Can someone please explain dpi/ppi to me?

[bridezillah]

Ok Im not an idiot. I thought I knew how it works but I am totally lost. I have some pics from my wedding in digital format. The pics are 9M JPEG files, 72 ppi, 28.44 inches wide, 21.33 inches tall. If I do the math the way it makes sense to me then this original image is 2047.68 x 1535.76 pixels. Am I figuring this right?


I plan to crop them and upload to printroom.com to give it a shot instead of printing myself. (OR should I put on a cd and take to one of those computers at Target?)I want to do all 3.5x5 inch prints and on printroom they say the minimum resolution is 800x600 for a 5x3.5 inch print (for excellent quality) Does this mean the ppi would be 171.43x160 respectively or am I figuring this all wrong.

I want to make sure I am figuring this correctly before I send it off to be printed. By doing the math this way, it looks to me like I should change my settings on the print size to 3.5x5 inches, and 171 + ppi. Is this right??
THANKS!

[Photo-John]

bridezillah-
I can't really say whether you're an idiot or not. I'll take your word for it that you're not.

Resolution can be confusing. Especially when it's for printing. The basic rule is, 300 dpi, at print size, is the correct resolution for printing. That's the professional standard. But, it's not that critical. Printroom gives you minimal resolution recommendations to cover their butt, should you be unhappy with your prints. You can safely ignore their recommendations as long as you have a file between 200-300 dpi, at print size. I use Printroom all the time and I generally give them files at about 250 dpi, at the final print size. That means, a 3.5 x 5 inch print will be 1250 x 875 pixels. You have more than enough resolution to do that. And if you need to go larger, don't be afraid to use Photoshop to "interpolate" (resize) your image. I just wouldn't recommend going larger than double the original pixel dimensions.

Unless your image is already sized to the correct final dimensions, the resolution Photoshop (or other software) lists doesn't mean much. That's why Printroom recommends 800 x 600 pixels, rather than listing a dpi. They're telling you the total pixel dimensions, because that's the number that really counts. The best way to check your resolution is to open your file in Photoshop, right-click on the blue bar at the top of the image, and check the "Document Size" and "Resolution" numbers. If the Document Size is the size you want to print, and the resolution is between 200 and 300 pixels/inch, then you're good.

I hope that wasn't too confusing. If it was, or if you just have more questions, let me know and I'll do my best to help you sort it out.

[bridezillah]

Your explaination makes sense. all the numbers mean what I thought they did but I wanted to make sure. I was getting so confused! Thanks for your help!!

[The Ever Optimist]

bridezillah-
I can't really say whether you're an idiot or not. I'll take your word for it that you're not.

Resolution can be confusing. Especially when it's for printing. The basic rule is, 300 dpi, at print size, is the correct resolution for printing. That's the professional standard. But, it's not that critical. Printroom gives you minimal resolution recommendations to cover their butt, should you be unhappy with your prints. You can safely ignore their recommendations as long as you have a file between 200-300 dpi, at print size. I use Printroom all the time and I generally give them files at about 250 dpi, at the final print size. That means, a 3.5 x 5 inch print will be 1250 x 875 pixels. You have more than enough resolution to do that. And if you need to go larger, don't be afraid to use Photoshop to "interpolate" (resize) your image. I just wouldn't recommend going larger than double the original pixel dimensions.

Unless your image is already sized to the correct final dimensions, the resolution Photoshop (or other software) lists doesn't mean much. That's why Printroom recommends 800 x 600 pixels, rather than listing a dpi. They're telling you the total pixel dimensions, because that's the number that really counts. The best way to check your resolution is to open your file in Photoshop, right-click on the blue bar at the top of the image, and check the "Document Size" and "Resolution" numbers. If the Document Size is the size you want to print, and the resolution is between 200 and 300 pixels/inch, then you're good.

I hope that wasn't too confusing. If it was, or if you just have more questions, let me know and I'll do my best to help you sort it out.

As per the calculation of 250 dpi required for a quality photograph, an 8X10 picture would require 2000X2500 pixels image which converts into a 5 megapixel CCD..... BUT i have seen snaps from 3 MP camera which are very gd.... so does this mean that normal photos are printed at less than 250 dpi also? 3 MP btw converts into 150 dpi
Also ...where does the difference between a .tiff and a JPEG come in? If i have 2 pictures at 2000X1700 pixels, one tiff and one jpeg, will the quality of the picture be the same?
Thanks a million... im all set to take digital prints for the first time so i really want to figure all this out!
Kunal