Scenario:

2 identical jpeg images (same physical dimensions, same content) with
one exception.  One has 200 PPI, the other 2000 PPI.  Both look
identical on my monitor in terms of quality (I know why this is the
case).  Both files take up the same amount of space on my hardddrive. 
Why wouldn't the image with the substantially larger PPI take up more
space on the harddrive?

On a related note, when those same images are printed at the same DPI
settings on my printer they look identical.  Shouldn't the image with
the larger PPI produce a better print, all other factors being equal?

And finally, if increasing the PPI of an image has no effect on either
its viewed quality or its printed quality, is PPI even relevant?  If
so, can you provide me with examples when it is relevant?

Hi Krutec:

Thank you for your question. The answer to the first of your questions
relies on the fact that Pixels Per Inch in graphics files is just a
simple value written into the file, and changing the PPI merely
changes this number, and nothing else. This is the reason why the two
files take up the same amount of space on your hard drive. It's also
because of this that the two are identical in quality on your monitor.
On the other hand, increasing the *size* of the image (say, from 100 x
200 to 200 x 400) would result in a higher quality image (that is,
bigger) on your monitor, and would result in a larger file size. But
merely changing the PPI will not affect the display on the image on
your computer monitor.

To answer your question about the DPI on your printer and the print
quality of the two images requires a better understanding of exactly
how PPI is used. This should also answer your question about if and
when PPI is relevant. PPI is used when scanning documents as well as
when printing documents to paper. When we set the PPI on a scanner, it
tells the computer how many pixels it should scan per inch of area.
Doubling the PPI would quadruple the size of the image, and also
quadruple the file size of the scanned image. So, the same piece of
paper scanned with 200 PPI would appear to be four times larger
compared to an image scanned with using 100 PPI, due the higher
resolution of the scanned image.

Now, when printing, the printer uses the PPI value in the jpeg file
along with the size of the image to find out how big the image should
be on the paper. For example, a 100 x 500 size image at 100 PPI tells
the printer that the image should be 1 inch by 5 inches. When you
printed those two images on your computer, what *should* have happened
is that the one with 2000 PPI be printed 100 times smaller than than
the one with 200 PPI. If the image you used had a size of 2000 by
1000, then the 200 PPI image should have printed out to 10 inches by 5
inches, while the 2000 PPI image should have printed out to 1 inch by
0.5 inches. I'm not sure why the two images looked identical when
printed out, because I'm not sure what program you are using to print.
Different programs may not take into account PPI when printing.
However, most major photo editing programs such as PaintShopPro and
Adobe Photoshopshould have printed the two images as I described here.

Here's something I took from my Paint Shop Pro 7.0 Help file:

"Image Resolution

Laser and ink-jet printers work by applying dots of black or colored
ink to the paper. While the image resolution is measured in pixels per
inch (ppi), the printer resolution is measured in dots per inch (dpi).
To determine the printed image size in inches, the printer divides the
image size in pixels by the pixels per inch. An image with a high
resolution appears smaller when printed at its original size than an
image with a low resolution. For example, if the size of your image is
600 x 300 pixels, it is 12 inches by 6 inches at a resolution of 50
ppi (600/50 by 300/50) and 6 inches by 3 inches at 100 ppi (600/100 by
300/100).

You can change the size of the printed image by changing the Scale
value in the Page Setup dialog box. However, this can lower the
quality of the print, as it enlarges each pixel. You will get better
results if you create the image using a resolution that is appropriate
for the printer.

Use the following guidelines to determine the image size for printing:

·	For a printer that prints 300 dpi use an image set at 72 to 120 ppi.

·	For a printer that prints 600 dpi, use an image set at 125 to 170
ppi.

·	For a printer that prints 1200 dpi, use an image set at 150 to 200
ppi."

I hope this answers your questions!

Google Search Strategy:

"pixels per inch" jpeg
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=%22pixels+per+inch%22+jpeg&btnG=Google+Search

"pixels per inch" jpeg file format
://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&safe=off&q=%22pixels+per+inch%22+jpeg+file+format&btnG=Google+Search

"pixels per inch" jpeg file format specification
://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&safe=off&q=%22pixels+per+inch%22+jpeg+file+format+specification&btnG=Google+Search

If any of the above was unclear, please don't hesitate to ask for a
clarification. I would be more than happy to clarify my answer.

Best Regards,
blader-ga

---

Clarification of Answer by blader-ga on 07 Jan 2003 14:14 PST

Here's some additional links that you may find very helpful to clear
up any questions you still have:

Choosing Image Quality and Resolution
http://www.nikon-euro.com/nikoneuro_en/hit/general/en/HIT_gen_en_9.htm

Changing Image Size in Photoshop
http://de.uoregon.edu/help/changePS.html

Image size and Resolution
http://www.espressographics.com/text/imagesize4.html

About Image Size and Resolution
http://adobedoc.kanisasolution.com/Photoshop6/Help.htm

Best Regards,
blader-ga

if the two files under disscussion print out the same and have the
same file size on the hard disc then the image size of the 2000 ppi
file is 10 times that of the 200 ppi file. ie image size 10" X 10" @
2000ppi will be the same as 1" X 1" @ 200 ppi.

That is incorrect. The file size of a 200 PPI file at 10'' X 10'' will
be the same as 1'' X 1'' file at 2000 PPI. The image size of the 200
PPI will be 10 times that of the 2000 PPI image, once printed out.