There are two things going on here with numbers. The important one is the total number of pixels (pixel dimensions), expressed like "4800x3600 pixels." The other number is called resolution
and it's defined as the number of pixels per inch in the final product. So an image with the pixel dimensions 4800x3600 will have a resolution of 16x12 inches at 300 pixels per inch. Note that resolution
requires BOTH the inches
and the pixels per inch
numbers, otherwise it's meaningless.
Please also note that changing the "Resolution" does not have any effect on the Pixel Dimensions. I can define the resolution of the above photos as 180 pixels per inch and get a print of 26.67x20 inches, but note that the pixel dimensions don't change. (I can also define the resolution as "1 pixel per inch" and make a 4800-inch long print, but it might not look so good...)
So when Jeff talks about resolution dropping below 180 pixels per inch, what that means is the your final print dimensions in inches will be large enough that the pixels per inch are <180. For example, trying to make a 36x48 inch print from our file above will result in a resolution of 100 pixels per inch from the original file. In that case, what you want to do is increase the pixel dimensions
enough so that you have a resolution
of 180 pixels per inch.
Is this clear as mud yet?