printing and dpi

[emanresu]

I did some (re)search on the net, and found out that pictures are displayed on the screen @72DPI, but in order to make a print, I need 300DPI.  That means, if I have a picture with 1024 pixels along one dimension, it will be somewhere around 3 inches when printing.  Is 300DPI the absolute minimum print resolution?  Because I was hoping to make some decent size prints such as 8x10

[Bob Atkins]

I think you mean PPI (Pixels Per Inch) rather than DPI (Dots Per Inch).

300 PPI is generally regarded as pretty high quality - in fact for most prints there would be no point in going past 300 DPI since the difference in image quality would be hard to see.

It's hard to say what the minimum PPI should be for good quality, but I've made some 8x10 prints at 150 PPI and the results have been pretty good. I wouldn't want to go much lower than that, though you can probably get "acceptable" quality down to about 100 PPI.

Screen resolution isn't 72 PPI on most systems. For example my monitor display area is about 12" across and my video resolution is 1280 pixels wide, so I get 1280 pixels in 12", or about 107 PPI

The "72 dpi" or "72 ppi" standard comes from some older measurements on Apple PCs and isn't really applicable to most monitors these days, especially not on Windows systems.

There's a bit more on PPI, DPI etc. at http://www.bobatkins.com/photography/digital/digital_image_resizing.html

[KeithB]

For larger prints you can use less ppi since folks are viewing them from a further distance.  I have heard 180-220 ppi for larger prints.

You can always print a few test images on smaller paper and check it out.

Here are some good articles by Ctein on that:

http://theonlinephotographer.typepad.com/the_online_photographer/2010/01/how-sharp-is-your-printer-how-sharp-are-your-eyes.html

http://theonlinephotographer.typepad.com/the_online_photographer/2010/05/more-on-printer-sharpness.html

[emanresu]

I knew it! I knew Bob would have already had something written about it - only that I didn't expect it to be under the topic of resize.  Thanks both, Bob and Keith.

DPI and PPI.  I think PPI only makes sense on a monitor, and in print industry, people usually talk about DPI.  But in any case, they are sometimes used interchangeably (and incorrectly).  DPI is more consistent than PPI, because the size of a pixel varies from monitor to monitor, since different screen sizes may have the same pixel count.  But as far as I know, in print industry, a point is a point, and a 10pt character is always this big.

The reason I am asking is that my school offers poster printing on those big HP plotters for something like 10 bucks.  They are probably still not as good as professional photo printing, but are definitely far better than our consumer-grade inkjets.  So I have been thinking about combining some good photos into a giant poster and print it there, but I need to make sure to print at the correct resolution so it will look good.

[KeithB]

DPI is a number for the printer, not the image file.  A pixel consists of a single point made up of 3 components (usually red, green and blue).  An inkjet printer cannot lay the dots on top of each other, so it must combine several dots to make up the information contained in a single pixel.  Therefore, a 300 dpi printer might actually only have 100 ppi resolution.

This is why you still feed your printer a 300 ppi image even if it can print at 2400 dpi.  It must use up a lot of those dots to portray the information given in the single pixel.

[emanresu]

on photoshop, if you click on image > image size  there will be a box for you to specify DPI, and by default it is 72.  I am just having trouble understanding that.

[Bob Atkins]

It's just a historical accident I think. It may be connected to the fact that there are 72 "points" in an inch when "points" is a typography measure of character size, so 72 point letters would be about an inch high.

So 72 dpi/ppi seems to have become some sort of default setting for images, even though it really has no meaning for monitor display.

[emanresu]

Being thick as I am, I am just more confused now.  Maybe it makes more sense to have a unit such as DPP (dots per pixel) or PPP (points per pixel) for monitor displays?

[Bob Atkins]

Not really. you get the best image whan a pixel on the screen represents a pixel in the image. You get that by default whenever you display an image at full size. Very few monitors/operating systems/programs take any notice at all of PPI (or DPI) settings.

The only practical use of PPI is in printing, where the PPI setting can determine print size. Images don't really have a DPI setting, that's a printer property. You can have multiple dots per pixel if you want.

It's far more confusing than it needs to be. Basically just ignore DPI and PPI totally as far as display on a monitor is concerned. Forget them.

For printing, use 300 ppi or more when you can. If that doesn't give you a large enough image you can drop the ppi setting down to 150 or so and still get a decent print, but below 100 ppi you'll notice quality will suffer quite a bit.

[emanresu]

thanks for the summary Bob. I will do some experimenting with the image print and see what turns out good.