effective.

So before you send an image to your printer, it’s best to first size the image to the print size that you want, then to your printer’s native resolution, and finally apply sharpening. This prevents your printer from resizing the image and helps ensure that your sharpening remains effective.

There’s one problem. It can difficult to find out a printer’s native resolution. The manual probably doesn’t include that information, and you’ll be hard-pressed to find it on a printer vendor’s website.

I can give you some general specs, though. Epson inkjets typically have a native resolution of 360 pixels per inch. Canon and HP printers typically have a native resolution of 300 pixels per inch.

The best way to find out if one resolution works better than another is to output test prints. Proper sharpening and resolution choice is especially critical when you enlarge images, but a little testing should answer the resolution question.

Related articles

• Image resolution–the key to excellent prints (macworld.com)
• Digital Photography Tutorial on Resolution (brighthub.com)
• How To Resize An Image In Photoshop And Keep It Sharp (blogs.sitepoint.com)
• How many megapixels do I need? (theglobeandmail.com)