Oh boy.. Luminous landscape isn't that clear.. They don't really explain the "72 DPI image for monitors" thing...
I've got time enough to rant tonight.. (Please excuse any typos or grammatical errors).. Note where I say DPI (dots per inch), I also mean PPI (pixels per inch) I'm using the terms interchangeably.
The DPI setting in a digital bitmap image file (like we get from our cameras), is NOTHING more than the the number of pixels in the image divided by the proposed print size. Note the term PRINT. The inches in DPI refer to inches of PAPER.. When you set the DPI of an image, ALL you are changing is the value of a few bytes in the file header.
These few bytes placed in the header are read by your printing software.. These few bytes tell your printer how big (in inches) to print the image. What you AREN'T doing when you change DPI only, is changing the arrangement or quality or number of the pixels in your file.. No matter what the DPI setting is, your pixels always remain at the same "quality"
Now the proof 
Work along with me.. Grab a 3072 by 2048 image and load it into Photoshop, or the Elements program that came with your camera. Go through each step to get a feel for what I'm saying and to confirm what I say is true..
If you load your 3072 pixel by 2048 pixel image by 180 DPI into PS or Elements, and bring up the Image Size dialog box, you'll see that the IMAGE size is 3072 x 2048, and the DOCUMENT size is 17.067 by 11.378 inches @ 180 pixels per inch.
Note they put the pixel dimensions and document dimensions in seperate boxes. They took the time to outline the two settings so they would be seperate. This was done for a reason.
The IMAGE is what you have on your hard drive. It's simply a number of pixels.
The DOCUMENT is what will appear on paper when you print it. Inches and DPI go in the DOCUMENT settings box not the IMAGE setting box.. The DOCUMENT is what you'll be printing. Inches and DPI are printing terms.
Anyway...
If you pull out a calculator, you will see that:
3072 pixels divided by 180 DPI = 17.06 inches..
2048 pixels divided by 180 DPI = 11.377 inches..
This is exactly what PS or Elements is showing you.. (rounded slightly).. I'll say again.. DPI is NOTHING more than the pixels in the file divided by the PROPOSED print size..
Try this.. Make sure the resample image box is unchecked.. (You'll see why later).
Change the width (in inches) to 6 and change the height to 4 inches.
Look what happened to the DPI.. PS or Elements is showing you it's now 512.. Lets do the math:
3072 pixels divided by 512 DPI = 6 inches
2040 pixels divided by 512 DPI = 4 inches
It works again.. The DPI of a file is nothing more than the proposed print size.. It's only the pixels in the file divided by the desired size in inches. Now.. Play around.. change the values in the pixels and print size boxes to anything you want. You'll see that the DPI is ALWAYS the number of pixels divided by the proposed print size. It will never be anything else because it can't be anything else.
OK.. Regarding unchecking the resample image box.. (This box is probably WHY the DPI concept is SO confusing).
If you change the DPI of your 3072 pixel image to 72 DPI, you'll see it becomes 42.667 inches by 28.444 inches.. (Lets check the math again)
3072 divided by 42.667 inches = 72 DPI
2048 divided by 28.444 inches = 72 DPI
Suppose you want your 3072 pixel wide image to be BOTH 72 DPI **and** 6 inches wide..
Sorry.. You can't. This is mathematically impossible.. You can't spread 3072 pixels evenly across 4 inches and wind up with only 72 pixels in each of the 4 inches..
I showed above that a 3072 pixel wide image printed at 6 inches has to be 512 DPI... A 3072 pixel wide image set to 72 DPI has to be 42.667 inches wide.
So HOW do we get an image that's 6 inches wide at 72 DPI ?
Lets do the math:
72 DPI times 6 inches = 432 pixels. Or in other words, if you spread 432 pixels evenly across 6 inches, you'll have 72 DPI.. That's the only way you can do it.
So HOW do we get our 3072 pixel image down to 432 pixels ?
Easy.. Check the resample box.. (It's there for this very reason). Now put in a width of 6 inches and a height of 4 inches. Enter 72 DPI in the resolution box.. Look at the number of pixels in the image.. Presto your image has been resampled to 432 x 288 pixels.. (You just threw one heck of a lot of pixels away, but now it will print at 6" x 4" @ 72 DPI
Lets double check the math:
432 pixels divided by 72 DPI = 6 inches
288 pixels divided by 72 DPI = 4 inches.
HEY.. Look at that.. We did all that complicated stuff yet the proposed print size is still nothing more than the image size in pixels divided by the DPI
To sum up:
DPI is a printing term.. It's your PROPOSED print resolution not your current image resolution. The value is there for your printer software to read WHEN you print it. It has nothing to do with the number of, arrangement or quality of pixels in your file.
When you check off the resample box, changing the DPI and clicking OK does TWO things..
1) You are changing the DPI --and--
2) You're changing the number of pixels in the image.
This is where a *lot* of confusion comes from.. Changing the DPI and resampling are completely seperate operations.. Adobe just bundled these things together for convenience.
But because of this, people believe that they get 3072 pixel images fitting nicely in their 1024 pixel wide screen by setting the DPI to the "monitor resolution" of 72..
NO NO NO.. [B]It fits because you resampled it..[B] Look at the example we tried above.. Our 3072 pixel wide image became 432 pixels wide after changing it to 72 DPI AND resampling it.. A 432 pixel image WILL fit nicely in a 1024 pixel wide screen.. But note... without resampling, the pixel count would have remained the same.
It was the resampling that made the image fit.. Not the changing of the DPI.. If you uncheck resample, and only change the DPI, then the image will stay the same size and won't fit in your screen. As a matter of fact, with resample turned off, you can set the DPI to anything you want and the image will remain exactly the same on the screen. There will be no visible change. All you are changing in this case is the print size.
Read here for a more detailed explanation of the 72 DPI screen thing.. with actual examples:
http://www.scantips.com/no72dpi.html
Changing DPI by itself (without resampling) does nothing to the pixels in the image.
When you resample an image you ARE changing the number pixels... But note..It's the resampling that changes the pixels, not changing the DPI.
Canon adds a value of 180 DPI by default.. They could have made it 300 DPI or 621 DPI or 247 DPI, or 229 DPI or whatever they felt like adding.. It doesn't matter because they aren't resampling the image when they add it..
You really don't have to worry about DPI until you actually PRINT your image on paper. Then by all means change it to what you need. Until then, the DPI is useless since all it refers to is printing.
DPI is nothing more than the number of pixels in a file divide by the proposed print size in inches.
I know I've repeated myself a lot.. But I've never seen such a simple concept so misunderstood
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For you techno types, here is the structure of a JPEG header (TIFF is similar) Note that the Xdensity and Ydensity are represented by only two bytes each.. THIS is all you change when you change DPI without resampling. You don't effect the pixels in the image..
typedef struct _JFIFHeader
{
BYTE SOI[2]; /* 00h Start of Image Marker */
BYTE APP0[2]; /* 02h Application Use Marker */
BYTE Length[2]; /* 04h Length of APP0 Field */
BYTE Identifier[5]; /* 06h "JFIF" (zero terminated) Id String */
BYTE Version[2]; /* 07h JFIF Format Revision */
BYTE Units; /* 09h Units used for Resolution */
BYTE Xdensity[2]; /* 0Ah Horizontal Resolution */
BYTE Ydensity[2]; /* 0Ch Vertical Resolution */
BYTE XThumbnail; /* 0Eh Horizontal Pixel Count */
BYTE YThumbnail; /* 0Fh Vertical Pixel Count */
} JFIFHEAD;
Here's more reading if you're interested:
www.w3.org/Graphics/JPEG/jfif3.pdf
