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It is very simple: dpi is merely an interpretation of how large your file will print on paper. You could create a file with 1 pixel, and print it at a 2.5 by 2.5 inch size by setting the ppi (we talk about ppi when working on screens, not dpi) in Photoshop to 1.
So guys, please: STOP abusing dpi when you talk about image resolution relative to the actual physical pixel. It's PPI, not DPI!
When you print an image, it depends on the paper quality, the inks, (yes, even the atmospheric conditions in a place), and the print method what resolution you will need for the best quality (there's more factors, but let's keep it simple).
"300dpi" is often coined as a standard for most press work - but you must understand that it also depends on the TYPE of artwork you intend to print. Prepress persons often grow tired after a while explaining things to the layman, so it's a "gentleman's agreement" to tell any client "just deliver your artwork at 300dpi". What they are actually saying is that a good acceptable quality print will be possible with an image delivered to them at the required minimum size at 300ppi. The truth of the matter is that if you worked at the optimal resolution and ink settings, there is a good chance you could arrive at a great or beautiful quality print. A lot of printers have no time for this, (regularly lacking knowledge/interest as well), and leave it to automated processing. Besides, clients (like you) lack the knowledge anyway, so the less time spent on these matters, the better.
Just deliver "300dpi" work. No questions asked. 
But not all artwork should be delivered at 300ppi. For example, text and black and white sharp artwork will/can be printed at 1200dpi or more, depending on the press tech. This is because colour work must be rasterized, meaning: any hue of colour consisting of a mix of cyan, magenta, yellow, and a shade of black (for shadows and darks) will be simulated with dot patterns (we call these halftones):
This means you LOSE a lot of the true resolution possible when you print colour work, as opposed to if you just printed with one full ink like black. Actually, high quality press work can have a physical print resolution of 300 lines per inch (LPI) (some cases even more) - and this is used to print those halftone patterns at a dot resolution of 300dpi.
(Read up on LPI)
Black body text does not need to be turned into a halftone, and is therefore printed at a much higher effective resolution. Pure black, that is - so the print raster can be fully utilized (up to the point of what the paper type can process in terms of ink).
However, it also depends on the paper used: newspapers work with an lpi of 85 or so. So images are also not required (nor wanted) to be delivered at 300ppi in that case. Due to the high absorption rate of that type of low-quality paper a 300dpi halftone would just blur anyway - and smear due to too much ink. So they are rasterized at a much bigger dot pattern half tone.
Most people do not realize that printing an image with a far too high ppi will probably print at a lesser quality than an image that has been prepared with the optimal ppi. (Which has to do with several things, but let's not get into that for now)
Still with me?
Now, suppose you have some comic artwork. If you inked the lines with pure black (very probable) for high quality glossy paper you best work at the very least at 700ppi for your required dimensions.
Here are some guidelines**:
- B&W art printed art on interior (vellum) paper stock: no higher than 600-700ppi
- B&W art printed on cover (glossy) stock: 700-800ppi
- Four colour line art printed on interior (vellum) paper stock: no lower than 300ppi; recommend between 400-500ppi.
- Four colour line art printed on cover (glossy) stock: no lower than 400ppi; recommend between 400-600ppi.
Keep in mind that you can also separate the black and white art from the colours - in that case the black and white art must again be at the higher ppi. (This process yields by far the nicest prints - but takes the most effort, because you must treat the pure B&W inking and colour work separately in two different layers, ideally having the ink work as vectors).
So... what does it all mean for you? For full colour artwork (digital paintings, comics) you best work at 600ppi. This will allow you to scale down if required, and still have enough rez for glossy work. Better a bit too much, than not enough is my motto. If you are unsure whether your artwork might be printed at greater physical dimensions on glossy paper (perhaps you are a famous digital (comic) artist who will have his work collected at some time in a big luxurious art book) you may want to work at those dimensions at 600ppi.
This is also a reason why digital painting/drawing software like Krita and Clip Studio offer 600ppi as a standard for new documents)
For B&W work (inks for example) 800ppi is a good resolution, but even better would be to work with vectors or the "printers' standard" of 1200ppi. This why I prefer Clip Studio (Manga Studio) for inks - because the ink tools feel like bitmap drawing, but have the incredible advantage that you can output your work at any resolution you would ever need. At the best quality. Unfortunately Photoshop doesn't do true vectors, so if you intend to stay with Photoshop for inking and b&w work you best work at a minimum of 800ppi (for more flexibility I would set it to 1200ppi).
(Clip studio knows that B&W work is often advised to be set to a ppi of 1200, and also offers this as a standard - I use this ppi setting myself for B&W bitmap inking)
So here is the math:
paper size (final printed size) @ 600ppi (full colour work).
paper size (final printed size) @ 800ppi (B&W work) | or work with/convert to vectors if you can/want. Lettering should be done in a vector/dtp app anyway.
Let's suppose you want to prepare your full colour artwork for a deluxe graphic novel format, glossy paper: 9" x 12¼" @ 600ppi results in: 5400px x 7350px
Next, the question about bit depth. I work at 16bpc whenever I can, because it allows for subtler control over colours, and I do a lot of artwork meant to be viewed on screens (like games, and stuff). However, generally for print 8bpc is enough. But again, it depends a bit on the type of artwork. 16bpc can make a visual difference, so if you have memory and cpu power to spare: by all means work in 16bpc.
Finally, one or two things about Photoshop: for inking and drawing I find Clip Studio a far better proposal, because it allows you to draw directly in true vectors. So I do all my inking and B&W work in Clip Studio. Also, the draw/inking feeling in Clip Studio is unsurpassed, and blows Photoshop out of the water. (I am not alone in this, btw: I know quite a few comic book artists who decided to make the switch from Photoshop for this type of work).
The fact that you can scale your line work up and down without any loss in quality is something that is hard to beat.
Also, Clip Studio has a built-in stabilizer, which can be a great help achieving those smooth curves (incidentally, Krita also offers this option, as does Gimp, as does the latest beta of Photoline). For photoshop there is the Lazy Nezumi plugin (which costs extra $$).
I always say: the right tool for the right job. Photoshop is not that great for inking and B&W work - an artist has to be realistic about these things 😉
Last, but certainly not least: discuss these concerns with your printer. An expert printer will generally be pleasantly surprised (well, not always...) when you ask him/her how to achieve the best quality print, and work with you in achieving this. Remember, the "300dpi" is merely a standardized requirement to prevent most problems. But depending on the paper, the inks, and so on, a higher resolution (ppi) may yield much better looking prints.
Anyway, I hope this cleared up some things. And stop using dpi when talking artwork resolution settings in Photoshop - it's PPI 
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Remember, your document has no dpi - it's ppi. The distinction is important. You work with pixels when setting the resolution, and dpi is merely the interpretation of the ppi into printed dots.
Anyway, the resolution of you file counts. You are correct in that. You are also correct that by increasing the ppi (but NOT resampling the actual pixels) your work will print bigger on paper. So a 5 by 5 inch colour image at 300ppi, when changed to 600ppi will print twice the original size - at the expense of halving the resolution. You will start to see grains and blurry pixels/artefacts.
Even if you do upsample: there is just no real information to base the higher rez version on, so it will become blurry. Same result when printed. Bad quality.
So, in a nutshell: work at the optimal ppi at the paper size you require.
No, large banners the size of buildings are NOT printed at huge pixel dimensions of thousands of ppi - what counts is the distance between the viewer and the print: this is why 300dpi printed matter for colour work in books and magazines works at 300dpi: we can no longer see the dots, and it the colours look homogeneous, i.e.smooth.
As for your question about the 72dpi printed version versus the 300dpi printed version: the first one will print much larger, but from a close distance the pixels will be quite obvious. Looking from a greater distance (like a banner/billboard) it looks the same as the 300dpi version.
Next time you are waiting for the bus or public transit, walk up to one of those billboards, and examine the print - you will notice that the coloured artwork looks dreadful up close. Like a blown-up 300dpi print
And although many people have theories about dpi/ppi, printing, and the lot: in truth it is quite simple, and most people I encounter have no idea about the real theory behind printing and prepress - they invent their own theories.
Read up on this in the book I linked in the previous post - it explains everything quite well.
boo radley 2013 wrote:
sorry, this is all just a blur now.
Herbert2001 wrote:
With apologies to Jackie Chan for the unforgivable segway, but his picture made me think of the news item last night, that it is about to become mandatory that everyone in North Korea addopts the same hair cut as leader Kim Jong Un. Especially the women . Actually, I think it has since been reported that it was a hoax, but it was a lovely story while it lasted.
JJMack wrote:
Oh and just reviewing the numbers if you happen to have an old IBM T221 display laying around you could display all the image actual pixels on screen. if to install it in a portrait orientation. The display displays 3860x2400 pixels 9.2MP. In portrait mode it would display 2400x3860 pixel your 2400x3000Pixel image would fit but because of the display resolution the image displayed would only be a 204DPI image.
Goodness! That's a serious display, and a new one on me. It doesn't look like it would easily stand in portrait mode, but I'm sure it would be doable
http://www.amazon.com/IBM-T221-22-2-3840x2400-Monitor/dp/B00006HS5R
A nice side benefit of the 1920 x 1200 display I now use for my second monitor in portrait mode (an idea that came from this forum), is that I can so easily drag a portrait mode image across to preview in much more detail, so this IBM beastie would be very impressive.
thanks JJ. I've read cave heiroglyphics with blindfolds on and had a better understanding of what I was reading as opposed to what you just posted, but that's due to my own lack of understanding of dpi... i appreciate the help, but I do think I finally have some sort of a grasp on this now.
As I had said from the start, it makes no difference if I'm working in 72dpi or 300dpi so long as the pixel quantity is the same. The difference will be the document size (in inches), and that can be re-sized when going to print - so yes, I have learned something new today. So in essence, dpi is something I should eliminate from my vocabularly entirely, and replace the word with 'pixels' - so long as I keep an eye on the document size first (inches) and then convert if necessary. Hopefully I get it now. thanks also for all the additonal help here.
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