Help understanding difference between MPs and DPI

[lgoldfarb]

I shoot with a Nikon D40x and organize and process my photos using iPhoto '08 (I have Lightroom, but haven't spent much time with it yet). I noticed that when I edit images using iPhoto, the changes are not made to the original file, but to a jpeg that iPhoto creates from the original. While the resolution remains the same, the copy is always 72 dpi. Again, the total pixel count is the same (3800 x 2600). So, does it matter what the jpeg dpi is? Does it have any relevance at all?

 

[astrostu]

Short answer: No.

Longer answer: It only matters when (if) you want to print it, in which case you would want to change the ppi (pixels per inch ... dpi (dots per inch) is only meaningful when printing). In general, you would want to change the ppi to greater than 150 if you want to print it.

However, I would assume that if you sent off the photos to be printed at any photo place worth its salt, they would effectively scale it to the proper dimensions and increase the ppi so that it's still printing the same number of pixels. But, I don't know this for a fact since I send stuff off at the desired size and ppi so's to eliminate that being a potential problem.

 

[glaston]

Sounds like Apples making iPhoto more like Aperture using their unique versions control.
It's great once you get used to it.
There's most likely a place to edit the setting that dictates the jpeg file that iPhoto creates.
You can probably set it to be a different file format also.
I can't see Apple locking people into using jpegs with iPhoto.

Anyway, 72dpi is the standard for a computer monitor. As astrostu said, it's all about printing. So don't send someone a 72dpi jpeg if you know they're gonna print it.
Use a tiff file with an embedded print profile using 300dpi or greater if you can.