Greetings Google Researchers:   Please help me understand, in simple
terms, digital image sizing.   I'm using a MAC G4, OS 10.2, Photoshop
7.   What is the relationship of pixels to inches?   How can I be
certain that when I import an image, via scanner, for example, that I
can resize the image so that when saved, and then attached to an
email, the recipient will receive the image in the same size?

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Request for Question Clarification by missy-ga on 12 Nov 2002 10:24 PST

Hi Seattle!

When you say "receive the image in the same size", do you want the
recipient to be able to print the image out in the same size as the
original scan? (For expample, you scan an 8x10 photo, send it to your
Mom, and she prints out an 8x10 copy?)  Or are you referring to the
size of the sent image?

--Missy

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Clarification of Question by seattle-ga on 12 Nov 2002 12:53 PST

Hello Missy!   Perhaps the best way to clarify is to give an example
of what I'm trying to do.  Currently, I am into a little project of
taking old 35mm slides and digitizing them.  For this I am using an
Epson 1650 scanner, with transparency adapter, and Photoshop 7.  There
is some software in between the scanner and Photoshop 7 referred to as
TWAIN.  I have no trouble getting the photo into Photoshop, but then
the confusion begins.   It seems that some menu commands for changing
image size refer to pixels, others to  inches.    Suppose that I go in
and enter a document size of 4 inches by 6 inches.  Then when I look
at the resized image on the screen, it may be close to 4 by 6, but not
exactly that, if measured with a ruler.   I think that I could do
better if I more clearly understood the relationship between
resolution, pixels, and inches.   Hope this helps, if not, let me know
and I'll try again.   Thank you!

Hello Seattle!

I couldn't help but giggle at your referring to your project as
"little".  That's an understatement if ever I saw one! :)

I maintain several web pages and was completely confounded by image
editing when I first started out.  Thankfully enough, there are plenty
of resources available to help out until experience sets in and takes
over.  It's very frustrating to try to resize when pixels and inches
are used interchangebly - nobody ever bothers to clearly explain how
pixels and inches are related unless you want to dig for definitions.

"Pixels" is what refers to how your images display on screen. 
"Inches" refers to image re-sizng for print.  Though they *can* be
related, they are two different settings, and PhotoShop's steep
learning curve doesn't help much.

Let's start with a couple of definitions, to differentiate between
your digital images and those you print out:

"PPI (pixels per inch) is the number of pixels displayed in an image.
A digital image is composed of samples that your screen displays in
pixels. The PPI is the display resolution not the image resolution.
(Adobe Photoshop uses PPI and Corel Photo-Paint uses DPI for image
resolution so it's no wonder everyone is confused.)

DPI (dots per inch) is a measure of the resolution of a printer. It
properly refers to the dots of ink or toner used by an imagesetter,
laser printer, or other printing device to print your text and
graphics. In general, the more dots, the better and sharper the image.
DPI is printer resolution."

Resolution Inch by Inch - Overview
http://desktoppub.about.com/library/weekly/aa101800a.htm

The more pixels per inch displayed, the larger the file size will be
*and* the larger the picture will appear on screen.  The way a picture
displays on screen will also be affected by the monitor's resolution -
what looks like a perfectly reasonably sized picture on your monitor
set at 1024x768 may look like it's monstrously large on someone else's
monitor set at 640x480.

Here's a good representation of size differences:

"The first thing you might notice is the difference in size of the
images. Why do image one and two appear so different in physical size
when I only changed the resolution? Because the monitor displays in
pixels and 300 pixels per inch takes up more screen space than 96
pixels per inch. (And they would take up differing amounts of screen
real estate if your monitor is set to 640 x 480 or 800 x 600, etc.) So
what happened with image 3? It's only 96 SPI but its physical size is
3" x 3" (300 pixels square) so it displays at the larger size."

Resolution Inch by Inch - Part 3: pixels per inch 
http://desktoppub.about.com/library/weekly/aa101800c.htm

When resizing an image for purely digital display, pay attention just
to pixels.  Inches are meaningless in digital display, and should be
referred to only when dealing with actual print size.

You won't lose any pixels by attaching your file to an e-mail and
sending it off, just keep in mind that your receipient might have a
different screen resolution than you do - people most commonly keep
their monitor resolution set to 800x600.  To avoid having your
pictures look gargantuan to the recepient, re-set your own monitor
resolution to 800x600 before resizing your images.

Here's an example of an image with an approximately 4x6 print size:

http://www.darkfriends.net/princessmoo/extras/schiller.jpg

(This is Schiller.  He's 15, and loves to be photgraphed.)

The actual print size of this image is 6.19 x 4.65 inches.  If you
pulled the image up on my monitor (set at 1024x768) and measured with
a ruler, it would come up to slightly less than that.  Pull it up on
my kids' monitor and do the same, it will come up to slightly more, as
their monitor resolution is set lower (800x600).  It has a *display
size* of 446 x 335 *pixels*, no matter which monitor it's viewed on,
and will print at a size of 6.19 x 4.65 no matter where it is printed.

There's a great tutorial explaining the difference between scanning
and editing for digital display and scanning and editing for print at
Scantips.com.  The chart and tutorial cover units of measurement,
which should be used for each display medium, and also discusses
editing tips to help you make the most out of your digital images:

A few scanning tips
http://www.scantips.com/basics01.html

Nik Multimedia also has a good explanation of when to use pixels and
when to use inches, including screenshots showing PhotoShop dialog
boxes, and an explanation of screen resolution and how it affects
image size:

A Closer Look At DPI
http://www.nikmultimedia.com/welcome/windowsxp/CloserLook.shtml

That's Not How It Looked On The Screen
http://www.nikmultimedia.com/welcome/windowsxp/NotHowItLooked.shtml

For something a little less confusing when it comes to image resizing,
I highly recommend Paint Shop Pro:

Paint Shop Pro
http://www.jasc.com/

PSP allows you to compare Print Size (inches) to Digital Display Size
(pixels) with just 2 clicks (View --> Image Information), giving you
valuable information while you're still editing, to ensure you get the
size you want.  PSP is free for 30 days, and is a helpful tool when
trying to figure out PhotoShop.

I hope this helps you out!  If you're still a little confused or need
a bit more information, don't hesitate to ask for clarification.  I'll
be happy to assist you further.

Good luck on your project!

--Missy

Search terms: ["pixels per inch"], [ image editing ]

Thank you Missy!  You did a super job of answering my question.  I
feel like I've moved way up the learning curve just reading what you
have written, now I will go take a look at the links for more in-depth
learning.    I appreciate your help!