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Never Enlarge Photos in Photoshop

Never is a slippery word. There’s almost always a work-around.

My fiancee’s daughter came to me this week with a question about printing her husband’s 50th birthday t-shirt (or invitation; all I really knew was that she wanted to enlarge his photo). I told her enlarging a photo could be problematic, and I suggested she choose a different image.

Here’s why. When you shoot a photo using your cell phone camera (or even a high-end digital camera), you save the image at a specific resolution, expressed in pixels per inch. If you then open the resulting image file in Photoshop and increase the viewing magnification from 100 percent upwards, again and again and again, you will eventually see the pixels (colored squares) of which the image is composed.

If you choose the correct resolution for your intended use, this is not a problem, because the human eye is forgiving, and at the proper resolution the pixels will be below the threshold of visibility.

Halftone Line Screens

For instance, if (as was the case with my fiancee’s daughter) you are reproducing the photo through some kind of printing technology such as offset lithography, you would want to choose a resolution for the photo that is twice the halftone line screen you plan to use. This would usually be 300 ppi (pixels per inch, but also often expressed as dots per inch). If you print your image at the 100 percent size at which you took the photo, this should be adequate to avoid seeing the pixels (called pixellation). The 300 ppi notation assumes the printer is using a halftone line screen of 150 lines per inch. (As noted in prior PIE articles, traditional halftones are grids of dots of various sizes that create the illusion of various tones of ink from a light tint to a dark shade.)

The bottom line is that if you save your photo as a 300 dpi (or ppi) image, you’re usually safe for offset printing, laser printing, etc.

That said, if you shoot the photo and then reduce the size of the image, that’s just fine, too, because you wind up making the pixels smaller (i.e., even less visible). Unfortunately, if you enlarge the photo, the resolution will drop, and you will begin to see the pixels. If this is an important job (like my fiancee’s daughter’s husband’s image, or an annual report photo), that would be a problem.

The Proper Resolution

As an aside, if you are producing images for the internet, you don’t need this high a resolution. For the internet (i.e., anything on a computer screen), 72 dpi is fine. In this case, human eyesight is not acute enough to see the pixellation. However, the same holds true about saving the image at 72 dpi and then enlarging it. Making a photo larger even on the computer screen will degrade the image.

Scanning photos (rather than taking the photos with a camera) would work the same way. Scan your images at the proper resolution, and then don’t enlarge them. In fact, it’s ideal to scan the images at a size that will be close to the final printed size.

Why is that? Let’s say you go in the opposite direction and scan a photo at 600 dpi when you only need to scan it at 300 dpi. This will create a much larger image file, which will slow down your page composition software and the printer’s prepress software slightly but unnecessarily (more, or less, depending on the size). And you will get no benefit from this.

One final note you might find interesting is that when you buy images online for use in printed publications (and for online use as well), they are often very large. For instance, when I write the PIE Blog posts, I usually add a photo at the top of each article. When I receive a few relevant photos and open them in an image editor and then reduce their resolution from 300 dpi to 72 dpi for on-screen use, their size usually grows to about 60" x 40".

Reducing the resolution increases what was originally a 10" x 15" image to a mammoth 60" x 40" image. So I reduce the size to 10" wide (to fit the PIE Blog design grid), and the image gets much smaller (in terms of computer storage size as well as virtual height and width). This makes high resolution images load faster on screen.

Upsampling Photos

Now earlier I mentioned that there’s always a reason to break a rule. Once I did look up how to upsample photos. (This is the technical term for enlarging an image by increasing its size and also its resolution. And this is different from just enlarging the photo, which absolutely would make the pixels visible. Upsampling decouples image size from image resolution. It means, for instance, enlarging a 5" x 7" photo to an 8" x 10" image while maintaining the necessary 300 dpi resolution for printing. Downsampling would be the opposite, making a photo smaller and keeping it at 300 dpi (as opposed to reducing the physical printed size and letting the actual image resolution rise).

Upsampling (let’s say enlarging the image from 5" x 7" to 8" x 10" at 300 dpi) just makes up (or creates) Photoshop image information. It tries to do this intelligently, but that’s not the same as actually using visual picture information (real pixels next to other pixels captured by the camera you used). This is one very important reason not to scan or shoot an image and then enlarge it. It’s called "interpolation," and it means letting the computer add intermediate pixels of color between other actual color pixels captured by the camera.

With all of this in mind, there is one way to increase the size of a photo without making larger pixels (i.e., without interpolation) and even with interpolation (jumping from 5" x 7" to 8" x 10" for instance while maintaining the 300 dpi resolution). That is to do it in very small increments. I personally did this once successfully.

A Work Around

By this I mean increasing the size of a photo (let’s say) 105 percent again and again while maintaining the 300 dpi resolution. I personally would not use this for a large image. I’m sure there is a downside to doing this. I would also look closely at the image on the computer monitor (the resolution and crispness of the image, as well as any increase in contrast between light and dark areas). Try this at various levels of magnification up to lets say 400 percent. If you don’t see any degradaton in the image after stair-stepping it up 105 percent repeatedly, and if you don’t include it as a major focus of your page spread design, sometimes it may be a solution.

But my fiancee’s daughter’s increasing a 72 dpi (presumably) image of her husband for printing (at 300 dpi) would have added halos, odd juxtapositions of dark and light tone (like a posterization). It would have made him look ghoulish.

In general, the best response to enlarging and upsampling is to just say "No."

[Steven Waxman is a printing consultant. He teaches corporations how to save money buying printing, brokers printing services, and teaches prepress techniques. Steven has been in the printing industry for thirty-three years working as a writer, editor, print buyer, photographer, graphic designer, art director, and production manager.]