The old rule of thumb about DPI used to be higher DPI Better print output. 

Now if you buy a Digital Camera, the output is usualy at 2048 x 2048 @ 72 DPI.  So if going buy the old rule of thumb that High DPI is better, that would mean the digital photos shold come out crappy but they dont when printed.

Also a lot of artists here work on Resolutions of 5000 to 10000 Pixels for high printer output.

This old timer needs to be updated.  It took me a long time to drill into my head, that Screen Resolution and DPI Resolution were two sepreate things.

What is throwing me off is new Digital Cameras that produce 72 DPI pictures but at higer screen resolutions.

Jpgs aren't really a pixel format. They are more like the signals your analog TV gets for it's red green and blue guns. You can stretch jpgs up to 120% and they will create new pixels. There is also software built in to antialias when you do. Better yet you can compress them.  If you take your 2048 square image at 72 dots per inch and squeeze it down to fit an 800dpi page you have actually bumped the resolution up to 187 dpi if you squeeze it down to the normal page width of 600 dpi the resolution goes up to 246 dpi. If you printed your 2048 image at native 72dpi resolution you would need a piece of paper 28.444... inches square. Since you are printing the image on a regular piece of paper you are automatically shrinking it and increasing the dpi in the process. If you display the image on your monitor set at 1280 x 1024 and it fits top to bottom the resolution is 144dpi not 72.

Here are a couple of resource links covering Resolution, DPI, Etc.

http://en.wikipedia.org/wiki/Image_resolution

http://www.fotofinish.com/resources/centers/photo/resolution.htm

Pixels control actual available image detail, wether they're stored with a mathematical algorythm that eventually converts to pixels, or stored on a per-pixel basis. DPI applies to the detail with which your printer prints. 
You could print 4 pixels at 600dpi on a sheet of A4 paper, and it'll simply print a representation of 4 huge pixels using 600 drops of ink per inch to do so, but it wont interpolate the 4 pixels or anything. The result will just be 4 big squares, each representing a pixel.

Shrinking an image with many pixels to a small piece of paper doesnt increase the DPI, it just changes the amount of dots used to paint one pixel onto the paper (=dots per pixel). The DPI is a quality setting of your printer. 
More dots = better print quality = the ability to 'paint' more pixels onto paper = the reason why most professional print artists render many pixels.

Generally, you could state that the theoretic ultimate level of detail you could render and print would be to have a pixel for each microscopic drop of ink. The more DPI, the more pixels you can theoretically represent accurately. More pixels then dots, is useless.

Your printer uses the DPI  associated with an image file info to detrmine the final printed image size. This can be overridden in most printers to increase or decrease the print size in the print settings dialog. 
For use with a 4 colour dot-on-dot halftone process 300 dpi is about where you have a break-even with most screens in terms of dot size and the colour gain experienced by the paper stock.
You can go lower than this (down to 100 dpi typically) on an inkjet if you can take advantage of error diffusion halftoning. This technique employs a slightly random distribution of the print dots as opposed to the rigid grid set by standard halftoning. This allows for a bit of optimisation giving superior result at lower dpi.
At least that's the way I understand it.

It's information overload: you either think in pixels (pixel elements, helllllooooo?!) or in some real dimensions like cm or inches AND the density of the dots within such distance (dpi, dots per inch, but in some countries also dots per centimeter,...)

The reality is that in the end it's all about the dots. You can only see the dots that are there. So it's all about pixels really. cuz you paint pixels, Once you have so many you can slap a fictitions dpi value on it but it won';t mean a thing until some software or driver or printer decides to use it along with that other parameter: the real work dimesion.

real-worl-dims x dpi = total dots, aka pixels

Seriously, do you think there's anything different in a file, I mean in the file, just because you change from 72dpi or 100 dpi to 300 dpi? I mean just changing that piece of header info. Nope it doesn't touch the pixels. You still have 8000 of them if you started with 8000 of them. But what you're now saying is that you'd like 300 of them packed into each inch, not just 72. So, the image will span across a shorter distance cuz you'll have exhausted the available 8000 pixels (or dots) in a shorter distance when you pack 300 per inch vs. when you pack only 100 or 72 of them per each and every inch.

 Well, that's my thinking. I never work in two dimensions when a single dimension does the exact same thing: I mean why worry about dpi AND inches when in the end you get the resulting number of dots in the form of pixels. It's all about pixels.

You can load a 3000 pixel wide image into Irfanview and change the DPI setting from 100 to 300 dpi and save it. That won't prevent you however from changing it back to the original, or printing it at different dimensions, smaller or larger in dimensions on paper. Yu can always have it resample on-the-fly to match the desired dimensions on paper.

Well, theoretically anyway. Unfortunately some software thinks that you really care to drop 2/3 of the pxiels when changing the dpi from 300 down to 72.

 Internally, it's still all about pixels though.

oh well, but that's just me.

Quote - The old rule of thumb about DPI used to be higher DPI Better print output. 

Now if you buy a Digital Camera, the output is usualy at 2048 x 2048 @ 72 DPI.  So if going buy the old rule of thumb that High DPI is better, that would mean the digital photos shold come out crappy but they dont when printed.

Also a lot of artists here work on Resolutions of 5000 to 10000 Pixels for high printer output.

This old timer needs to be updated.  It took me a long time to drill into my head, that Screen Resolution and DPI Resolution were two sepreate things.

What is throwing me off is new Digital Cameras that produce 72 DPI pictures but at higer screen resolutions.

very good point, that the dpi setting is just a variable stored in the file header. Dpi only really takes on a serious meaning when you are printing (especially standard dot-on-dot commercial presswork).

Incarnadine is right (I fix printers for a living, so I should know..;) There's all sorts of 'tricks' to printing; sometimes they even make different sized dots, just to make life interesting..;)  i've seen up to 1200dpi (in the price point I work at; we don't have commercial art printers...well, not many, anyways..;), but 300-600 is about the norm for most laserjets/inkjets you would own.
 

Here is one of the responds I got on my website/forums.

"Having worked in newspaper layout....I disagree. You really do need a higher dpi when printing something for it to look decent, even if it is a digital photo. I think it's gotta be the camera settings and/or software. Check your camera manual again. My Nikon is only 6 MP now that I think of it but I get 300 dpi pics because I have the pic quality set to the highest."

I have a Canon Rebel XT and evn at 8MP quality set to Medium I still get 72DPI Photos just at higher screen resolutions like 2500x1450 etc..

I just to think I had this DPI and screen resolution thing figured out, but I havent.

There does not seem to be a set rule of thumb.

If I were to take a 72 dpi file to my printer; he'd boot me out the door, after he finished laughing...
300 dpi minimum, and CMYK color format.
That's inches x 300 = picture's pixel size.
So when I made my 36" x 60" posters, I had to render at 10800 x 18000 pixels.

When I print at home I use RGB and could care less about the dpi.

Just throw money at it.

:-)  just kidding btw. Well kinda...

72 DPI is way low for printing professionally. Try getting a better camera maybe. Maybe that's where one should throw money at.... hrm.

It all depend how you are printing.  For use with a high resolution inkjet  (2880x1440 ink droplets per inch) via Corel's photopaint, I can achive profesional quality below 300 dpi. For commercial (4 colour offset via photolithographic plates) 300dpi is a good compromise between the lines per inch of the screens used to create the four halftoned images necessary to create the dot-on-dot colour. Also when laying out a commercial job, the image size is determined by the area (usually in inches or cm) of the image within the layout program.  300 dpi has enough dots so that the set of the evrlaid dots does not blur the image unaccceptably.

I used to do graphic arts, darkroom and presswork for a commerical job printing business through highschool, university and through the following year while I hunted for an engineering position.

Bookmarking this thread.  Thank you, Ana-X for asking.  I still can't get my mind around it but this should help me.

What makes for a good DPI also depends on viewing distance. A glossy magazine might have up to 600DPI, but a poster you're going to view from 10ft away - then 72DPI is probably more than enough.

Also is your printing program keeping the DPI, or is it scaling stuff to fit on the page? (In which case it's just using overall resolution and ignoring scaling that affects the DPI.) If you're using camera software it could be shuffling around the actual DPI using EXIF data embedded in the .jpg even. (Some cameras have a generic web-ready 72DPI in the regular .jpg data fields, but add more proprietary info than what's normally showed in properties.)

For cameras, DPI on output files means diddly. You're going to want overall resolution. (Not much point if a camera is 640x480, but saves at 300DPI that's a postage stamp.) After resolution (or perhaps before it) the important factors are color trueness and noise.