why do 72 ppi images look good on a display but bad in a print?

mcluckie wrote in post #9537265
What a bunch of lame answers.
It's freakin' late for me, and what i wrote below isn't my best linear lecture by far, but I was compelled to try to get this info off because of responses like "thats the way it is."

Monitors display 72-75 pixels per inch. A pixel is the smallest "picture element", hence, the term.

Print output is dots per inch, dpi. Similarly, lpi is lines per inch. Printed halftones are measured in lpi. It takes a 2400 DPI imagesetter to produce a FULL RANGE of gray at 150 LPI.

Here's the math: (Resolution divided by the line screen) squared + 1 = number of grays that will be output (256 optimum, as in all the gray information in a photoshop channel).

BTW, You can tell even a 600 or 1200 DPI output device to generate ANY LPI you want, but you will get far less shades of gray. Banding will occur, but at the LPI you chose. A 1200 dpi imagesetter can only produce (if I recall correctly) a 106 LPI film doing 256 shades of gray.

Your answer is here somewhere. Offset printing an image at the standard 150 LPI (again, requiring a device that can do 2400 DPI to get a full range of tones), requires an image resolution (at 100% of usage size) of twice the line screen; everyone knows the famous 300 dpi. (Use the formulas above to prove this.) A good looking screen image (72 dpi) is roughly 4x too small.

Actually, of the 4 CMYK seps, the range of LPI per channel ranges from 120-150, at different angles, to avoid moire. The pattern of dots (or diamonds, or squares, or whatever the operator chooses) is called a rosette. Each photoshop channel can hold a max of 256 shades of gray — channels are just grayscale info with a color over it.

Line art is different. Since there is no halftoning involved, the art needs to have the SAME res as the output device. 2400 DPI output needs 2400 PPI line art.

If this thread is still going tomorrow, I'll try to be clearer when I wake up. The short of it is, there's no halftoning going on in your monitor. The difference in image presentation has to do with the halftoning process.

Thank you. I agree the answers have been lame. I don't know why people answer a technical question that they don't know the answer to.

You explanation is a little confusing but I'll do some further research into things you are addressing so I can better understand it. Thanks for taking the time to answer.

channel_49 wrote in post #9540123
Not sure if anyone has answer this properly, but... dpi stands for Dots per inch, which
specifically refers to print, monitors don't have a dpi. People refer to it as 72dpi simple to show
the amount of pixels per inch of the actual monitor's size, however it doesn't work the same
way as in print.

One important thing to remember is that monitors are back lit, papers are not, the
constant refreshing and far viewing distance makes the monitor appear as sharp, where as
up close it isn't. The same principles apply for color in print vs color on monitor. Of course you
must also factor in that CMYK is a subtractive system, where as RGB is an additive system.

Since we're not talking about color, here is how dpi really works. In photoshop, when you see
dpi in the image settings, that simple refers to how big you can print said image on paper, at
said resolution. 300 dpi would mean that for every inch, you are printing 300 pixels, each pixel
being a single dot. The actual size of the image will be in the first box, the dimensions in pixels,
whatever dpi you set it as, the actual size will not change, that how big the image is. This is
not to say that it doesn't get upsized, most people just set the image to 300dpi when it does not
have enough information so photoshop will interpolate said image to the pixel dimensions so
that it can print at 300dpi. Since there is no real image information, it must be generated by
interpolation, which will look... bad.

So, for a standard 8.5"x11" print @ 300dpi, your image needs to be 2550px x 3300px,
for a standard 11"x17" print @ 300dpi, your image needs to be 3300px x 5100px,
for a 24"x36" print @ 300dpi, your image needs to be 7200px x 10800px. Although you won't need
the poster size @ 300dpi, because you never view it up that close anyways, same as a monitor.

Why print at 300dpi? Very simple, it is at that resolution the human eye cannot distinguish
the dots, try looking at some printed text @ 300dpi very very close and try the same on a monitor,
you will notice that the paper is sharper, you cannot distinguish any imperfections within the
text. Modern monitors have fast enough refresh rates that you can't really call them "not sharp"
as well, but I'm sure you've noticed when you take a picture of a monitor,
you can notice the dots.

Simple put, its because a monitor is not static, while a piece of paper is static.

---

For those arguing about what monitor dpi is, its very simple, just divide the resolution by the
physical size of the monitor, and you will get the dpi. As you change the resolution, that dpi changes,
it's very simple, and not really a fixed number.

Pixels are pixels, inches are inches, dpi is just a conversion unit to get from one to the other,
and it changes as the two units its converting changes. Without interpolation, your image is just
X by Y pixels and it will not change, you can downsize it or upsize it, but that information will be
resampled by the program and generated, thus it won't look "good".

Thank you. This is very helpful!

channel_49 wrote in post #9540184
He knows exactly what you're talking about.
That's why you trust a GD with print, and since I'm also a GD, I can say that his answers
are the most accurate out of everything posted here... Newspapers are exactly 150-200 dpi
just like he said, I know because I work in editoral...

Thank you.

I measure my image size in 2 ways, PPI and Inches.
To determine the Viewing size I combine the 2 measurements.
For web, a 5x7" @ 72 PPI will display on a 72 PPI monitor at Approximately 5X7", and look good.
Pixel count is 360 X 504 = 181,440.
For a print 5X7, I use 300PPI,1500 X 2100 PPI = 3,150,000 Pixels.

My printer uses a 300 PPI file, and prints 5760 x 1440 DPI.

DPI is a reference to the size of ink droplets.

BTW, i think TZALMAN had your answer a few posts back.

Imagesetters are certainly commercial. A Komori is a printing press. You usually need to run film on an imagesetter to make a plate for a printing press. There are direct-to-plate options, but imagesetters are certainly commercial. A web press (2 words) only means an offset press that runs paper like a toilet roll. Phone books are done on a web press. The option is sheet-fed; a press that takes a page at a time. Much higher quality

Ask ANY web designer (not the psuedos here) and he'll tell you that sites are done for a 72 ppi screen. OF COURSE you can set the res at whatever you want on your monitor. 72 ppi is the STANDARD, this century and last. Do you really know a web deigner that designs for an 88 ppi display?

DAMphyne wrote in post #9540383
DPI is a reference to the size of ink droplets.

The droplets are always the same size. DPI=dots per inch. It's how many dots that are in a square inch that varies.

mcluckie wrote in post #9540463
Ask ANY web designer (not the psuedos here) and he'll tell you that sites are done for a 72 ppi screen. OF COURSE you can set the res at whatever you want on your monitor. 72 ppi is the STANDARD, this century and last. Do you really know a web deigner that designs for an 88 ppi display?

That is definitely true. I don't design a site mock in photoshop at anything other 72dpi. I did know a guy who did his designs at 300dpi and then downsampled for the web. His rationale was that his work would then be suitable for print if it was ever needed. I always told him it was a waste, and simply made his files larger for no reason, but he insisted.

mcluckie wrote in post #9540463
Imagesetters are certainly commercial. A Komori is a printing press. You usually need to run film on an imagesetter to make a plate for a printing press. There are direct-to-plate options, but imagesetters are certainly commercial. A web press (2 words) only means an offset press that runs paper like a toilet roll. Phone books are done on a web press. The option is sheet-fed; a press that takes a page at a time. Much higher quality

You are correct on that, my mistake on the earlier post. I was thinking of something different. It's been so many years ago that I delt with a printer that used film to make the plate that I forgot about them. I've been dealing with printers that go straight from digital to plates for a good while now.

Todd Lambert wrote in post #9540496
That is definitely true. I don't design a site mock in photoshop at anything other 72dpi. I did know a guy who did his designs at 300dpi and then downsampled for the web. His rationale was that his work would then be suitable for print if it was ever needed. I always told him it was a waste, and simply made his files larger for no reason, but he insisted.

lol, I got to admit, I'm one of those guys, but I do it for other reasons. It just bugs the hell out of me working at 72dpi. I much rather create at 300dpi then downsample to 72dpi

Designers that can only center type, do gradients and drop shadows maybe. Phone book ads and menus. I've been a designer forever; annual reports, corporate identity, capability brochures. A designer would never say that

It's LPI, the halftone screen used. Newspapers ARE LPI, the image res needed is dpi. The DPI of film (or direct-to plte) needed to get those 256 shades of gray is not the same. The image resolution you need is 1.5-2x the line screen of the output device.

output / paper and typical LPI
screen printing 35-65
laser printer / photocopier
(copier or matte laser paper} 50-90
laser printer / photocopier
(coated paper) 75-110
quick printer
(uncoated or matte bond paper) 75-110
offset printing
(newsprint) 60-85
offset printing
(uncoated paper) 85-133
offset printing
(coated paper} 120-150 +
high quality offset or gravure
(such as glossy magazines) 150-300

And, yeah, I know of samples per inch, but no one (I didn't think) uses that term, unless you're a drum scanner operator

mcluckie wrote in post #9540463
Imagesetters are certainly commercial. A Komori is a printing press. You usually need to run film on an imagesetter to make a plate for a printing press. There are direct-to-plate options, but imagesetters are certainly commercial. A web press (2 words) only means an offset press that runs paper like a toilet roll. Phone books are done on a web press. The option is sheet-fed; a press that takes a page at a time. Much higher quality


Ask ANY web designer (not the psuedos here) and he'll tell you that sites are done for a 72 ppi screen. OF COURSE you can set the res at whatever you want on your monitor. 72 ppi is the STANDARD, this century and last. Do you really know a web deigner that designs for an 88 ppi display?

DPI on the web doesn't matter, I'm a web designer (as well as a GD), I can tell you
we don't "do" things in 72dpi. We do things in pixel dimensions, dpi DOES NOT MATTER
for web usage, there is no inches on the web.

When you design for web, you design by the size of the website in pixels, dpi
does not come into play, no one makes websites to print.

No web designer that I know of (trained from school anyways) "design" in Photoshop anyways.
The term "designing" means Illustrator or InDesign... or like what I do, notepad++ for web.

mcluckie wrote in post #9540543
Designers that can only center type, do gradients and drop shadows maybe. Phone book ads and menus. I've been a designer forever; annual repors, corporate identity, capability brochures. A designer would never say that a newspaper is 150-200 dpi. It's LPI, the halftone screen used. The DPI of film (or direct-to plte) needed to get those 256 shades of gray is not the same. The image resolution you need is 1.5-2x the line screen of the output device.

output / paper and typical LPI
screen printing 35-65
laser printer / photocopier
(copier or matte laser paper} 50-90
laser printer / photocopier
(coated paper) 75-110
quick printer
(uncoated or matte bond paper) 75-110
offset printing
(newsprint) 60-85
offset printing
(uncoated paper) 85-133
offset printing
(coated paper} 120-150 +
high quality offset or gravure
(such as glossy magazines) 150-300

And, yeah, I know of samples per inch, but no one (I didn't think) uses that term, unless you're a drum scanner operator

No you've probably been a printer forever, not a designer.

channel_49 wrote in post #9540558
No you've probably been a printer forever, not a designer.

I'm NOT a printer, but I buy $100,000 of it a year for clients. I used to spend $50,000 a year in film, so we bought our own imagesetter. Look at portfolios.

Oh, and I teach DESIGN at a university, have a MS degree in design and a BS degree in photo from the Institute of Design, and do consulting on 3 continents. My clients are all Fortune 500 companies.

channel_49 wrote in post #9540548
DPI on the web doesn't matter, I'm a web designer (as well as a GD), I can tell you
we don't "do" things in 72dpi. We do things in pixel dimensions, dpi DOES NOT MATTER
for web usage, there is no inches on the web.

When you design for web, you design by the size of the website in pixels, dpi
does not come into play, no one makes websites to print.

No web designer that I know of (trained from school anyways) "design" in Photoshop anyways.
The term "designing" means Illustrator or InDesign... or like what I do, notepad++ for web.

Yeah, I agree.. web is all about pixels, however when you open a new photoshop file, you have to choose an image size setting, which normally is 72 pixels per inch.

I do disagree with your statement about designing though. Ever web designer that works for anyone commercially, uses Photoshop to design in. They don't use Illustrator or InDesign. In fact, we laugh at those who do, because they are generally wannabe web designers who are crossing over from print design.

The only other acceptable industry standard for web design is Fireworks (which I've tried to use since Macromedia days, but just hate it). Layered Photoshop files are THE standard.

And, personally, I wouldn't put much stock in web designer trained solely by school. Schooling means absolutely jack squat when I hire web designers and developers. They either learn the stuff on their own and are committed to learning forever, on their own... or they are a paper-toting youngin that thinks they know everything about the web. They're usually pretty easy to spot and I generally show them the door quite quickly.

dugcross wrote in post #9540485
The droplets are always the same size. DPI=dots per inch. It's how many dots that are in a square inch that varies.

Well, according to the advertisements, My printer has smaller drops than most other printers.
How does that fit?