DPI

Introduction

DPI or dots per inch is a parameter of scanners, printers and other IO devices. The overall quality of an image you scan, or print is very dependant on DPI. DPI is not a complicated concept. It simple refers to how many dots per inch the input/output (I/O) device can do. Scanners and printers will have a max DPI number. Generally you can select numbers below the max to allow smaller images and faster processing.

Scanning

It is very important to consider what you want to do with the image when you are done when yougo to scan. This will help define the DPI or the scan you want to do.

Scanning for the Web or display on a screen

This is actually the least demanding from a DPI point of view. Each point scanned by the scanner becomes a pixel in the image. So the first thing you need to decide is how big you want the image to be when done. The following is a list of common screen sizes. So keep these in mind when scanning. If the image is too big for the persons display settings then they will not be able to see the whole picture at once.

640x480
800x600 (Most commong)
1024x768
1280x1024(Very uncommon)

There are other sizes but these are the most common.
If we are scanning photos typical sizes for photos are the following:

negative 1x1.5 inches
snap shot small 4x6 inches
snap shot medium 5x7 inches

So when scanning for these sizes here are the DPIs you need to do at min.

Image Size	1x1.5 Neg	4x6 snap	5x7 snap
640x480	427	107	91
800x600	533	133	114
1024x768	683	171	146
1280x1024	853	213	183

If you want a good quality image personally I double these and them resample down using a quality package like Photoshop.

Scanners

Scanners can be had today very inexpensively at 300 and 600 DPI but you start to pay much more as you go above this. As you can see by the table even an inexpensive 300DPI scanner is adequate for scanning photographs (not negatives) even for large images. By the way before you wory about negatives and the DPI needed to scan negatives most scanners need a special adapter to allow them to scan negatives. Check to see if the scanner is capable of hadling negatives before worrying about the DPI it requires to do so.

Scanning negative to print

Scanning to print is a different chore and is much more demanding. Let's assume you are wanting to scan a 1x1.5 inch negative and print it. The next table shows the DPIs needed.

Print DPI	4x6 snap	5x6 snap
100	400	500
200	800	1000
300	1200	1500
360	1440	1800
600	2400	3000

As you can see scanning to print from a negative can require quite a high res scanner.

Scanning a 4x6 snapshot to print

The next table shows the DPIs needed.

Print DPI	4x6 snap	5x6 snap
100	100	125
200	200	250
300	300	375
360	360	450
600	600	750

Digital Cameras

Digital cameras are all the rage right now. They have come a long way in the last 2 years and I expect them to continue to improve. Digital cameras pic up images by having elements (often called CCD arrays) for each pixel. The quality of the image is measured in Pixels (or Mega pixels). A Mega Pixel camera is one which is greater than 1 Mega Pixel. As with scanning you need to pay attention to what you want to do with the image once you have taken the picture.

Printing from a digital camera

The next table shows you the number of Megapixels the camera should have to print at a specific resolution.

Print DPI	4x6 snap	5x6 snap
100	0.2	0.3
200	0.9	1.3
300	2.0	3.0
360	3.0	4.3
600	8.2	12.0

Digital Cameras

Today 1.3 MegaPixel cameras are reasonably inexpensive. When you cross over to say 4 MegaPixel these are pretty pricey cameras.

Digital Image

Once an image has been scanned DPI means nothing. It means something when you go to print, it means something when you scan but an image is defined by the pixel size of the image. A program like Photoshop will assume and you can configure the DPI of the image so that when you print it you know how big the image will be, but a digital image has no DPI.