PPI - Pixels Per Inch
DPI - Dots Per Inch
in the world of imaging, PPI and DPI are used almost interchangeably.
They are two quite different things and the confusion of the two leads
to more confusion than almost any other single thing in digital imaging.
first things to understand is that in digital imaging, the only thing
that really counts about a file is how many pixels are in a file. Terms
like megapixels, DPI and file sizes in megabytes only confuse the
issue. In the end, all digital images are simply X pixels by Y pixels
big (by Z bits of colour data but we can ignore that for now).
This is the only absolute measure of the quantity of information in a file (nb it has nothing to do with the quality of information in a file!)
In almost all cases, unless you are talking about physically how your
printer is laying ink down on the paper, you are actually dealing with
PPI - pixels per inch.
Dots Per Inch
is an old printing term and has almost no place in modern digital
imaging. DPI is a measure of how many tiny, tiny droplets of ink a
printer is laying down in its dither pattern to form one inch of a
print. Most Epsons, for example, operate in Photo mode at 1440 DPI (can
be as low as 720 DPI and as high as 5760 DPI). You tell the printer
which mode to print in in the driver - this is usually camouflaged by
the use of modes 'Photo', 'Best Photo', 'Photo RPM' or 'Fine',
'SuperFine'. In the bigger printers you can usually choose the DPI
directly as, say, 2880 DPI. Incidentally, almost all printers operate
best for general photographic usage at around 1440 DPI. The higher DPI
modes like 'Photo RPM' are useful if you're printing really high key
(i.e. all light toned) shots, but next to useless for general printing.
In fact, they are worse than useless, they are positively damaging -
way too much ink is layed down on the paper, resulting in seriously
impaired shadow detail. And of course the more ink you use, the more
ink you pay for (an Epson R800 running in 'Best Photo' mode is less
than half as expensive to run as one left in 'Photo RPM' mode for
instance, and in 99.9% of photos, you'll get worse quality from the
'Photo RPM" mode!).
Pixels Per Inch is the
key term. It is a description of the logical number of pixels from your
original image (X pixels by Y pixels, remember) that will be used to
tell the printer to print one inch on paper. Assuming a sharp original
shot with good technique (see resolution discussion
below), the higher the PPI, the better the quality print you can
achieve - this is testably true even well beyond most claims of 360 PPI
being the most you need ... 600 PPI images can easily be seen to be
much sharper again if this data is available at good quality from the
PPI is a logical term - changing
the PPI of a particular file does not in any way affect the file itself
- it is simply a decision about how many pixels of the available pixels
you will use to print an inch on page. You can choose any number you
like - from 1 to infinity. The de facto standard for high quality,
photographic printed images is 300 PPI - that is, for each inch of the
printed image, there must be 300 source pixels to use.
is why the 'resample' check box, in the Image Size dialogue, is the
single most important (and dangerous) control in Photoshop! When
you resample an image, you are actually changing the number of pixels
in your image (i.e. changing the value of X and Y) - adding some or
throwing them away. You should only do this if you are making an
explicit and informed decision to do so, because no single other thing
will affect the quality of information available to you from your file
An example will make all this clear:
Say we scan a 35mm tranny at 4000PPI - this will result in a file that has 5400 by 3600 pixels.
now want to make a print of 12 by 8 inches. This means the PPI we have
available from our file for a print of this size is 5400/12 = 450 PPI
(or 3600/8 = 450). We are choosing to use 450 pixels to represent one
inch of our print.
The printer driver will now translate those 450 logical pixels into 1440 physical dots per inch (DPI) and produce a very high quality print for us.
we wanted to know the maximum print size we can achieve at good
quality, and we know from experience that given a sharp original
outputted on an Epson inkjet, 240 PPI is sufficient, we can calculate
the maximum print size by taking the total number of pixels available
to us (5400 on the long edge) and dividing it by the PPI required to
give us, in inches, the size of the print:
5400/240 = 22.5 inches
3600/240 = 15 inches
our final print will be 22.5 by 15 inches, with 240 pixels used to
represent each inch. And of course, the printer will actually use its
1440 dots per inch to actually print that image to paper.
Hopefully that is clear - it can be a bit confusing at first. Just in case, you can read another version of the same stuff here to get a different perspective on it.