(This is a four-part series on picture/image, screen, and printer resolution and how they all work together, with the hopes that it will clear up Dots Per Inch (dpi), Pixels Per inch (ppi) and why things look different on your computer monitor than they do when you print them out. Taken from a list I’m on.)

On Nov 28, 2009, at 11:24 PM, Mark wrote:

Tracy,

While you are at it and if you don’t mind, an ordered, complete explanation, like you so wondrously can give, of printing, viewing and other designations, especially LPI and PPI and how they all fit together in the whole picture would be a super help.

Thanks for this fascinating thread.

M

well…. 🙂

There are whole chapters in books written on the subject, so it’s much easier to just answer individual questions than it is to spend a day trying to compose a lesson in it!

How about an outline? (With significant technical oversimplification…)

Digital images are really long lists of numbers. You cannot hold up a file to the light and see an image. The numbers come from an interpretation of a light-sensor’s value. Those are called photo sites, and it’s an incredibly tiny bit of electronics that emits a current when it’s struck by photons (light). The more photons that hit it, the more electricity it gives off. (OK: strictly speaking this isn’t really what’s going on, (it’s more like a photon receptor/bucket) – but let’s live with this version anyway.)

The light sensitive part of your camera (what used to be film) is a flat plate of millions of these things, arranged in a rectangle. So, for example, if your camera is a 12-megapixel one, then (let’s say) the checkerboard of that plate is 4000 x 3000 (since 4000 x 3000 = 12 million).

So, you’ve got 12M little sites each giving off some amount of electricity. Notice that this doesn’t have anything whatsoever to do with colors yet… just 12M different levels of electrical current.

To get color out of that, the little photo sites each have a colored filter on top of them, either red, green or blue. (There are twice as many greens as the other two, but don’t worry about that.) They are arranged in a nice pattern (the Bayer pattern) like a checkerboard. A red filter only allows the red wavelengths thru; the blue only allows blue, and green allows green. (Why RGB? Because that’s what our eyes see, and the combination of all three make up all the other colors.)

So, the net result is that you get a current which represents the amount of red, green or blue light (depending on the filter) for any given cell.

When you click the shutter, the light exposes all 12M photosites, and the current is captured by an Analog to Digital (A/D) converter, which has the fairly simple task of assigning a number to the amount of current captured at each site. In little cameras, that number can be between (say) 0 and 1000, or in more expensive cameras it can be between 0 and 16000. (What we’re really talking about in doing that is how small a difference can be accurately recorded: do you have 1000 shades or tones or do you have 16,000 of them?)

So what you end up with then, inside the camera, is a file which is a huge table of these 12 million numbers, each one representing one photo site.

Now, one of two things happens: either that file is left alone and just sits there on the memory card, without any further interpretation. In that case, it’s in its ‘raw’ form – unchanged… or… the software built into the camera starts massaging it. It will bump up some numbers based on the other numbers that are near by, (“sharpening” the “image”); adjust the numbers based on the overall “temperature” (color temperature) and all kinds of other wonderful things you can do with a computer and numbers. (Yes: inside your camera -is- a computer.) All that is well beyond the scope of this little ditty… except this one thing: if your camera is going to give you a “jpg” file, then it also compresses that whole range of numbers (0-1000, or 0-16,000) down into 0-255.

Yes, you’re right: it throws away a huge amount of data.

And one way or the other (raw or jpg) you eventually download that list of numbers to your computer.

I’ll break there for tonight. It’s well past my bed-time, and I’ll resume some time tomorrow, and cover PPI (Pixels per inch; Lines per inch; LCD and CRT screens, and output for the web, photos and books.

Meanwhile, here’s a definition: A pixel is a picture element; the smallest “dot” of information. One of those numbers from that table of numbers we just downloaded from the camera. (Again, that’s not quite the proper definition, but it’s close enough for this discussion.)

Mañana.

Tracy