DPI setting

timibaltazar  wrote I think I have the general idea on how I should do it.

Go forth and do Good Work After a while all this becomes second nature. I still maintain that there's a high degree of semantics involved in this, which is why these discussions tend to go on forever, like a medieval scholastic dispute. How many angels can perform a square dance inside of a pixel?

It might be easier if you just read it literally: pixels per inch. You have so many pixels in your file, and so many inches of paper in front of you. The rest is math.

I posted this message several months ago and it may be helpful here.

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To create constancy and eliminate the current confusion in definitions, I would like of offer a standard regarding terms for use in all Forum posts. They refer to dpi, lpi and white balance. If I am lucky, in the future these terms will be as universally treasured as the flag, home and the loyalty of a fine dog. 

PPI: Pixels – picture elements per inch viewed on a monitor. I think we are safe here.

DPI: The number of inkjet printer ink droplets. It is measured in picoliters and may appear among a given machine’s specs. When printing, the number of droplets varies throughout the tonal scale with few ink dots deposited on a surface to create the lighter tones and more of them deposited to create the darker tones. We might refer to this variation in the number of dots as Frequency Modulation (FM) and it is the only time the term DPI should be used. Promise me.

LPI:  Dots deposited on a surface by offset lithography or letterpress printing. When viewed from a distance, the dots seem to fuse and create a smooth gradation of tones. On the printed page smaller dots create the lighter tones and larger ones create darker tones, but the number of dots throughout does not vary. Only dot size varies. We might refer to this as Amplitude Modulation (AM). 

Why LPI?

Some halftone history: Before the introduction of computers in the graphic arts, the device used to create halftones consisted of two sheets of glass, each with fine parallel opaque black lines -- perhaps 150lpi – glued together (one vertical, the other horizontal) to create a grid of small clear windows. When this device, called a glass halftone screen, was positioned in front of high contrast film (not in contact with the film) in a camera and exposed to an image, it broke the light reflected from the subject being photographed into an array of various sized dots -- a halftone negative that was then used in plate-making.

Today, glass screens and their successor -- plastic contact screens of a checkerboard-like design (which are placed in contact with the film) are gone but the term lines-per-inch (lpi) -- a vestige of the glass halftone screen era -- remains to designate the number of dots to the linear inch in the image on the printed page. That is why a 150 dots-to-the-linear-inch image on a printed sheet, for example, is referred to as a "150 line halftone." To repeat: a halftone is an example of Amplitude Modulation (AM), different from an inkjet’s FM.

Now, on to White Balance (and don’t even try to talk me out of this) is defined as the neutral quality of only the highlight values in an image. The neutral quality of tone elsewhere in an image -- sidewalk, schnauzer, slate roof -- should rightly referred to as Gray Balance. These are not two arbitrary definitions: they are terms that have been used in the graphic arts for about a century. The camera manuals are wrong. There. I said it and I'm glad.

If you concur, raise your hand.

Yeh to be honest, it grinds my gears when someone says DPI and PPI aren't interchangeable, it's definitely interchangeable in the apps provided, it's a direct translation.

It's the internet police that go to town on the do-gooders who accidentaly say dpi and ppi in lax situations. It's essentially the same thing, yes DPI is print and PPI is print, but it's the same thing essentially. I know someone will come along and give me a very technical run down, but I don't care.

Nobody ever mentions LPI!

It always gets lost in these conversations!

While stictly 300 is a good guess at the ppi required it actually depends a lot on the image subject, printing method and a few other variables.

Typically 300 ppi is a good rule of thumb.

If it's a grayscale image you can probably go about 1/3 of that.
A foggy image you could probably go about half that
Anything with little definitition is about half that too.

For printing on litho machines plates are made with lines per inch (LPI) and these LPI are key key to printing resolution.
Newspapers are pourous and have a typical LPI of 80-100
Magazines or low quality magazines have a typical lpi of 100-150
Magazines mid-high end have a LPI of about 150-175
And High End coffee table/art type books typically have a LPI of 175-200

The rule of thumb most people go through is that that the LPI must be x2 to get the optimal PPI for printing (or DPI in print)

This is where the 300PPI rule of thumb comes from, as most printers set their LPI to approx 150 for most jobs and 150 x 2 = 300.

But that's a bit of misnomer in terms of the figure - as the x2 rule is erroneous.

The actual figure you should multiply by is 1.41 (recipricol)

This is due to the halftone dots in questions, which are printed in a square shape.

During the process the halftone square needs to be rotated 45 degress to avoid Screen Clashes called Moire (http://the-print-guide.blogspot.com/2009/12/moire.html)

It's 45 degrees because technically that's all a computer algorithim can do - it can faux rotate at 15/30/45/60/90 etc. but in technical computer terms 45 degrees is the only true rotation for very techincal reasons.

When you rotate a square 45 degrees it's now exactly 1.411111 times the size it was from horizontal cross

Say when the square is square shape and it's 1 across the top, and 1 down the side.

When it's rotated 45 degrees it's now 1.41 from tip to tip.

And that's why you multiply the LPI of the print by 1.41 to get the true DPI (PPI) for placing your images.

Digital printing is completely different process, it doesn't use plating or halftones, it uses toner (powder) and electrically charges the paper in the places the powder sticks to.

In essence, it's a lot more forgiving than lithographic printing and you could go as low as 150-200 - but you would have to check that with your print provider.

I'm confused... So, when they say submit an artwork that's 1280x1280 with 200dpi... I don't really need to change the resolution setting in photoshop (when opening a new file)? I just go draw with a 1280x1280 with 200ppi setting?

Sorry I don't really understand, tried researching but ended up more confused.