Scanning Resolution: DPI & PPI Crash Course

What Is DPI & PPI?

At Nostalgic Media, we get more questions about image scanning, resolution and file formats than anything else. Hopefully, we can cut through some of the misinformation. We'll keep it simple:

DPI = Dots (of ink) Per Inch. Use DPI when you're printing.

DPI, that's dots of ink on a piece of paper. The more dots of ink that are laid down within an inch, the sharper the image will appear. Sharpness is resolution.

PPI = Pixels Per Inch. Use PPI with monitors and TVs.

Similar to DPI but for monitors and TV screens. The more pixels per inch on a screen, the sharper the image will appear.

Tips On Photo Scanning

If you never plan to make enlargements to your scans, stick with 300 DPI

If you expect to enlarge or crop your images, scan your original photos at 600 DPI

300 dpi is a standard benchmark for an excellent print. 
200 dpi will still produce a good image. 150 dpi can be acceptable if you're viewing the print a few feet away.
Most desktop printers and photo labs print digital images at less than 300 dpi. Why? Often, the ink jets on those printers aren't capable of putting 300 dots of ink in a given inch. Also, it takes longer and uses more ink when printing at a higher dpi, so some printers are set at a lower dpi for speed and economy.
At 20 inches, most people can't see more than 170 dpi.

Researchers at the University of California determined that the average person viewing an image at a distance of 20 inches will detect about 170 dpi. Your ink jet printer may be printing from 120 to 240 dpi. Prints from a one-hour lab are about 220 to 240 dpi. How about a slick fashion or travel magazine? Those pictures can be about 180 to 250 dpi. A billboard viewed from a quarter mile looks good at 15 dpi. As distance increases, resolution can decrease.

Printed Media = DPI Resolution
Photo from a one-hour lab
200 - 260 dpi
Consumer ink jet printer
150 - 280 dpi
Glossy color magazine
180 - 240 dpi
160 - 200 dpi
Large billboard
12 - 30 dpi
So why should you care about high resolution?
It depends. If you have 4x6 snapshots, then 300 dpi scans are fine for archiving. You can still print a good looking 7x10 enlargement.
For a small wallet-size picture, scan at 600 dpi so you can enlarge it and retain more detail. Got a photo of a group of people? 600 dpi will allow you to zoom in and crop. Slides and negatives are smaller so they're scanned at a higher dpi rate. Better to have too much resolution. You can always go down in size; you can't go up without losing quality.
Recommended maximum print sizes for scanned media
Scanned Media 300 DPI - Excellent 200 DPI - Good 150 DPI - Acceptable
4x6 photo: 300 dpi
4x6 photo: 600 dpi
8x10 photo: 600 dpi
35mm slide: 2000 dpi
35mm slide: 4000 dpi
6x6 negative: 3000 dpi
4x5 negative: 2000 dpi
TIFF or JPEG file format: JPEG wins
At Nostalgic Media, we deliver all photo, slide and negative scans as JPEG images. JPEG is the most universal format. The images from your digital camera are JPEG. 97% of the pictures on the internet are JPEG. You can easily send JPEG images by email. JPEG images can be viewed and shared on nearly all digital devices.
TIFF images are specialized and used for high end applications such as printing and professional photography. TIFF files are massive; up to five times larger than JPEG images; often too big to send by email. Furthermore, TIFF files cannot always be opened on common digital devices. You'll likely need special software such as Photoshop to open and view TIFF files.
We can deliver TIFF files for an additional charge. Contact us for the details.

PPI = Pixels Per Inch. Use PPI with monitors and Tvs

A 21-inch monitor is about 105 ppi and
looks sharp at 20 inches.

A 40-inch HDTV is about 48ppi.
It looks sharp when viewed at 10 feet.

A scanned photo has more resolution than you will see on a computer monitor or TV
A pixel is an individual dot on the screen. More dots mean higher resolution. Monitors cannot resolve at the same sharpness as a printed photo. This chart shows that the pixel density increases as the monitor gets smaller. However, you will view the image at a greater distance as the monitor size increases. You'll view your iPhone (326 ppi) at about 15 inches. You'll watch that HDTV (48 ppi) on a couch ten feet away.
Digital Media = PPI Dimension (pixels) Resolution
40-inch HDTV
48 ppi
24-inch monitor
92 ppi
21-inch monitor
105 ppi
Laptop: 15" monitor
141 ppi
iPhone: 4.7" screen
326 ppi