DPI and PPI

Wayne wrote:
In article ,
says...

...


Why? Because of constrained dpi values. It's very difficult to create a
raster dot that consists of 255 microdots while still ensuring that this
raster dot is visually small enough to blend into an even shade of gray
together with its neighbours. In fact to do this would require an output
device that can print over 30,000 individual microdots per inch if you
want the smoothness of a magazine print. Magazines print at around 130
raster lines per inch for smooth photos, making the formula 130x255 =
33,150 of required microdots per inch. That kind of sharpness is
impossible at present and probably will be for a long time coming.




Bas, I dont know anything about commercial printing, but FWIW, I must reject
all of your numbers as ridiculous.

All of your numbers apparently put all of the microdots to create one tone
value on a straight line, instead of a circle or square or diamond. For just
one example,

"Magazines print at around 130 raster lines per inch for smooth photos, making
the formula 130x255 = 33,150 of required microdots per inch."

1/33150 inch is roughly one micron, which is going some for ink dots on
magazine paper.

I do know it is 256 tones, which would be a 2D shape on paper, either a 16x16
matrix, or a similar circle (with appearance of varying diameter), probably on
a 2400 dpi screen for 130 lpi.

16x16 dots allows 257 tones, and a 2400 dpi screen would allow 2400/16 = 150
lpi work. 10x10 dots would allow 2400/10 = 240 lpi, but only 101 tones.
There are many ways to arrange it, circles, squares, diamonds, ovals, but
256x1 seems very creative.

I think you meant 130 lpi x 16 = 2080 dpi screens, but didnt know it.

...


So let's say we just made a photo using a Nikon D70 that outputs images
of 3000x2000 pixels (rounded off). That means you can print a black and
white image made with a D70, using an 8000 dpi image setter, retaining
all your shades of gray, that's (8000/255)x3000 = 93,000 inches wide.
That's positively HUGE and you won't (usually) see any pixelated edges!


Agreed 93,000 inches is huge.

But unless your dots are really 255x1, it should be 8000/16 = 500 lpi, needing
500x1.5 = 750 ppi images (I'll say ppi just for you). But 3000 pixels / 750
ppi = 4 inches.

I seem to be 92,996 inches short - which is nearly 1.5 miles short, so I'm way
off. But I certainly do get the idea

This is the most creative thing I have ever read.


Like I said, there are probably mistakes in there. When I actually do
publish this, I'll go over it again. Especially the figures, which are
only given for example's sake. What you say about raster dimensions is
correct, in my hurry to write this up I didn't take the real spatial
dimensions of the raster dot into account.

Will post an URL to a corrected version here soon. Thanks for noticing!

Bas

In article ,
says...

Like I said, there are probably mistakes in there. When I actually do
publish this, I'll go over it again. Especially the figures, which are
only given for example's sake. What you say about raster dimensions is
correct, in my hurry to write this up I didn't take the real spatial
dimensions of the raster dot into account.

Will post an URL to a corrected version here soon. Thanks for noticing!

Perhaps I was too subtle. The most fundamental basics do seem quite
important. But it is just like the ppi vs dpi thing, in that some people
can only explain things in the way they imagine it might be, or ought to be,
vs explaining how things actually really are.

Good luck with that.

--
Wayne
http://www.scantips.com "A few scanning tips"

This is a fascinating and confusing thread. Like others in this thread,
my print agency has said they "require 300dpi images" for printing on
their large format printer, which it appears is a meaningless
requirement per se. Surely their printer doesn't have to print only at
that resolution, and their software must be able to resize the image so
it effectively prints at a lower resolution?

What I want to know is whether I can get decent print quality from the
camera shop's 300 dpi Epson large format printer if I get them to print
a 7000x2600 pixel image at 32x12inches? That would give a resolution of
over 215 dpi, which should still be high enough for it not to look
pixelly, right? Or do I have to interpolate it in Photoshop up to
9692x3600 pixels to make it 300dpi at that size (from its present
nominal 72 ppi label originally put there by the camera)? Which one
would more likely give a better result?

Greg

Wayne wrote:
In article .com,
says...


Hi There

I have a somewhat technical question about PPI and DPI. I'll do my
best to ask my question as clearly as possible. I understand that all
digital cameras take images at 72 PPI which is similar to 72 DPI?

However according to Microsoft Office Picture Manager the images that I
receive from different cameras have very different DPIs - ranging

from 72 to 300. Since I need to print these images they have to be at

least 300DPI. So my question is - is there a way that one can change
the PPI/DPI your camera takes photos with. I've notices that when I
change the resolution setting on my camera, my photos still come in at
180 DPI.




No, the camera does NOT take pictures at 72 ppi. It is very confusing,
but the only possible answer is that the camera takes pictures
dimensioned in pixels.

A 3 megapixel camera creates images maybe 2048x1536 pixels.
A 6 megapixel camera creates images maybe 3008x2000 pixels.

The only dimension of the digital image is in pixels. Period.

Then when you print on paper, the paper does have a dimension in inches
(or centimeters), and you space these pixels to fill the paper at xx
pixels per inch... whatever it takes, but best is at least 200 ppi, up
to 300 ppi for the best try.

For example, if you had one image dimension of 2048 pixels, and if you
printed that dimension to be 6 inches on paper, then it is printed at
2048/6 = 341 pixels per inch. Necessarily so, to space the 2048 pixels
over 6 inches of paper. Space the same 2048 pixels over 10 inches of
paper, to get 2048/10 = 205 ppi. You can print it any size you desire,
so long as you are happy with the ppi resolution you get with the number
of pixels that you have - spaced over that paper size.

The ppi value in the image file does NOT affect the image pixels. The
ppi number has no effect on the creation of the image. The ppi number
is only one number stored away somewhere separately in the file, but it
is just a number, nothing more, and not really related to the actual
pixels. You can change it at will.

PPI in the image file is just a "what if" number, so that these pixels
can show a print dimension (inches) **IF** printed at 72 ppi or 180 ppi
or 300 ppi. That size is probably NOT same as your eventual choice -
you will likely do something different when ready to print it.

The camera has no clue what size you may want to print the picture on
paper, so the camera typically did not guess any ppi number, and just
leaves it blank. But then when you ask the photo editor to see that
number, it is blank, so it makes something up to show you, often 72 ppi,
or 96 ppi, etc. No good reason, no meaning to it, it just does. This
too-low guess of 72 ppi gave printed sizes up around 2 or 3 feet, which
is normally ludicrous. So some cameras started intentionally storing a
higher number like 180 ppi or 300 ppi, just so that the printed size was
more realistic, just to hide the 72 ppi number. But the 180 ppi or 300
ppi value is still a meaningless guess that has no effect. The image
size is dimensioned in pixels.

Until you decide how large to print the image, and invent a more
appropriate number, the ppi number is still just a wild dumb meaningless
guess - because the camera has absolutely no clue what size you may
print it later. When you are ready to print it, you will size it more
realistically to fit your goal. When you say to print it 4 inches size
or 6 inches size or 10 inches size, you are changing the number that
will be used as ppi - to space those image pixels over that much paper
dimension.

Digital images are dimensioned in pixels (not in inches).

Dpi and ppi have the same meaning when about an image (instead of a
printer). dpi is just the historic name, and it means pixels per inch.
It does have a different meaning if about printers, but if the context
is about images, the two terms have the same meaning. Pixels per inch is
the only possible meaning then, if about images.

Digital newbies seem to prefer ppi now, helps them to grasp the
concepts. Which is fine, it is exactly pixels per inch, nothing wrong
with saying ppi. Just not everyone does. Some of them even wish to
argue the acceptability of the old term dpi for pixels per inch, when
the fact is, dpi is the accepted existing term for many years before
those newbies showed up. Which is just a preference, and which is NOT
important, but since both ARE used, then it is important that we all
must understand it either way, or else we won't understand much of what
we read about it.

On 7/27/06 1:34 AM, greg wrote:
This is a fascinating and confusing thread. Like others in this thread,
my print agency has said they "require 300dpi images" for printing on
their large format printer, which it appears is a meaningless
requirement per se. Surely their printer doesn't have to print only at
that resolution, and their software must be able to resize the image so
it effectively prints at a lower resolution?

What I want to know is whether I can get decent print quality from the
camera shop's 300 dpi Epson large format printer if I get them to print
a 7000x2600 pixel image at 32x12inches? That would give a resolution of
over 215 dpi, which should still be high enough for it not to look
pixelly, right? Or do I have to interpolate it in Photoshop up to
9692x3600 pixels to make it 300dpi at that size (from its present
nominal 72 ppi label originally put there by the camera)? Which one
would more likely give a better result?

Except for a reference to the shop's 300 dpi Epson, [which will print at
far higher than 300 dpi], all instances of dpi should be converted to
ppi for accuracy and clarity.

As to how the print will look with either method, I'd print at home a
crop of each one and decide for myself. It's likely that both will look
fine, perhaps indistinguishable, and that your large format print at 215
ppi will look fine.

--
John McWilliams

greg wrote:

This is a fascinating and confusing thread. Like others in this thread,
my print agency has said they "require 300dpi images" for printing on
their large format printer, which it appears is a meaningless
requirement per se. Surely their printer doesn't have to print only at
that resolution, and their software must be able to resize the image so
it effectively prints at a lower resolution?

What I want to know is whether I can get decent print quality from the
camera shop's 300 dpi Epson large format printer if I get them to print
a 7000x2600 pixel image at 32x12inches? That would give a resolution of
over 215 dpi, which should still be high enough for it not to look
pixelly, right? Or do I have to interpolate it in Photoshop up to
9692x3600 pixels to make it 300dpi at that size (from its present
nominal 72 ppi label originally put there by the camera)? Which one
would more likely give a better result?

If they want 300 dpi and it's a 32x12 print then give them 9600x3600 pixels,
otherwise they'll find some way to screw it up and then blame _you_.

And find a different shop that actually reads the docs on their printer
before they set it up--300 dpi is a weird number for an Epson--their
transports are generally designed for multiples of 360, not 300, it's HP
that works in multiples of 300.

Whatever they are using probably _can_ scale for you, but that would require
that one of their minimum-wage employees actually do something other than
push a button, and if he did manage to get your image to print he'd leave
the system in a state where his counterpart on the next shift wouldn't be
able to undo it.

If you want them to do the scaling for you you really need to find a proper
service bureau and not use a camera shop for this purpose.


Greg

Wayne wrote:
In article .com,
says...


Hi There

I have a somewhat technical question about PPI and DPI. I'll do my
best to ask my question as clearly as possible. I understand that all
digital cameras take images at 72 PPI which is similar to 72 DPI?

However according to Microsoft Office Picture Manager the images that I
receive from different cameras have very different DPIs - ranging

from 72 to 300. Since I need to print these images they have to be at

least 300DPI. So my question is - is there a way that one can change
the PPI/DPI your camera takes photos with. I've notices that when I
change the resolution setting on my camera, my photos still come in at
180 DPI.




No, the camera does NOT take pictures at 72 ppi. It is very confusing,
but the only possible answer is that the camera takes pictures
dimensioned in pixels.

A 3 megapixel camera creates images maybe 2048x1536 pixels.
A 6 megapixel camera creates images maybe 3008x2000 pixels.

The only dimension of the digital image is in pixels. Period.

Then when you print on paper, the paper does have a dimension in inches
(or centimeters), and you space these pixels to fill the paper at xx
pixels per inch... whatever it takes, but best is at least 200 ppi, up
to 300 ppi for the best try.

For example, if you had one image dimension of 2048 pixels, and if you
printed that dimension to be 6 inches on paper, then it is printed at
2048/6 = 341 pixels per inch. Necessarily so, to space the 2048 pixels
over 6 inches of paper. Space the same 2048 pixels over 10 inches of
paper, to get 2048/10 = 205 ppi. You can print it any size you desire,
so long as you are happy with the ppi resolution you get with the number
of pixels that you have - spaced over that paper size.

The ppi value in the image file does NOT affect the image pixels. The
ppi number has no effect on the creation of the image. The ppi number
is only one number stored away somewhere separately in the file, but it
is just a number, nothing more, and not really related to the actual
pixels. You can change it at will.

PPI in the image file is just a "what if" number, so that these pixels
can show a print dimension (inches) **IF** printed at 72 ppi or 180 ppi
or 300 ppi. That size is probably NOT same as your eventual choice -
you will likely do something different when ready to print it.

The camera has no clue what size you may want to print the picture on
paper, so the camera typically did not guess any ppi number, and just
leaves it blank. But then when you ask the photo editor to see that
number, it is blank, so it makes something up to show you, often 72 ppi,
or 96 ppi, etc. No good reason, no meaning to it, it just does. This
too-low guess of 72 ppi gave printed sizes up around 2 or 3 feet, which
is normally ludicrous. So some cameras started intentionally storing a
higher number like 180 ppi or 300 ppi, just so that the printed size was
more realistic, just to hide the 72 ppi number. But the 180 ppi or 300
ppi value is still a meaningless guess that has no effect. The image
size is dimensioned in pixels.

Until you decide how large to print the image, and invent a more
appropriate number, the ppi number is still just a wild dumb meaningless
guess - because the camera has absolutely no clue what size you may
print it later. When you are ready to print it, you will size it more
realistically to fit your goal. When you say to print it 4 inches size
or 6 inches size or 10 inches size, you are changing the number that
will be used as ppi - to space those image pixels over that much paper
dimension.

Digital images are dimensioned in pixels (not in inches).

Dpi and ppi have the same meaning when about an image (instead of a
printer). dpi is just the historic name, and it means pixels per inch.
It does have a different meaning if about printers, but if the context
is about images, the two terms have the same meaning. Pixels per inch is
the only possible meaning then, if about images.

Digital newbies seem to prefer ppi now, helps them to grasp the
concepts. Which is fine, it is exactly pixels per inch, nothing wrong
with saying ppi. Just not everyone does. Some of them even wish to
argue the acceptability of the old term dpi for pixels per inch, when
the fact is, dpi is the accepted existing term for many years before
those newbies showed up. Which is just a preference, and which is NOT
important, but since both ARE used, then it is important that we all
must understand it either way, or else we won't understand much of what
we read about it.


--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)

Neil Ellwood wrote:

On Wed, 12 Jul 2006 06:16:19 -0400, Mark B. wrote:

Cameras have resolution, i.e. 2544 x 1696 not ppi (pixels per inch).
dpi, dots per inch, is a printer term; it refers to the density of the
ink dots
that are used to create the image on paper. You've no doubt noted specs
on
printers - 1440dpi, 2880dpi, etc.. The two terms aren't related. Send
an image to your printer at 72ppi, and even if the printer is at 5760dpi,
it'll
still look terrible - it'll just use more ink. Anyway, none of the
cameras
I've had have a ppi setting. Just set the ppi at 300 for the image size
at the time of printing.

Mark
Actually lenses have resolution, not cameras which really are just the
container/holder for the lens.

The sensor places an upper bound on the achievable resolution. A lens with
infinitely high resolution can't make a 4-pixel sensor produce a decent
image. Digital is not film.

--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)

John McWilliams wrote:

Dalene wrote:
My version of Microsoft Office Picture Manager (Office 2003) does not
specify, anywhere I can find, the Ppi for any Jpeg or Tiff open in it.
It
also does not seem to have any way of changing that


In Microsfot Office Picture Manager if you click on properties and then
more picture properties you will find this information. It would be
helpful if someone would check this and can let me what this means?

I'm not using this software to edit pictures. I'm sending the pictures
to a printer for printing in a Journal and the printers are telling me
that only the one at 300DPI are big enough (and yes they are the same
resolution).

Here are the picture properties according to Microsoft Office Picture
Manager

Photo 1
Dimensions: 2048 x 1536 pixels
Size 1.68 MB
Horizontal Resolution: 72 dpi
Vertical Resolution: 72 dpi
Bit Depth: 24

Photo 2
Dimensions: 1536 x 2048 pixels
Size 753 KB
Horizontal Resolution: 300 dpi
Vertical Resolution:300 dpi
Bit Depth: 24

The photos were taken on different cameras - however I'm not sure what
cameras. I can find out though.


The key numbers are the "dimensions" expressed in pixels.

It is wrong of MS to use DPI at all in this context.

In that case it is also wrong of them to use "PPI" or "glocka per klackwhoo"
or any other unit as the number is arbitrary.

However, even when
converting dpi to ppi for the so-called resolution, the figures are next
to meaningless.

Actually they aren't. The establish the size at which the image is to be
printed. Not all applications use this information, but some do.

These photos are in fact the same resolution at any given size; the H
and V are simply reversed.

They have the same number of pixels but when printed at the specified size
the printed images will have different resolutions.

Do you have access to Photoshop?


--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)

Roger N. Clark (change username to rnclark) wrote:

Don Stauffer wrote:

However, for people just taking pictures, and not doing photogrammetry,
the ppi of the basic chip is NOT important. While the poster being
referred to is not exactly correct, in saying that a CCD camera does not
HAVE a ppi, I do agree with folks who say that the inherent ppi is
meaningless. I know of no camera nor image processing software that
uses the inherent ppi of the camera in processing.

I disagree that it is meaningless, regardless of photogrammetry
needs. Example: I want to make 10x10 inch prints from my cameras.
Camera 1 has a 1/4 x 1/4 inch sensor. Camera 2 has a 1 x 1 inch
sensor. Camera 1 needs to have the focal plane image magnified
40 times. The camera 2 image needs only 10 times magnification.
Hmm which camera is likely to perform better?

Insufficient data.

Perhaps we don't see the true ppi numbers so the manufacturers can
hide the facts, just like the try and hide the true sensor size
with specs like 1/1.8" for the sensor.

What "facts" are they "hiding"?

I'd rather rely on test charts and measurements of the actual performance of
the equipment in hand than on the number of pixels per millimeter on the
sensor.

Roger

--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)

Dave Martindale wrote:

imbsysop writes:

This figure is (however) of limited
practical use.

ROTFLMAO .. kindly express the resolution in PPI of a 7Mpx and 1/2.5"
sensor .. ?

For the X axis and this sensor this must be something around 13546 PPI
! LMAO, speaking of a "limited use" ! I like the logic of some of my
fellow planet inhabitants ! braindead humbug !

The actual PPI of the sensor is very useful if you want to do any
calculations involving actual image distances. For example, if you
have mounted the camera on a telescope, and see two features a certain
number of pixels apart in the image, you can determine their true
angular separation in the sky if you know the sensor PPI and the
telescope objective FL. Or if the camera is mounted on a microscope
and you know the microscope objective magnification, you can measure
distances in the subject.

Now how many photographers actually _do_ this in the real world? Not
astronomers or surveyors or the like, but photographers for whom the image
is the end product and not a means of performing a measurement?

--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)

John McWilliams wrote:

On 7/13/06 10:27 AM, Wayne wrote:
In article , ess says...

It's not *flat* out wrong.

PPI and DPI are frequently used interchangeably. There are established
and respected institutions use DPI when referring to the pixel density
of an image file.

For example; The JPEG file interchange format (JFIF), specifications
refer to dots per inch in the X and Y resolutions.

JFIF uses a block of bytes at the beginning of the data stream that
define
certain paramaters. The X and Y pixel density are included in this
block. The resolutions are referred to as dots per inch, (or dots per
centimeter).

Turn to page 6 of this document:
http://www.w3.org/Graphics/JPEG/jfif3.pdf You can see the actual
specification there.

DPI is also used as default in the TIFF standard: (See page 38 in the
PDF below)

http://partners.adobe.com/public/dev...tiff/TIFF6.pdf


To summarize. When Microsoft Office Picture Manager reports a TIFF or
JPEG file has a particular DPI, they are in full compliance with the
written standards for those images.

These "standards" you quote are almost 15 years old. Much has changed
since then.

Snipped bits out

But newbies just getting it sorted out the first time do tend to invent
their own rules sometimes, and some of the vocal ones love to shout Wrong
if it
doesnt match their own limited understanding. Which is a bad thing,
because what is wrong is to advise everyone that dpi always means
something other
than it does mean. That is far from helpful. Much more helpful to
correctly advise that dpi and ppi are used interchangeably, since of
course, they are.

They were, but that doesn't make it correct to do so today, even though
it's a common mistake.

Since these so called standards pre-dated the advent of digital
photography as we know it today, there was (then) little incentive to
distinguish between the two. Now that we have printers in the home that
commonly can print in excess of 720 dpi, now that we have cameras that
can routinely produce 4x6 images in excess of 600 ppi, it's time the
distinction was made clearly.

To not do so is ultimately more confusing to newbies, and MS should be
corrected on their incorrect use of dpi.

Please note that I have left scanning out of the equation entirely, as
it increasingly is not the method used by NG regulars.

Don Quixote, the windmill _is_ going to win.

--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)