what is the difference between "resulotion" i.e 1600X1200 pixels and "PPI"?

Question

I mean, what does 300 PPI mean for a photo with a "resulotion of say 1600X1200 pixels, and how come this PPI value doesn't change when you enhance the resulotion?

Answer 1

Pixels Per Inch (PPI) or Dots Per Inch (DPI) have nothing to do with the resolution of your picture - PPI is a measurement of print quality.
If you print at 300 dpi (great quality), you'll print 300x300 dots per square inch. Every dot corresponds to a pixel in your jpg file, so the consequence is that a 1600x1200 file will become a 5.3 x 4 inch print.
If you use 200 dpi, the same file will become an 8 x 6 inch print.
To answer the second part of your question - same thing, the other way around - when you enhance the resolution of your jpg and keep the dpi constant, you'll end up with a larger print. (a 3200x2400 file at 300dpi = 10.7 x 8 inches)

Answer 2

We had a discussion (as well as you can on Yahoo! Answers) a few weeks ago and I saved my thoughts. Take what you need and leave the rest.
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Resolution - PPI - DPI

You can not control the pixels per inch in your camera, as this is a function of the sensor. I have found the following ppi for SOME cameras by the following manufacturers. I wonder if this specification is available anywhere. I have four camera manuals in front of me and I can't find it in any of them.

NIKON - 300 ppi
CANON - 180 ppi
PENTAX - 72 ppi

We "discussed" this by way of question and answers a while back and decided that the ppi file produced by the camera really did not have any effect on the capability to produce a good print. If I take a photo with my wife's 6 MP Nikon and view the image on the screen, it will start out as 300 ppi. If it's a full frame image of 3008 pixels x 2000 pixels, the SCREEN image would be pretty big if I viewed it full sized, but it would print 10.0" x 6.6". If you crop a 5x7 out of this, keeping it as full-frame as possible, it becomes 400 dpi.

With my brother's 6 MP Pentax, the image is 2816 px x 2112 px, but it's at 72 ppi. If you print this without cropping, it would be 39" x 29"!!! If you crop a 5x7 out of this, though, it becomes 402 dpi resolution, which is virtually identical for the Nikon 6 MP camera.

With my son's 6 MP Canon Powershot A620, the image is 3072 px x 2304 px, but it's at 180 ppi. If you could possibly print this without cropping, it would be 17.1" x 12.8". If you crop a 5x7 out of this, though, it becomes 439 dpi resolution, which is a bit better than either of the other 6 MP cameras.

In other words, regardless of the sensor's pixel-per-inch output, 6 MP cameras produced the same resolution when it came time to print comparable images. How about a camera with more pixels?

If I take a photo with my 10 MP Nikon D200 and view the image on the screen, it will start out as 300 ppi. If it's a full frame image of 3872 pixels x 2592 pixels, the SCREEN image would be pretty big if I viewed it full sized, but it would print 12.9" x 8.6", if I had the right paper. If you crop a 5x7 out of this, keeping it as full-frame as possible, it becomes 516 dpi, or higher resolution than the 6 MP cameras. We knew that is would work out like this, though, didn't we?

The REAL question is, what difference does this make in the real world? If we agree that 300 dpi is an excellent resolution for prints, as long as you end up with at least 300 ppi after you are done cropping on the screen, you will get an excellent print as far as resolution goes. All four cameras exceed that amount. I usually resize "down" to 300 dpi before printing, because you can't tell the difference above that resolution anyway. Most people would be quite happy with a 200 dpi print, so go ahead and crop a bit. If you get a print resolution too much below 200 dpi, you will start to see digital artifact in the print and it will be objectionable.

Answer 3

ppi is pixels per 1 inch (in a straight line).

resolution is pixels per X,Y area

ppi is a resolution per inch.