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Pixels, Printers, Video - What's With That?

We could subtitle this: Details that no beginner wants to know. However the point is: You'll never grasp digital images until you get it ... until you know what digital images are, what to do with them, and how to do it. These basics are pretty much about the single issue: How do I use my image, how to I make it be the proper size for viewing, for printing, or for the video monitor? All this is really quite easy, digital is just a new concept. It is like learning to drive - once you learn an easy thing or two, it is a skill helpful for life. You will simply just know.

We just gotta know about pixels, and if any mystery, a short primer is here: What is a Digital Image Anyway?

Seriously, once we accept that pixels actually exist, then all this stuff is rather easy. It's all about pixels.

This page tries to be a quick summary of the digital concepts, about how things work. The answer to virtually any question about image size starts with one of these basics. To be able to use digital images well, we need this understanding. This may perhaps be written a little like an argument, refuting the dumb incorrect myths we may have heard about how digital works. The concepts below are instead what you need to know to use digital images properly. It is actually rather easy to grasp, if you get started right.

The Most Fundamental Basic Digital Concepts - Essentials to Know to Use the Images

FWIW, I am saying dpi for "pixels per inch". I am aware that nowadays, some instead prefer to say ppi for same thing, but I am also aware it has always been called dpi. Yes, I am aware that printing devices have another second use for dpi, meaning ink drops per inch, including halftone screens. If interested or confused about dpi, see more details here.


Common Case Examples

A scanned image has different creation concerns than a digital camera image. At any one setting, the camera makes images of the one size, dimensioned in pixels. The pixels are already defined and created, and it is what it is. However, the scanner requires we specify scanning resolution to create the image size our goal requires, so there is a little more to it.

The one overall rule remains: Printing is best when we have about 300 pixels per each inch of paper to be covered (300 dpi). Note again that dpi is NOT a property of the image, it is simply "just a number" inserted into the image file to establish print size (the inches are on the paper). Review the dpi section above again.

Example: Printing a digital camera image

Plug your own numbers into these examples.

A ten megapixel compact camera image might be 3648x2736 pixels (3648/2736 = 1.33, which is 4:3 shape). 8x10 paper is 10/8 = 1.25, which is 5:4 shape. Printing 8x10 inches can compute either 2736 pixels / 8 inches = 342 dpi, or 3648 pixels / 10 inches = 365 dpi. The aspect ratio, or shape, of the image and paper are different.

Most photo labs will choose the first option. But if you first crop the image yourself to 8x10 paper aspect ratio, you can choose what you will get. See the Image Resize page for more details.

A DSLR 12 megapixel image might typically be 2,848 x 4,288 pixels in size. 3:2 shape. Digital images are dimensioned only in pixels, and to use that image in any way, these dimensions are all important. And the shape can be important.

If we upload it to an online photo lab and specify to print 8x10 inches, this implies printing at 2848 pixels / 8 inches = 356 dpi (and the long ends will be cropped). Their chemical printer is probably set to print at about 250 dpi capability, but this small excess will work OK. Possibly you cropped the image a bit smaller first anyway (cropping away excess blank space at borders improves many pictures).

But if we ordered a 4x6 inch print size, this one implies printing at 2848 pixels / 4 inches = 712 dpi. Which is absolutely outlandish, so the lab will first resample it smaller, to about 250 dpi size. No harm done, except your upload was much larger and slower than is reasonable for this goal. You could have prepared it better. See the Image Resize page for more details.

Example: Viewing an image on a HDTV or monitor, including web pages

Images for a video monitor are pretty small. A HDTV screen might be 1920x1080 pixels, or about 2 megapixels. More pixels cannot help it. Few computer monitors are larger. A Large web page image might (arbitrarily) be 900x600 pixels, about half a megapixel. Presenting a 12 megapixel image there works, but is pretty lame, since the viewing software must resample it smaller (every viewing) to appropriate size to fit the screen, which will be slow, and unnecessary. Viewing our original images one time on our monitor is one thing, but if on a web site (which intends to show it many times, and it has no use for a 12 megapixel image, unless maybe the site offers to sell large prints), we really ought to resample it to our proper smaller goal size, one time, preparation done right, instead of requiring every web visitor viewing to download all the excessive bytes and resample it again and again (which is truly lame). Most regular image hosting sites will do this resample for you, but on your own site, YOU have to do it first.


Example: Scan 35 mm film and print 8x10 inch on paper

Basics: Scanning Resolution for Enlargement

The way things work is this:   (if bothered by the numbers, just skip to the last paragraph below, Method B there).

One subtlety, important to some critical users:   The above works of course, quite well, but this computation may result in computing and scanning at an odd number like maybe 2642 dpi, or even 2700 dpi. The scanner sensor hardware can only scan at specific values (integer divisions of maximum resolution specification), like perhaps 1200 dpi, 2400 dpi, 4800 dpi, 9600 dpi (these will be its menu choices). This requires the scanner do that, and then resample the scan lines, and step the carriage motor unevenly. Perhaps of little concern for some, but when convenient, a common technique is to scan at the next larger integer division menu value (1200, 2400, 4800, 9600 dpi, menu choices), and then resample smaller to desired size at higher quality with a photo editor, which has all the data available at resample time.


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