Someone wrote to me with a question about printing getting a digital photo printed in a magazine:
Attached is a photo that blows up very well on screen. However, I keep getting told by journal publishers that they need them with more pixels. Can it really be so that sufficient resolution on screen will not turn out to be sufficient resolution when printed (in the same or even smaller size)?
Attached was a 1700x1200 pixel image.
You should give print publishers the largest possible image size so they have the flexibility to reduce or enlarge it as needed.
Let's see if I can explain it.
Every output device, such as a monitor or printer, has a native pixel density, called pixels per inch, or PPI, and also incorrectly referred to as dots per inch (DPI). A monitor is typically somewhere around 75-100 PPI. A printer's PPI will be stated by the manufacturer; for example, 600 ppi or 1200 ppi.
Let's take a 600ppi printer as an example. A resolution of 600ppi means the printer prints 600 pixels per inch in each direction. Each square inch of paper is filled with 600x600 little droplets of ink.
If you take a photo that's 1000x800 pixels and print it at the printer's native resolution, how big will it be?
It will be 1000/600 inches by 800/600 inches, or about 1.7x1.3 inches. Each pixel in the image becomes a tiny droplet of ink on the paper. That size is fine for a postage stamp or a business card, but what if you want to print a 4x6 inch picture?
A 4x6 picture would need 4x600=2400 pixels in one dimension and 6x600=3600 pixels in the other: a 3600x2400 pixel image. In other words, in order to print out a 4x6 image so it appears the sharpest and most detailed on the printer, you need to send it an image that's 3600x2400 pixels.
"But!" you protest. "3600x2400 is 8.6 megapixels. That's more than my digital camera has. You mean my $1500 digital SLR can only print small 4x6 photos? How can I print out anything bigger than about a 4x6 image if my camera's sensor doesn't have that many pixels?"
Well, here's where you can fudge things a bit. The calculations are still fundamentally the same, but things get a little blurry, literally.
The human eye simply can't see anything as detailed as 600ppi. There aren't enough rods and cones in the eye to do it. So although the printer is capable of printing 600 pixels per inch, only about 300-400 of them are actually visible.
Couple that with limitations in print technology itself. You know from personal experience that when you touch ink to paper (say, when you a black marker), the ink spreads a little. It's no different in an inkjet printer. The printer may be spitting out 600 little droplets of ink for every inch, but they smear together such that the effective resolution of the printer is around 250-450 ppi, depending on the paper and ink being used.
Let's settle on 300ppi as a nice average of both eye resolution and printer resolution and rework the previous example.
A 4x6 print at 300dpi is 1200x1800 pixels. In other words, you can print out a 1200x1800 pixel image at 300ppi and it will look pretty much the same as the 600ppi image. But you can't get away from the native resolution of the printer. The printer can't actually print anything at 300ppi. So here's what actually happens:
- You direct the computer to print the 1200x1800 image, scaled to fit 4x6 inches. The computer calculates that to be 300 ppi.
- The printer driver takes the 1200x1800 image and scales it up to 2400x3600 by doubling each pixel in both directions. The image is now scaled to the printer's native resolution.
- The printer prints the 2400x3600 image. It occupies 4x6 inches of paper. It comes out looking pretty good.
Let's say now you take that same 1200x1800 image and direct the computer to make an 8x12 inch print out of it. What will happen?
- The computer calculates the resolution to be 150ppi.
- The printer driver scales up the image to the printer's native resolution of 600dpi by quadrupling each pixel in each direction. The image is now 4800x7200 pixels.
- The printer prints the 4800x7200 pixel image onto an 8x12 inch print.
But what does the print look like? Is it sharper, blurrier, or the same as the 4x6 print? A: at only 150ppi, it's blurrier than the 4x6 print because the same 1200x1800 pixels now effectively occupy a larger area. Each pixel had to be printed bigger to fill the area, so it appears less sharp.
Do you see what's happening here? An 800x600 pixel image looks great on your monitor displayed as a roughly 8x6 inch image on the screen, whose native resolution is around 100ppi. The image is as sharp as it's going to be on the monitor, so you think "Wow! I'm going to print this out!" Although it would look great printed as a 2.6x2 inch print (300ppi), it would look lousy printed as a 4x6 print (133ppi) and really crappy as an 8x12 (66ppi).
So back to your magazine publisher. Let's say they want to print your flower picture as a 4x6 in the magazine. Let's also assume they have a really high quality printer with good, glossy paper whose effective resolution is 400ppi. How many pixels should your image be to make the publisher happy?
4x400=1600, 6x400=2400. Your image should be at least 1600x2400 pixels. More pixels would give the publisher some flexibility, since it would allow them to print the photo larger without having to reduce the pixel density. Scale your image to at least 1600x2400 pixels. Don't worry about PPI or DPI; just give the magazine an image at least 1600x2400 pixels.
Here are some common rule-of-thumb output resolutions:
75-100 ppi: screen
200: fax machine
300: ca. 1990 B&W laser printer
300: effective resolution of a consumer inket printer
350-400: limit of human eye
400: Costco 1-hour photo
400-450: high-quality offset color printing press
600: modern B&W laser printer
Hope that helps!