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Blow It Up!

By Mike Pasini, Editor
Imaging Resource Newsletter

Sometimes wouldn't you just like to blow it up? Your image, we mean.

"Sometimes wouldn't you just like to blow it up?"

Well, you can.

The trick to making the biggest possible image is to calibrate the one factor that is almost always neglected in those exquisite mathematical formulas used to derive the largest usable print size. And that factor is (drum roll) . . . viewing distance.

You've seen a billboard? Up close? The image decomposes into large halftone dots. The mind struggles to reassemble what the eye has taken apart (in a manner of speaking).

But walk back a few paces and you get the picture.

The same general principle applies here. If you're not holding the image in your hand, you can really blow it up. To 30 x 40 inches even.

Or a service bureau can. If you don't have a wide-format printer in the basement.

To find out just how big you can print your picture, open it in your image editing program and set the image resolution to 25 (yes, 25) pixels per inch, without resampling. Your width and height dimensions should change to reveal just how large the image will print. A megapixel image is more than you need to get a 30 x 40 poster, for example.

The trick is that this only works if you view the poster from no closer than 15 feet.

"A megapixel image is more than you need to get a 30 x 40 poster. The trick is to view the poster . . . from no closer than 15 feet."

We found a helpful chart by Fayeq Oweis at where Oweis does just this sort of thing commercially. He recommends 100 dots per inch for images viewed from 1-5 feet, 72 dpi for 5-10 feet, 50 dpi for 10-15 feet, and 25 dpi for further than that.

Oweis calls pixels dots just to make life simpler for customers trying to scan images.

Before you take your image to the service bureau, Oweis recommends you make two more changes: convert it to CMYK mode (which is how printers print) and save it as a TIFF file.

You can try a proof of concept at home. Just set your image resolution to 25 dpi (and let the dimensions balloon) then crop a 200 square piece of your image (an eyeball is fun). And print that on your printer.

Awful, isn't it? You can see every pixel in the image. It may even look like an abstract painting. But wait. Tack it on the wall and start backing up. Ah ha. See what we mean?

To rid your image of the jaggies, just lower the resolution of the halftone screen (try 50 lines per inch) until they disappear.

And for more on this subject, read the article "Pixels, Dots, and Inches: How Big Can I Print It?" It will help you find the largest size you can print an image intended for normal viewing.


This article is reprinted from The Imaging Resource Digital Photography Newsletter,
Advanced Mode Column, published December 3, 1999