What's all this about DPI and INCHES and RESOLUTION?
Before we get all technical ...
What's this DPI stuff all about?
In an attempt to shed a small puddle of light on this business of dpi in raster/bitmap files, I've created a set of test files you can use to determine how your particular app deals with dpi.
My personal opinion is that DPI=BS, at least when we're talking about data in bitmap files.
DPI or PPI is relevant when scanning ... how many points should the scanner sample per inch of original?
DPI and LPI are relevant when printing ... how many dots will the printer resolve for every inch of image printed?
DPI can be important when you're displaying an image on a video monitor ... how many pixels/dots of space can the monitor display in the amount of space you've allowed for the image?
But while the image is on your disk, a raster image is a collection of numbers. Each number represents the color of a single pixel or dot. How many DPI are there? Who knows? We know how many dots there are, but until we know how many inches we're going to display or print those dots to, DPI is meaningless.
To belabor the point, I created a simple raster image in Photoshop. When it asked how big I wanted the new image, I told it 7 inches wide by 2 inches high at 72pi. It told me that this worked out to 504 by 144 pixels, which was just fine by me. OK.
Remember those two numbers -- 504 by 144.
How did Photoshop come up with them? 72 dots per inch times 7 inches is 504 dots or pixels. Ditto for the height ... simple multiplication.
I saved the image as an LZW compressed TIF called 72dpi.TIF
I "resized" the image to 300dpi with the Resample option turned off. When I changed the size from 72dpi to 300dpi, Photoshop declared it to be 1.68 by .48 inches. Fine. You do the math this time. I'll wait right here.
If you came up with 504 by 144 pixels, give yourself an A for Attention, which you're clearly paying.
OK, we save this one as 300dpi.TIF then play the same game again. This time let's make it 12dpi. Now Photoshop thinks the image is 42 inches wide (and I SWEAR I didn't do that deliberately even though I am a rabid Hitchhiker's Guide fan). Same deal ... 42 inches times 12dpi is how many pixels? Bingo ... 504!
Beginning to see the pattern? No matter what DPI you set (or, as it happens, what size in inches) so long as you don't let Photoshop resample the image up or down, it's STILL 504 by 144 pixels.
Let's save this one as 12dpi.TIF before we forget, then have a look at the file sizes. You'd think that a 300dpi file would be higher resolution than a 12 dpi file, and because of that a lot bigger, right?
Sorry. All three files are exactly the same size.
And if you use a file compare utility, you'll find that the data is byte-for-byte identical too. Almost. There will be 8 bytes that are different; these are where the image's "size" and "dpi" are stored.
Wait. If the size and dpi are stored but the images are the same in each case, what's going on here?
Simple. Having the size and dpi stored in the image can be a real convenience. Say you want to scan a 5x7 photo and have it import into your DTP program at 5x7 automatically. Most apps respect the size info if it's in the file, so it's handy to have it there.
The size stored in TIF and other such files is a handy lie that you, your scanning software and your other programs all agree on. But again, since the rest of the bytes in the files are the same, it's clear that the image itself hasn't changed any; when we change the dpi or size settings in Photoshop, we're just changing the white lie we're agreeing to tell one another, us and our software.
I've saved this same file as TIF, JPG and GIF, each in 12, 72 and 300 dpi versions. Try importing them all into your favorite DTP, presentation or word processing programs and watch how they're automatically sized there. You may notice that it doesn't behave quite the way you'd expect in all cases.
[Note: Later versions of PowerPoint will import images at slightly different sizes than what's described for PowerPoint 97 below - no matter. The basic principles are the same.]
In PowerPoint 97, for example, the GIFs all come in at the same size. That's because GIF files don't carry any sizing or DPI information. PowerPoint's left to figure out some reasonable size on its own. On my system, it brings them in at 4.2" scaled down to 83% or 3.5" Why 83%? I haven't a clue. But 4.2" is 504 pixels divided by 120 dpi (my video resolution), so at least that part makes sense.
The JPGs are a different story. JPG carries DPI/Size info so the 72dpi version comes in at 7" wide by 2" high as you'd expect; the 300dpi version comes in at 1.68 x .48, again as expected, but the 12dpi JPG comes in at 3.5" x 1". Hello?
That's just PowerPoint trying to help. You don't really want an image to come in at 42" wide on a 10" slide, do you? No. It seems that if the image will fall off the edges of the slide, PowerPoint makes its next best guess ... number of pixels divided by video resolution, so this one comes in at the same size as all the GIFs, 3.5" wide.
The TIFs? Evidently it doesn't understand something about the latest TIF format, because it ignores the sizing information in them and brings all three in at ... you guessed it ... 3.5" wide.
Your program may behave entirely differently. That's why I'm providing these files ... so you can see for yourself.