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Detangling DPI by Raeistic Detangling DPI by Raeistic
TUMBLR FRIENDLY VERSION HERE
Let's talk about DPI.
I've heard people ask, "If I make an image in Photoshop and I want to bring it into After Effects, what DPI should it be to be good quality?"
Or on Deviantart: "I made my canvas 2500x3000 pixels @300DPI."
No. That's not how it works xD

DISCLAIMER: This guide is targeted mainly at people who work digitally, for digital/web output, set their document size via pixels, and rarely print, but still set their DPI thinking it changes something about the quality of their digital pixels. Obviously if you work specifically for print, DPI is going to matter a lot more to you than pixels. In that case this guide is not really meant for you, but the information still applies nonetheless.

Advanced cropping, resizing, resampling in Photoshop from Adobe, in case you still don't understand and/or want to read about more advanced specifics regarding image resolution.

OTHER CLARIFICATIONS:
PPI = Pixels Per Inch

PPI is often used interchangeably with DPI. They are not, however, the same thing. PPI is device-dependent while DPI is image-dependent. (Though if you want to get technical about it, PPI is for screens (Screen Resolution), DPI is for printers (Print Resolution), and the image resolution is neither DPI nor PPI, it's the 'Image Resolution,' measured in pixels-per-inch. The DPI and Image Resolution are so closely related though that they may as well be the same thing, I refer to them as the same thing, and for the sake of this guide I am also referring to them as the same thing, since most people know it as DPI anyway). I know, crazy right?

PPI is for screens. A screen with a higher PPI will fit more pixels into every inch of its screen size, giving you higher quality visuals. The higher the PPI of a screen, the smaller the pixels get, and newer monitors tend to have higher PPI (retina displays, anyone?). It’s why, when you got that shiny new Windows computer, your icons/windows/everything all seemed to be smaller than the ones from your old one.They show you that same 256x256 image in their new, smaller pixels, resulting in a higher quality, less pixelated look. More info about Pixel Dimensions, Image Resolution, and Screen Resolution at Adobe here.

Also, the Resampling section applies only to bitmap images. Resampling images (ie changing their pixel dimensions) is okay for vector graphics, since they're scalable essentially indefinitely without losing quality. But you already knew that, right? ;]

EDIT: Adjusted the wording in a couple of places in the hopes of making it a bit clearer.
EDIT2: Added links for more info, adjusted more wording.

Rosie the Riveter painting by J. Howard Miller, retrieved from the website of the Virginia Historical Society. This work is in the public domain in that it was published in the United States between 1923 and 1977 and without a copyright notice. More info here and here
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Daily Deviation

Given 2013-04-17
Detangling DPI by ~Raeistic ( Suggested by L0NES0ME and Featured by PirateLotus-Stock )
:iconshizuru1412:
Shizuru1412 Featured By Owner May 7, 2014
Umm, I have a question, does that mean most people who do digital art does the artwork with 300 dpi from the beginning?
And is there difference if say, someone started with 72 dpi but change it to 300 dpi in the middle?
Thanks for this, it's very helpful. :3
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:iconraeistic:
Raeistic Featured By Owner May 8, 2014  Professional Digital Artist
I think you might have missed the entire point of the guide ^^; The idea is that for artists who purely work digitally, their main concern should not be the DPI number, it should be the pixel dimensions. The DPI is just a number to guide them in what to make the pixel dimensions, should they ever want to print their image.
For example, I do digital artwork and rarely print. Therefore, I don't really pay attention to the DPI when I start a new art piece. I just set my canvas size in nice, even pixel dimensions, like 3000x2000 pixels or 1500x2000, etc. If you're setting the document size by pixels, the number that you put in the "Resolution" box when you're first creating a document doesn't alter the document at all. Go ahead, open up Photoshop and try it. A 1000x1000px canvas will be 1000x1000px regardless of the number in the "Resolution" box.
Only now, say you are about to create a new piece of art that you KNOW you want to print. You should then be thinking, "How big do I want to print this?" Say you decide on 18x24 inches. How do you know how many pixels to make your digital canvas in order to get a good quality print at 18x24 inches? That's where the DPI/Resolution number comes in. A "Good Print Resolution" is 300DPI, meaning you want enough pixels so that you can pack 300 into EVERY INCH OF PAPER. How do you figure that out? Simple math. You multiply the number of pixels by the number of inches (for example, 300 pixels per inch into 24 inches, that's just 300*24). But Photoshop and the DPI makes it easier for you than having to do the math every time. You just set the canvas size in INCHES and put the size you want to print it at (18x24), and then you tell Photoshop, via the "Resolution" box, that you want it in 300DPI, meaning you want to be able to pack 300 pixels in every inch. Then when that canvas is created, it'll give you exactly the amount of pixels you need.

As far as changing the DPI/Resolution in the middle, if you change it with the "Resample image" box checked, that's actually going to change the amount of pixels in your image. It will blow your pixels dimensions up and add more pixels, therefore making your image blurry, or it will subtract pixels and make your canvas size smaller, depending on whether you changed the number up or down. Generally, this alters the actual quality of your image, so it's NOT usually what you want, but it could be useful in certain situations. But if you do this you will always have to repaint over the blown-up picture all over again, unless you want a really blurry, crappy result. This might happen if, say, you made an image that was only 1000x1000 pixels, but then you realized that you need more pixels in order to print in good quality on a large piece of paper. You could then change the canvas size and DPI to match what you want, but you would have to paint the art all over again, because, sorry, your original piece of art was not high enough resolution. Technically, because you resized the image, now it does have enough pixels for the proper resolution, but it's "Fake Resolution," since it was automatically done by the computer, and it doesn't have Real Details, you get it? 
Anyway, alternatively if you change the DPI/Resolution in the middle WITHOUT the "Resample image" box checked, then the only thing you're going to be changing is what size the art piece will print at. So if you made a 1000x1000 pixel image, and then you printed it but it printed really big and really crappy and pixel-y, that probably means your "Resolution" was set to a low number. If you don't care what size it prints at, you just want it good quality WITHOUT having to repaint it, you can just change the DPI/Resolution in Photoshop but UNCHECK the "Resample image" box. That way, you're telling Photoshop/the printer to pack more pixels into each inch when it prints. It will still only have 1000x1000 pixels to work with, so the image will end up smaller once you print it, but it will look better and not pixellated. But either way, that's why it's typically a good idea to work with a large number of pixels, so that if you ever decide to print something, you can make sure Photoshop already has a lot of pixels to work with. The printing resolution, the DPI, can always be altered without consequence so long as you're not resampling the image, but the number of pixels is pretty resolute. Once you've painted something in a certain number of pixels, that's the amount of pixels it has, and it can't have more unless you repaint it.

Whoo, sorry for the wall of text, haha. I just wanted to make sure you fully understood everything.
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:iconshizuru1412:
Shizuru1412 Featured By Owner May 8, 2014
No, wall of text is fine, I appreciate it. :)
Thanks for your explanation, it's just that, I use sai so I don't get what a resample image is, here's what is shown on change canvas resolution option:
sta.sh/0vuqkw8y4eu
Should I checked the boxes? Thank you again for the reply, I'm not so sure about this kind of thing, generally I use dpi, since working with constant high dpi could end up with high file size... :/
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:iconraeistic:
Raeistic Featured By Owner May 8, 2014  Professional Digital Artist
Okay great, so, SAI still has the same functions as Photoshop in that regard, they're just named differently.

The top is your pixel dimensions (remember, those are what MATTER THE MOST, because once you paint something using a certain number of pixels, it's permanently using that number of pixels, and if you want to put it onto more pixels, you have to repaint it), the bottom is your resolution/DPI, and the middle is the size your image will print at. Right now you have it also set to pixels, so you can't see the results of the math. It's also technically locking you into the same thing that you would do if you UNCHECKED "Resample Image" in Photoshop. That means that, in this case, no matter how much you change the Resolution/DPI, it won't change either pixel dimensions. Try it and see, they'll stay the same. That also means that your file size will not get bigger, either. The pixels are what determine the file size of your document. At the bottom though, you'll see the numbers change. They're showing you what the printing size of your document will be at whatever DPI you change it to. That's ALL it affects.

Now, if you change the middle one to a physical printing size, such as "inches" instead, then if you change the Resolution/DPI there, it will RESAMPLE your image and add or subtract pixels, instead, in order to keep the physical printing size the same. If you want to not alter the pixels but you still want to be able to see how big your art will print in inches, then you can check the box that says "Constrain Pixel Size." Remember, your pixel dimensions are what's MOST IMPORTANT. The Resolution and the canvas size in inches only need to be theoretical, to help you figure out what your pixel dimensions need to be. 

The best thing you can do is play around with those settings and see how they all change each other. But like I said, I recommend you focus on the pixel dimensions, and not change them after you've started painting unless you have a very good, very specific reason or need to. With the DPI you can just mess around with it, keeping the "Constrain Pixel Size" box checked so that you don't mess with your pixels, and it'll help you see what sizes the amount of pixels you have will let you print at, at with Resolution/DPI/quality. (:
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:iconshizuru1412:
Shizuru1412 Featured By Owner May 9, 2014
Thank you very much, it really help!
I don't do prints, but I see some commisioners are doing their work with high dpi, and I was wondering wht's up with that.
Anyway, thanks for the lengthy explanation. :)
So, I need to keep the constraint pixel size checked, then?
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:iconraeistic:
Raeistic Featured By Owner May 9, 2014  Professional Digital Artist
Well, if you don't do prints, then there's no reason for you to need to change or even worry about the DPI, meaning there's also no real reason to keep "Constraint pixel size" checked or unchecked. When commissioners say they're working in High DPI, they either mean they have a lot of pixels, they're going to print whatever it is they're working on, or they're mistaken about what DPI means.
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:iconshizuru1412:
Shizuru1412 Featured By Owner May 9, 2014
I see, just to clarify, though. Is it better to keep 'constraint pixel size' checked or unchecked?
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:iconraeistic:
Raeistic Featured By Owner May 9, 2014  Professional Digital Artist
It's not really whether it's better or worse, it really depends on what you're trying to do. For example, say you made a piece of art at 500x500px, and you painted it, but then you realized that you wanted a bigger version for a website or something, and it needed to be 1000x1000px. Obviously, you need to NOT check "constrain pixel size" so that you can change those numbers to 1000 instead of 500. Of course, you'll need to repaint your image afterwards anyway, because otherwise it will be blurry, but still.

Alternatively, say you painted a picture at 2000x3000 pixels. That's plenty of pixels to print at a high resolution at a decent number of sizes. But then say you open up your image and it turns out it's in 72DPI and it's printing way too big and looking pixellated. That's just fine, you can go in, CHECK the "Constrain Pixel Size" box, and then just change the DPI to tell the printer to print at a smaller size and therefore a higher resolution, no harm done. 

So see, it's really just used for what you need. You just need to make sure that when you're changing those numbers, they're changing to do what you need, and not more (:
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:iconlawsdraws:
lawsdraws Featured By Owner Oct 28, 2013  Professional Digital Artist
Very helpful, thank you! 
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:iconraeistic:
Raeistic Featured By Owner Oct 28, 2013  Professional Digital Artist
I'm glad you found it useful (:
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