Updated: Dec 15, 2021
A brief detour into some digital concepts
Some questions go right to the heart of your understanding. When I was teaching Digital Graphics, I discovered a question which tested student's understanding of digital graphics.
That question was:
"How Big Is A Pixel"
If you can give a definite answer to this, you probably don't understand digital images!
That's because pixels don't physically exist. They are a mathematical idea. Like numbers, they can refer to physical things, but they aren't themselves physical.
Then think about this:
If I have a tiny digital image, say 100 pixels by 100 pixels, and I display it on my monitor, its size depends upon how far I zoom in. Okay?
I could draw a grid on the image, 100 by 100, to mark out the pixels. If the image is showing on my monitor at 100cm x 100cm, then each pixel is appearing at 100/100 = 1 cm.
But if I zoom out, each pixel is smaller. So on my computer, the pixels have no fixed size.
But What about the dots on my screen?
If you magnify your computer monitor, you'll see tiny dots (rectangular or circular or similar).
These dots are not pixels.
They're fixed in size, and are usually around 0.2 to 0.3 mm in size, and surrounded by narrow empty or black space.
They make a kind a sieve for the light, which is on top of your image (and its pixels). That enlarged 100x100 image above, might take up 10mm/0.2m = 50x50 dots per pixel.
And What About Printing?
Printers don't make pixels, either! They also make dots, which are even finer than those on your display: there can be 30 dots per mm on your bubblejet print, and 100 dots per mm on a laser print.
When the printer takes your image-made-of-pixels, it has to "map" those pixels into dots, to make the image at the size you've asked it.
If that 100x100 pixel image is printed on an A4 or letter-sized page - and let's say we'll print it at the same size that it is on our monitor - then each pixel is going to contain as much as 1000x1000 dots!
This May Be Interesting, But What Practical Use Is All This?
The short answer: Don't be fooled by what you see on your screen.
The image may turn out more or less sharp than you think, when you print it.
- if you've magnified the image on your screen, larger than it is going to print, it may mislead you into thinking it will not print well.
- if you've zoomed out, the image may appear sharper than it's going to look when printed (if the display size is smaller than the printing size).
So, How Do I Work Out Whether My Image Is Sharp Enough (high enough resolution)?
Here's a good rule-of-thumb: DPI = P / S
You need two numbers:
The length of the shortest side of your print, in inches (S)
The number of pixels on the shortest side of the digital image (P)
Divide the number of pixels on the shortest size of the image, by the actual size in inches, that you intend that side to print at. If that number (DPI) works out to less than 150, your image won't be good enough for hand-held viewing.
DPI = P / S
For example: if I want to print at 100cm x 100cm (about 8inches on a side), and my image is 2000x2000 pixels, then the calculation works out to:
DPI = 2000 / 8 = 250 (dots per inch) *
which means the image will be plenty sharp enough.
This assumes that your printer is capable of printing at 150 DPI or more - which is true for all modern bubblejet or laser printers, and certainly commercial printers.
Most good printing services will test your image for this. Often they will insist on 200 DPI for art images.
...And How Did Your Students Go With the Question?
My students mostly had blank looks. But some of them did think about it!
* PS - It works for centimetres, too.
If you measure the print in centimetres, then the number you get for DPI, should be at least 6.