Resizing photos?

I took some digital photos with the camera's (Panasonic DMC FZ30) settings on 5 Mpixel, Jpeg high quality. When I downloaded the photos and open one in Photoshop, I noticed that they where 2560 x 1920 pixels at 72 dpi (I did not expect the camera to take pics at 72 dpi while set to high quality jpeg!?) Anyway now I need to reduce the print size and increase the resolution to get decent prints? or at least that's what I think. Been unfamiliar with imperial mesurments (I'm used to metrics only) I'm always confused about the relationship between pixels, dpi, screen size in pixels, print size, etc. Here's what I done with my photos (I'm not sure if it was correct or necesairy)
In 'Image size' with 'Constrain proportions', 'Resample image' ticked, I changed the width in the 'Document size' to 17.5cm and then increased the 'Resolution' until the 'Pixel dimentions' was back to app the same value (13.9M) as before I changed the 'Document size' The resolution was then 380 dpi. When I got the prints, the quality was very good.
Can somebody explain the relation between resolution and print size?
Also when I set the view to 'Print size' and I activate the ruler, the dimention indicated by the ruler on screen does not correspond to the dimention as mesured with a physical ruler that I hold against the screen??

Thanks
Cheers

What your looking for is to simply change the resolution of your image. If you want high quality prints, you'll want atleast 300dpi, you have 380 so thats why it looks good. When your changing the resolution, you dont need to check "resample" unless you want to physically alter your photos, either by enlarging or down sizing.

-----------------------
Extremely Long Answer
-----------------------

Well lets start from the beginning here.

The quality of a photograph (independent of print & screen) is determined by how many pixels you have available to render your image. With a 1MP camera, you only have 1 million pixels available. Now if you took the same exact photo with a 5MP camera, you'd have 4 million extra pixels to get all the details the 1MP camera missed. The more pixels you have, the better the quality. Thats why your images are so huge ie. 2560x1920. So like 640x480 is considered low quality or sub-megapixel.

Now resolution (known as ppi, or pixels per inch) is how many pixels are displayed in an inch. Most devices, monitors/cameras etc. display images at resolution of 72 (or 96) ppi, regardless of the quality of the image being displayed (which is determined by physical size). I don't actually know where the number 72 came from, it's just a comfortable resolution to use. Imagine if your 5MP image was displayed on a 17inch monitor at a resolution of 1ppi. You would only be albe to view about 256 pixels at a time. You would have to scroll for hours, you wouldnt be able to see your full image, and this text im writting would be HUGE!! Now imagine a 600ppi monitor, you wouldn't be able to read this or any pixel font because it would be extremely small! Screen resolution (72ppi) is simply how an image is displayed for your viewing pleasure, it has absolutely no effect on the image itself.

Now where resolution and quality actually become useful is in print. Most of the time resolution (ppi) is referred to as dpi, which is wrong! Dpi stands for dots per inch (or drops of ink) and is completely independent of the computer. Only your printer has control of that, you don't. What resolution or ppi is actually used for is scaling your images for print. Resolution doesnt usually change your images (unless you resample) it just tells the printer how many of the pixels in your image will be rendered by the number of ink drops your printer uses on 1 inch of paper. So as a quick example, you could print out a 72ppi image @ 600dpi, or a 300ppi image @ 600dpi. Again 72ppi & 300ppi are the same exact images! Its just the printer used 600 drops of ink to represent 72 pixels from that image, or it uses the same ink to represent 300 pixels from that image.

Now 72ppi is screen resolution, which is considered to be low quality when printed on paper. 300ppi is the standard for high quality prints. Print quality also comes from how many drops of ink are used to render those pixels, but again that's usually out of your control. The quality of the photos (remember determined by physical size) helps in the printing process because the more pixels you have, the bigger the prints can be and the more pixels you can "pack" into an inch for a higher resolution. An 8.5x11in photo @ 72ppi, is just that, a low quality 8.5x11. Now if you wanted to print that image at high quality, you tell the printer to "pack" more pixels into an inch (same image, nothing changed), you'll end up with a high quality image, but only at 2x2 inches. With a 5MP image @ 72ppi, you have a 36x27inch low quality print. If you wanted high quality, you again "pack" more pixels into an inch and get a high quality 8x6inch print of the same exact photo. As an example of how print size/resolution is related...take a 100x100pixel image at 100ppi. If you "pack" 100 pixels into every inch of paper, you only have a 1x1inch photo. If you only pack only 10 pixels into every inch, you'll have a 10x10inch photo.

Hopefully you see now how resolution is independent of the actual quality of a photograph. It's nothing more than a scaling factor for viewing the image, whether on screen at 72ppi, or in print at 300ppi. Also i hope you see how DPI actually has little to do with any of this. Most people interchange the two terms (incorrectly) and it just causes a massive amout of confusion.

------------------------------
I hope some of the above makes sense, im tired and it's a bit hard to explain. Sorry if it's too long/simplistic. Hopefully everyone who comes across this thread will find it somewhat useful....that is if it makes sense....it's a bit complicated to explain though easy once you understand it lol

Ok Rodder454, I think I understand Its the number of pixels for a given photograph that determins the quality of the print. The number of pixels a camera can take depends on how many sensors they can pack in a CCD. My camera has a 8MP CCD so It can pack 8 millions pixels or 3330 x 2430. My monitor displays it at 72 ppi so the physical size on my screen would be 3330/72 = 46.25 in x 2430/72 = 33.75 in, if I print it at 300 ppi the physical size of my print would be 3330/300 = 11.1 in x 2430/300 = 8.1 in.
I'm thinking that I probably didn't have to modify the resolution of my photos as the photo lab would have 'packed' all the pixels into a given size (eg 11 x 8 in) giving me app 300 ppi resolution prints. M'i correct in my reasoning?

Once again thank Rodder454 your clearing this up for me.

Cheers

rodder454, i thank you. i never really understood why so many feel threatened, or constrained, when it comes time for print. any experienced Photoshop user can tell you that a high resolution photo is probably the single most important aspect in the editing process. though your post was extremely long (for a post), i read it in full and will use it whenever i'm in my closing stages of work: print.

and a kudos goes to you and your remarkable post. regularly, most people write and type like a 10-year old, but this was like a passage from a published, well-recognized magazine.

i figured someone must've had this problem...but i was lazy. it's rather embarrassing to find that it was, basically, the exact same problem word-for-word. sony > panasonic. w00t.

thanks...
i'm still confused on some parts. but........i get the important ones.

well which parts?