Alright,
. whew this is gonna be a long one so bare with me haha. First the reason why some say don't change the DPI at the end, is because of something called "resampling" this is typically an issue for digital photographers, not digital drawing. Resampling is when an image is enlarged and the pixels themselves are resized. This will cause issues, but changing DPI itself won't do this if you have your program set properly. Using PS for example, if you choose the resize option, it will open a window with screen size options for width and height. this is under screen size and is the pixels you are choosing ( like my 4000 x 4000px) the next portion says print size, this will be the inches or cm of the document print size and where the width, height, and units are displayed. units is the dpi and by default (at least in the US) is pixels per inch, but can be changed to CM for you proper people
. Anyways right under that there is usually at least 3 check boxes you can select depending on which PS you have. Mine says keep ratio (that would be aspect ratio), keep print size, and Resample. Make sure that resample is unchecked to avoid resampling. I always have keep ratio checked and that's all. So if you don't have resampling checked, the DPI has no bearing on anything. As long as the pixel dimensions aren't changed, the DPI will not affect the original quality of a photograph or your drawing. The only thing that is changing is the internal DPI setting of the image and the output telling the printer how many dots to put down. This leads into most modern home printers don't actually produce dots, but sprays on the ink somewhat randomly.
I didn't want to confuse you with DPI and such but in order to explain this to the best of my ability, I will now really confuse the hell outta you. DPI has nothing to do whatsoever with the quality of your image. huh? I know right?! DPI or dots per inch is the resolution. PPI or pixels per inch is a measure of quality. Not the quality of the image, but quality of the actual print. If you print the same image with a low dpi say 20dpi and one set at 600dpi, you probably will not be able to tell the difference without a magnifying glass. PPI determines the print size of the image. Resolution and quality are 2 completely different things. I know previously I was solely talking about DPI only and that is because it's just convenient conversion and generally accepted, but completely false in application. Kind of like when people say "blind as a bat" or that a lion is "king of the jungle" both well known and accepted phrases, but 100% false. Bats are NOT blind, and lions don't live in the jungle, but in the desert. If you want to get technical, there are a few lions in the jungles of India if I remember correctly but this is irrelevant to the discussion
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ANYWAYS, if you don't change the pixels of the image, the digital resolution will always be the same regardless if you put 10dpi or a billion. QUALITY of the print is how many PPI are sent to the printer and that is where 300dpi comes in as the generally accepted number. This is determined by the "quality of the image" when viewed at about an arms length away.
So the simplest way to think of it is the quality of your print is based on how many pixels the image contains, not the size of the pixels.
Now with all this being said, if the image being printed is 6000 pixels wide and printed at the same size (lets say 12 inches) if you have your DPI set at 20 and print another one at 6000, they will both be printed at the same PPI and no difference can be seen in quality. This is because of the printer. Printers remap the pixels into dots, and they all do it different ways. Some better than others. This is why most printers or clients paying for your digital work request a minimum of 300 DPI. This is so there is enough pixel dimensions to meet their PPI requirements (meaning there are enough pixels in the image to meet the output of their printer)
What usually happens when the quality of the print goes haywire, is the fault of the person printing it, not the work itself. Print shop employees are trained to look for the pixel dimensions of the image, and the DPI of the image which can be seen when they open it on PS to set it up for the printer. If the image isn't saved at 300 DPI they freak out and will try to increase the DPI of the image (like say I left it at 150 DPI which I work in and forget to save it in 300) and they won't uncheck the resampling box I was talking about earlier and so now the pixels have been enlarged over 4 times the original pixel dimensions and now its pixelated all to hell when printed, when if they left the dpi alone even if set at an extremely low 10 DPI it would have printed just fine. This is simply not knowing how to change DPI without resizing the image or accidently resampling the image and enlarging the pixels themselves instead of the actual image.
It's all about old school thinking. In older photo programs DPI was important because you had to adjust it in order to adjust the printed output size. These days the programs automatically do it regardless of the DPI setting and has rendered it almost completely pointless. Even though you are "adjusting" the DPI, the program is actually adjusting the PPI. The program isn't telling the printer to print x amount of dots per inch, but the x number amount of pixels per inch. Depending on the quality of the printer, it may actually print at a DPI 10 times more than your setting. DPI is more related these days to set the default size of an image on the web. This is why when you click on an image online and sometimes it is smaller to be easier viewed on the resolution of your monitor, but then you have the option to "view default size of image" and then BOOM it blows up and is full size and quality. DPI is used by your computer (or more specifically word processors and programs) to set the default size of the image.
Still with me?
Now onto scanning, where DPI DOES matter. muahaha!
DPI in a scan is used to adjust the amount of detail captured in the scan itself. Your DPI setting of your scanner directly relates to the final pixel size of the scanned item. For example, If the painting you are scanning is A4 (sorry have to stick with inches on this one) that would be 8.267 x 11.692, if you scanned it at 300dpi, the digital image would be 2480.1 x 3507.6 pixels in size (8.267 x 300 = 2480.1 and 11.692 x 300 = 3507.6) So in this case DPI does relate to quality, since the higher the DPI your scanner is set, the more physical information being collected from the scan. 300 DPI is basically universally accepted to produce the average high quality photo, thus where the "magical DPI myth" comes from. If you want to enlarge the scanned image, that's when higher DPI scans are preferred (600dpi -1000dpi and beyond) I usually wouldn't go over 600 DPI unless you plan on a massive enlargement (like blowing up just the eye in your painting)
When you run into issues blowing it up, this is because of the software (PS, painter, mangastudios whatever) and how it resizes the image. This goes back to unchecking resampling image (common with PS) and is different depending on the program. I usually use painter 12 and corel chooses to use "maintain original size" in lue of "resample image." You have to be careful with this because you would want to uncheck resample, but you would want to check maintain original size. You will see this a lot of you take a scanned image or even just taking a photo found online and trying to blow it up or "place" it onto your canvas. It will ask if you want to maintain aspect ratio or original size. If you uncheck it, the image will pixelate like crazy when printed.
The last piece of the puzzle is the DPI of the printer in question. Take out your pencils boys and girls! hehehe god I need a case of beer now
. There is no direct correlation between a pixel in a digital image and a printer dot. Your printer re-maps digital pixels into printer dots using whatever algorithm the manufacturer has chosen to use. This algorithm also has to do with the color output (RGB, CMYK) and why colors don't always match whats on your screen, but an entirely different discussion that I don't think my brain can handle at this moment haha. The quality of the print is dependent on the amount of data in the image (the PPI) and the quality of the printer itself, and the paper being printed on. If you are sending a printer say 100 PPI and the printer can print 1000 DPI, the printed image is gonna look good. Now if you send a printer 1000 PPI and it can only print 100 DPI, its gonna look like complete garbage. This is because of overprinting of the dots in an attempt to match the "quality" of the image you are requesting. So in order for you to determine what DPI to print your image, you have to find out what DPI your printer can handle and not exceed it. Then it depends on the printer heads, how the printer is made, the inks themselves and blah blah blah DPI, PPI, DPI, PPIDSFJLKSDD I'm done
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