Pixels and Dots (per Inch) - A review of DIgital Image Display

Back in 2004 I wrote an article in my Digital Rag, Pixels and Dots - Basic.
For some it was a bit technical. I'll try to bring the escence of it out in more fundamental terms here.

The article was in response to a question by my long-time friend, Gary Bannerman.

"When digital images directly downloaded from any digital camera show up in PC software such as Photoshop... the "image size" box shows this as 72 ppi ...   which, of course, is the optimum for a PC monitor.....   and it is easy to "save as" the image to 300 ppi or more...    but my question is whether the basic image we are starting with is 72 ppi.... or is that just a reading of how it shows on the monitor?

As you know, saving a truly inferior image to a 300 ppi or more resolution, merely demonstrates the pixelation in its worst form.

Bottom Line: what does 72 ppi mean in Photoshop after images are downloaded from digital cameras, because 72 ppi going to the printer invites disaster..."

Actually, Gary used digital camera words (PPI - pixels per inch) to refer to something that in the printing world is slightly different (DPI dots per inch) but many mix the two worlds so I'll try to clear things up.
In the printing industry, Dots Per Inch is the measurement - and it refers to the "screen" that is used to produce continuous tone (as opposed to line-art) images such as photographs. The typical newspaper of old was around 100DPI - photos were grainy.

In contrast, the typical shiny-paper magazine was anywhere from 600 to 1200DPI - photos jump off the page at you with their clarity and you are hard-pressed to see the dots they are made up of with the naked eye.

But how does this relate to cameras and the pixel width/height - and the Megapixel rating your camera now has?

It all comes down to some simple mathematics: Pixels divided by Pixles/inch = how big (in inches) the image will be when displayed.

In other words, an image that is 720 X 480 Pixels (width X height) will end up 10" wide by 6.6" high when displayed on your video monitor that is 72 PPI/DPI

but...

That same image will only end up 3.6" wide by 2.4" high if printed at 200 DPI on a color printer, and correspondingly smaller if printed at higher DPI (1.2" high at 600DPI for example)

The "DPI" setting in a digital image file can be changed - it is only an indication of either the intended use of the image - or the default setting imposed upon it by the last piece of software that touched it (or maybe the first piece)

So I can have 4 files - each with the same image in it - each with different DPI settings - and the information in each file is EXACTLY THE SAME! (except the one text field that has the DIP number) The only thing that will be different is that if I print each on a printer that reads the DPI filed and follows it, the image printed will be a different size for each. But if I hold each of the resulting prints up in front of me so that they fill the same visual field (hold the bigger ones farther away and the smaller ones closer - so that for example they cover 10 degrees of my visual field) the will all look exactly the same quality (allowing for the abilities of the printer)

The DPI field is an indication to the printer (the actual computer printer, or the master craftsman who runs the million dollar printing press) of how big you want the image to be when it is printed.

Let's turn this around and see what happens when I have a particular size of image in mind and a quality for that image - what do I have to do to make a photo file that will print the image at the size and quality I want?

First - let's decide on the quality. This is usually decided for us by the manufacturer of the equipment that will show/print the image.

Computer monitor manufacturers made the typical CRT about 72DPI and really good ones at 100DPI

LCD monitors are mostly 72 DPI but some as high as 200 DPI

Newspapers are around 100 DPI

Magazines may be from 300 DPI to 1200 DPI or more

Billboards may be from 1 DPI to 10 DPI

Silk-screen processes are from 10 DPI to about 30 DPI

T-shirt and cup printing processes are around 200 DPI

So you need to think about how big your final image is going to be - then multiply this size in inches by the DPI to get total dots/pixels (I'm simplifying a bit - but this will get you into the right ballpark) then go out and purchase the camera that will give you an image with that many dots across (and corresponding dots high).

What's that? You can't afford a camera that puts out images 96000 pixels wide for that 8"  wide, full-size, 1200 DPI ad for Gopher's Weekly?

Or you only have a 720x480 pixel image that you want to print 10" wide on a T-shirt at 200 DPI?

How do you do this without breaking the bank or telling the person who gave you the image that it can't be done?

You use your photo editor's "re-scan" or "pixel enhancement" or "scale image" tool to make a larger (in pixels) image from your small one - and hope that the result will look good. There are any number of free or commercial programs that will also make bigger pictures out of small ones. All use various techniques to hide the fact that they are making something out of nothing - adding detail to places where the detail was never recorded in the original - or was lost in an initial size reduction.

The most basic (and worst) is simply stretching the existing pixels into clumps that are larger but still retain exactly the same information - just bigger dots made up of typically square or rectangular clumps of pixels that are all the same color as the original one.

Other methods include interpolation of "what would be between" two pixels by averaging or other methods - and adding things like edge detection and sharpening to make the result look more pleasing to the eye.

Many professionals will end up getting in and actually "pixel editing" to manually bring out particular details - clean up the edges of text for example - get rid of "dithering" and sharpen edges - but this can be long and arduous.

The point of all this is to present an image that when viewed from the correct distance, will be pleasing to the eye. You don't normally walk up to a billboard and critique the fact that you can see the individual dots that make up the image - you view it from a passing car on a road up to a hundred feet away! Similarly you don't hold a magazine up to your eye and look a the wonderful patterns of dots - you hold it 14" to 20" away from your face.