Printing in high DPI / What about PPI

[andyw]

Hello everyone,

I am quite sure this is a trivial question for this pro Adobe / Printing forum, I am sure is not the first time someone here ask questions about digital imaging PPI and DPI, but I am one of those who still struggling to understand this theory. Here is my question and would be immensely thankful for your input!!!!!

1) I am printing some Photoshop files on a high resolution printer (HP Z3200 COMERCIAL PRINTER: Up to 2400 x 1200 optimized dpi from 1200 x 1200 input dpi with maximum detail selected). Does it mean that I need to set my Photoshop file to 1200/2400 dpi to get best quality prints on this printer? I am a bit confused when I hear that anything more than 350 dpi is overkill, at the same time printers specifications have their dpi much higher at 1200/2400. If 400 or 500 dpi is overkill why printers today claim to have output up to 2400 dpi?

2) I also have read quite a few articles on websites that talk about the relationship of PPI and DPI trying to figure out the best PPI and DPI combination for the size work I am producing in Photoshop. So far I still not sure at which DPI / PPI I should be working with. The image I am working right now is set at (PIXEL DIMENSIONS: Width: 6337 - Height: 5639 / DOCUMENT SIZE Width: 18 inches - Height: 16 inches at 350 DPI). The only hint that I am getting that this maybe a overkill is that the document is pretty heavy, but not sure if true.

Anyone can give me some tips on the above file settings or direct me to a easy website that explain DPI and PPI so I can figure out

THNAKS A LOT!!
Andy

 

[Lukas Engqvist]

The dpi is the size of the smallest dot the printer can produce, this dot is print or no print, on or off. Now to be able to print different values of a colour a pattern of lines is constructed, this is the LPI. If DPI is 1200, and LPI is 200, then you you have a square that is 6x6 subdots (at the DPI) for each dot (LPI) which is not enough to give you full 8-bit per colour. If you don't want visible dot pattern you need a high DPI printer if you also want to be able to print distinct shades of grey/colour.
So if, to make maths simple, we decide on 200 LPI, then we have 200 raster dots per inch, now this means that for colour sampling you are only using a 200 PPI information… but the RIP is usually smarter than that an can make fine tune the shape of the raster dot depending on other pixel information for details. This is why a PPI higher than the DPI can be used. Why not just test it and see? You can easily make a test with images of different resolutions and then print… judge which is best. Believe empirical data

 

[Greg_Firestone]

Hi Andy,

PPI = points per inch, DPI = dots per inch. PPI is used for describing on screen while DPI is used for describing print.

Regarding your resolution, what Luke said is correct. You have lines per inch (LPI) and within those lines are your dots. Even though a printer can do 2400x2400, it doesn't mean that we'll see the difference.

A general rule of thumb is to double the LPI to = your DPI. For example, many newspapers only print 100 LPI because you won't notice higher resolution on newsprint. So if you're making a file for a newspaper, 200DPI is fine (100 LPI x 2 = 200 DPI). For commercial printing, between 150LPI to 200LPI is most common. Therefore, your resolution (at the printed size) should be 300DPI to 400DPI.

You can have larger files with a higher resolution, but you simply won't be able to notice the difference when printed. Where you will see the difference is working on screen and file size.

One last bit - text and graphics are different than images. They are composed of curves and have no set resolution. This allows them to be scaled from a business card to an poster without any loss in quality. Images have a set resolution and can't do this.

Greg

 

[rmac2125]

Hi Andy,

Photoshop is a raster program and works in pixels per inch. Your HP is also rastered but works in dots per inch. The pixels per inch refer to a monitor / display resolution and dots per inch refer to resolution on a substrate (paper or printing plate). Greg is correct in saying that general rule of thumb for printing company's is 2 x lines per inch (printed lpi) = photoshop pixels per inch (ppi). Generally all high-end magazine printers print at or above 200 lpi and use images at or above 350 pixels per inch. I did say it was GENERAL rule of thumb that does not always have to be followed.

Because you are using photoshop you need to watch the pixels per inch of text / logos. I would recommend minimum 800 pixels per inch in photoshop If you are reproducing text / logos. Try and create any vector art, like text and logos, in illustrator / indesign so they are NOT rastered to pixels in photoshop. If NOT reproducing type in photoshop then 300 to 400 pixels per inch is good enough.

Rob

 

[andyw]

Hi Lukas and Greg,

I did read both your replay few times before starting to understand abit more about this issue. Now that you guys mentioned about lines per inches "LPI". Where do I find references to LPI in Photoshop? Shoud I be paying attention to "PIXEL DIMENSIONS"

Thanks Greg for explanation and "the general rule of thumb" and for sure I will take Greg advice of testing out diferent resolutions. Like the saying "Learning by Teaching"!

Thanks,
Andy

 

[gordo]

These posts on my blog might help as they include explanatory pics and applications:

The Print Guide: "DPI" and the misuse of graphic arts terminology

The Print Guide: Image resolution for printing - LPI vs DPI a.k.a. LPI vs PPI a.k.a. LPI vs SPI

best, gordo

 

[Stephen Marsh]

For Epson inkjet printers and contone photo content (not rasterized text, logos, lines etc), say at 720x720dpi the 100% size photo input resolution is often 1/3 to 1/2 of the printer resolution - so 240-360ppi for 720dpi output. You can often get away with lower input. For a 600 or 1200dpi printer such as many HP inkjets, 200-300ppi input at 100% size was fine (I used to use a DesignJet 5000).

Stephen Marsh

 

[Greg_Firestone]

Where do I find references to LPI in Photoshop? Shoud I be paying attention to "PIXEL DIMENSIONS"

Hi Andy,

LPI is related to the output device which is probably why you don't see anything in Photoshop regarding it. Photoshop is a creative tool.

I would concentrate more on Document Size vs Pixel Dimensions within Photoshop's Image Size window because you plan on printing your file at a set size and resolution.

Glad the comments here are helping.

Greg

 

[Stephen Marsh]

When resizing an image in Photoshop using the image size command, there is the "auto button" - which works with lines per inch or cm and uses the common x1.5 or x2 quality factors (screenshot attached).

EDIT: This command is intended for use with a traditional AM halftone screen on a printing press, not an inkjet printer (inkjet FM screening is more forgiving of lower resolution input).


Stephen Marsh

 

[Stephen Marsh]

I would concentrate more on Document Size vs Pixel Dimensions within Photoshop's Image Size window because you plan on printing your file at a set size and resolution.

Andy, I agree with Greg. In Photoshop, turn off the checkbox next to resample in image size.

Change the document width/height to the desired final 100% print size.

Then look at how many ppi the image has. For an inkjet output of photographic content, you should be safe with anywhere from 200-400ppi. If you have more ppi at final 100% print size than you think is appropriate, you could elect to turn resampling back on and reduce the ppi, leaving the print dimensions as they are. This will reduce file size and speed up processing, however for some image content you may introduce more issues when resampling down, you need to inspect the image content before/after resampling and decide if things are OK.

The image should also be sharpened for output. Rasterized text and logos and lines would of course require greater resolution, usually 600ppi or greater. Ideally text and logos and lines are vector rather than raster.


Stephen Marsh