Printing basics

Learning goal

You will be able to decide what size you can print a photo by looking not only at the file's pixel count but also at the distance from which the print will be viewed. From a file's pixel dimensions you will calculate the number of pixels per inch for different print sizes, and see how the smallest detail the eye can distinguish at a given distance enters this calculation. You will tell apart pixels per inch in the file and the ink dots per inch the printer prints, and know what resampling to enlarge a file does and doesn't do. You will solve the mismatch between the camera's aspect ratio and paper ratios with cropping or a border, and adjust output sharpening according to print size. When printing a face larger than life, you will think about how the skin's detail and noise will look, and ask whether the person agrees to their face being hung at this size.

The question

The baker wants to see the photo in which they hand bread to their neighbor big enough to cover the empty wall behind the counter, about a meter wide. Your friend from the club calls the print shop. The print shop asks two questions: how many pixels is the file, and how many centimeters will the print be? The file is six thousand pixels wide. Your friend remembers that three hundred pixels per inch are needed for a sharp print looked at up close; by this calculation the file is only enough for fifty centimeters.

They are afraid the one-meter print will come out blurry. The person at the print shop, on the other hand, smiles and asks whether people will stand in front of that wall, from what distance customers will look at the print. How big can you print a photo? Why can a file that is enough for fifty centimeters still look sharp at a meter?

Context

In the megapixel myth lesson you saw that the measure often used for a sharp print looked at up close is three hundred pixels per inch, and that by this measure a six-thousand-pixel side makes about fifty-one centimeters (Lesson 3.4). In the export lesson, too, we discussed print resolution and output sharpening as parts of preparing the file for where it will go (Lesson 17.9). This lesson adds the variable missing from that calculation: the distance of the eye looking at the print.

The three-hundred-pixel rule is a good starting point for someone holding a print in their hand and looking at it up close. But no one looks at a large print on a wall with their nose pressed against it. As the print's size grows, the viewer steps back and the detail the eye can distinguish shrinks. That is why the question of how big a photo can be printed has no single answer; the answer depends on where it will hang and how it will be looked at.

Simple explanation

What does viewing distance change?

Viewing distance is the distance between the eye of the person looking at a print and the print. Visual acuity is the measure of the smallest detail the eye can tell apart at a given distance. A value often used for normal vision is the eye being able to distinguish an angle of roughly one sixtieth of a degree. This angle corresponds to a detail that grows with distance: an eye looking from thirty centimeters can pick out a detail of about a tenth of a millimeter, while an eye looking from a meter can't pick out detail smaller than about three tenths of a millimeter.

Turning this rough calculation into pixels per inch makes the picture clear. For a print looked at from thirty centimeters, the limit the eye can distinguish is about three hundred pixels per inch; this explains where the three-hundred rule comes from. For a print looked at from a meter, the limit drops to about ninety, and for a print looked at from two meters to about forty-five pixels. Printing detail the eye can't distinguish doesn't make the print look sharper. Another habit often used in practice is that viewers usually look at a print from a distance close to its diagonal; a print one meter wide is usually viewed from a little over a meter.

How are pixels and dots calculated?

Pixels per inch (PPI) is the value that shows how many image pixels will fall on each inch of a file when it is printed; one inch is 2.54 centimeters. If a six-thousand-pixel file is printed at one meter, sixty pixels fall on each centimeter, and about a hundred fifty-two pixels on each inch. Since the limit for a print to be viewed from a meter is about ninety pixels, a hundred fifty-two pixels is comfortably enough for this wall. The pixels-per-inch tag written inside the file changes nothing on its own; what matters is the pixel count and the size it will be printed at.

Dots per inch (DPI) is the number of ink droplets a printer can lay on each inch of paper. Printers create the color of a single pixel by putting many small droplets of several different inks side by side. That is why the values of thousands of dots seen in printers' advertising describe not how many pixels the file should carry, but how fine the droplets are with which the printer builds color. The two concepts are often confused; when the print shop asks "how many dpi should the file be?", it usually means pixels per inch.

The camera's aspect ratio decides the size too. If a three-to-two frame is printed one meter wide, its height is about sixty-six and a half centimeters. Paper sizes, on the other hand, are often in other ratios; in one of the common paper series, the ratio of the long side to the short side is the square root of two, and a page of 21 × 29.7 centimeters is from this series. The photo is fitted to these ratios either by cropping a little or by leaving a border around it; composition decides where the crop is made.

Does enlarging add detail?

Resampling is increasing a file's pixel count by calculating missing pixels from their neighbors, or reducing it by merging excess pixels. Enlarging a file doesn't bring out detail that doesn't exist; it only spreads the existing detail over more pixels. A measured enlargement, especially in a print to be viewed from a distance, can keep the edges of pixels from showing; but a frame that is soft at fifty centimeters doesn't get sharp at a meter by being enlarged. Some programs try to add detail by guessing it while enlarging; this addition isn't real detail but a guess, and it can look artificial, especially in faces and textures.

As the print's size grows, output sharpening is also adjusted again. Because ink spreads a little on the paper, the print looks a little softer than on screen; in a large print sharpening is done with a wider radius, and in a small print more finely.

A large print carries another question for people. In a print one meter wide, the baker's face looks bigger than its real size; the pores, small marks and lines of tiredness on their skin become more pronounced than they ever were on screen. How much of this detail is retouched needs to be decided without taking the face away from its naturalness, and by talking with the person. A little noise in the shaded areas of dark skin can look like a pronounced grain in a large print; while reducing this, the skin's texture shouldn't be erased and the face shouldn't be made to look like wax. Asking whether the person agrees to their face being hung larger than life on a shop wall is also part of the printing decision.

Visual evidence

You calculate the file's dimensions: six thousand pixels wide, about a hundred fifty-two pixels per inch in a one-meter print. The wall is behind the counter; customers look at the print from one to two meters away. At this distance the limit the eye can distinguish stays below ninety per inch; the file is enough. The three-to-two frame comes to sixty-six and a half centimeters high at one meter wide; you place it on the print shop's roll paper leaving a border.

At the print shop you have a small test strip printed. Looking from thirty centimeters, the finest textures of the crust and flour are slightly soft; when you step back a meter, the softness disappears and the frame looks sharp. You increase the sharpening a little for the large print.

You show the baker their face on the test strip. They want the few small marks on their forehead to stay, and only the noise on their shaded cheek to be reduced a little. You reduce the noise just enough to preserve the skin's texture. In the one-meter print the baker's face stands a little larger than its real size, in its warm brown, the texture of their skin in place; the baker stands in front of the print and laughs.

Common mistakes

Summary

How big a photo can be printed depends not only on the pixel count but also on the viewing distance: visual acuity drops to larger details with distance, which is why the about three hundred pixels per inch needed at thirty centimeters falls to ninety at a meter and forty-five at two meters. The file's pixels per inch and the printer's ink dots per inch are different things, resampling doesn't add detail, the aspect ratio is fitted to the paper with cropping or a border, and output sharpening is adjusted according to print size. In a large print the face's detail and noise become pronounced; retouching and noise reduction are done preserving naturalness and by talking with the person, and approval is obtained for the face to be hung at that size.

Mini assignment

In this assignment you will calculate the print of a photo at different sizes and do a small test.

1. Write the pixel dimensions of one of your photos and roughly calculate the largest size for prints to be viewed from thirty centimeters, a meter and two meters.
2. For a print size you choose, calculate the number of pixels per inch, and write how the aspect ratio will fit which paper, whether cropping or a border will be needed.
3. Print a small part of the photo as a test strip at the large print size, or view it on screen at the same magnification; compare the sharpness looking up close and from a distance.
4. If there is a human face in the photo, write what the skin looks like at a large size; get the person's view on retouching and on the face being hung.

Check yourself

• Given a print size and viewing distance, can you calculate whether the file is enough?
• Can you explain the difference between pixels per inch and dots per inch?
• In a large face print, did you preserve the skin's texture and get the person's approval?

Glossary

Mini quiz

1. For a print to be viewed from a meter, about how many pixels per inch is the limit the eye can distinguish?

• A) One thousand two hundred
• B) Six hundred
• C) Thirty
• D) Ninety

2. If a six-thousand-pixel file is printed one meter wide, about how many pixels fall on each inch?

• A) A hundred fifty-two
• B) Three hundred
• C) Sixty
• D) Six thousand

3. What does the value of thousands of dots written in a printer's advertising describe?

• A) The number of pixels the file should carry
• B) The largest size of the print
• C) How fine the ink droplets are with which the printer builds color
• D) The thickness of the paper

4. What does enlarging a file that looks soft at fifty centimeters to a meter by resampling achieve?

• A) The frame gets sharp
• B) No new detail is added; the existing detail is spread over more pixels
• C) The colors get saturated
• D) The viewing distance gets shorter

Answers

1. D — At a meter, the eye can't pick out detail smaller than about three tenths of a millimeter; this is about ninety pixels per inch.
2. A — Six thousand pixels over a meter is sixty per centimeter, about a hundred fifty-two per inch.
3. C — The printer builds a single pixel's color with many small droplets; this value isn't the file's resolution.
4. B — Enlarging doesn't produce detail that doesn't exist; it only distributes the existing information over new pixels.

Summary card

• Print size = pixel count + viewing distance.
• 30 cm → ~300 ppi, 1 m → ~90 ppi, 2 m → ~45 ppi.
• PPI is the file's measure, DPI the printer's.
• Enlarging doesn't add detail; adjust sharpening to the print size.
• In a large face print, preserve skin texture and get the person's approval.

Bridge to the next lesson

Once the size was settled, the print shop asked about the paper this time and held out three samples. One was glossy and slick; the blacks were deep, the colors vivid, but the lamp's reflection shone on the surface like a stain. The one beside it was matte; there was no reflection at all, but the blacks leaned a little toward gray. The last was thick and slightly textured, more like a fabric than paper. The same photo took on a different character in each. Matte, glossy or fine art? The next lesson tells about how paper types change a photo's character.