abc1234 wrote:
Gene, what is not correct in what I posted?
"You probably will not see a difference between 240 and 300 at normal viewing distance. If you are emailing photos for viewing on a monitor or mobile device, you can probably go down to 150, 96 or 72 dpi and people will still be happy. The higher resolutions are for printing. Even though printers may boast 1440 dpi, the eye sees only about 300. And then export at the final aspect ratio without resizing and 300 dpi."
Not only will you not see a difference between 240 and 300 dpi, you won't see it between 12 and 600 dpi - as the two images I posted illustrate. The images are visually identical, and have exactly the same file size. You can download the 12 dpi image and be just as happy with it as you would if you downloaded the 600 dpi one.
The principal determinant of image quality and file size is the dimensions, in pixels, of the image, and the quality setting for the jpeg on export - not the "dpi" setting.
I print 40x60 images at about 40 ppi, which is not the same as 40 dpi - and no one complains about lack of clarity and sharpness. I will print the same image at 300 ppi when I do smaller prints, like 5x7 or 4x6. Viewing a print up close requires higher resolution, or ppi, because your eye can see the fine detail at 18" better than it can at 6', the average viewing distance for a 40x60. Required print resolution is always based on viewing distance. Billboards will look super sharp though they may be printed at 25 ppi. Up close, the dots that create the image will be the size of golf balls.
If I wanted to resize my image for email for instance, I would resample the image to a lower resolution. By this I mean to reduce the number of pixels in the length and height. Let's say I take the image I posted, which is 7317x4883px, and downsampled it for email to 1024x683. it would look like the images below. Note that I used 5 dpi to export the first one, and 2880 dpi to export the second. Do you see any difference at all between them? Both files are 170kb, btw.
If you compare the 1024x683px image to the 7317x4883px you will see a big difference - the file is bigger, at 5 mb I believe, and will display much bigger on the screen, as you would expect a .7mp image would compare to a 35.7 mp image, regardless of dpi.
So, other than your first sentence, regarding the difference between 240 and 300 dpi, which while technically is true, it's because there is no difference in image quality when setting dpi when exporting in Lightroom, not because, as you are hinting, there is a difference in resolution and it is imperceptible between 240 and 300.
I am not sure if this will clarify things or increase the confusion:
http://99designs.com/blog/tips/ppi-vs-dpi-whats-the-difference/But you are not alone in thinking that dpi and ppi mean the same thing.