The DPI myth
#1
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So, what’s the deal with the DPI thing? Well, it’s a hard to explain concept - DPI was created to solve an abstract problem, so the whole reasoning behind it is.. as you could guess, quite abstract. So, which problem you ask? Simple: a pixel does not have a physical size.

What?

Ok, hang on.
What is a pixel? By definition, a pixel is the smallest graphical unit we have on computers. So yes, it has a virtual size. You aren't going to get smaller than a pixel, nope. But that unit does not exists in the physical world - so it doesn't have a physical size. It exists inside your software, much like a byte of any other information, but it does not have any physical attribute to it.

Let that sink in for a bit: a pixel is a virtual-only unit.
Got it? Okay, good.

When you display an image on your screen, the pixel does have a physical size - the one your display’s attaches to it. But you can have a different display, with a different size/diagonal, displaying the same image with the same resolution: the image will appear at different sizes because of that.

Example: you can have a 13” laptop with full HD resolution. That is a 1920x1080 pixels display. You can have a 50” full HD TV that will display the same 1920x1080 image - thus making it look much bigger. Because you attributed different physical size to the same amount of pixels.

This amount of pixels is called resolution. The more pixels you have the more pixels you will have compressed in the same area. A retina display iphone has as much pixels in it as a lot of laptop displays that ranges from 17” to mid 20”s. So it’s a very high resolution display because it packs a lot of pixels in a small area!

So yes, when you think about it, a 50” full HD TV is not that very high resolution if you can have the same amount of pixels in it as something as small as a 10” display. What is happening? You’re making those pixels very big so it can cover all of the 50” area of the TV screen. usually, that leads to a fuzzier image. You start seeing each pixel, and it starts to look like a patchwork of small squares.

So!
Where does DPI comes in? It’s simple. When you have to print an image, you need something to do the conversion of pixels, that have no size, to something that will print into a predictable size. So DPI is just an unit created to give printable size to digital images.

It literally means how many pixels (called Dots back then) you’re going to squeeze into a square inch. That’s it.

Now, where the confusion arises: DPI means absolutely nothing to your computer. It is only ever used when you’re going to print something. Everything else your computer calculate as pixels. It makes absolutely no difference if you have a very small pixel size file but sets the DPI count to 600 in Photoshop.

Why? Because for high resolution images you need a high quantity of pixels. That is resolution, remember? The amount of pixels. If you’re working on a 1000x1000 pixels image, that is not a lot - so when you try to print it, you will have to stretch your pixels to cover the area, you image will look fuzzy and you won’t have a lot of details - simply because you’re making your pixels too big.

So this is it: a high resolution file needs to have a lot of pixels - so when you print it using high DPI, like 300, you will still get a big print out of the digital file.

300 DPI is usually a good print resolution for something you will hold in your hands - like a book, or a magazine. Some art oriented publications requires a bit more resolution (400 usually for those). So you set the DPI you want in photoshop and then you set the actual physical size of the image, and photoshop will generate a file with enough pixels for you. Magic!

Banners and other printed media that are meant to be seen from a distance doesn't require so much. Actually, a big billboard can have DPI count as low as 20. Yep, twenty, not a typo.

If you don’t have any idea if you will ever print this but you’re just starting in a portfolio piece, you may want to make sure you will be able print it after, so put enough pixels in there for a print. This will obviously make the file bigger and harder to work on, so don’t go to crazy with it, specially if you don’t have a good rig. It is a good idea to take a bit of time and browse through photoshop presets for new files. See how the pixel size and resolution relates to the print size. Input some custom values in there too, see how that works out.

Personally, I have a computer good enough to work on big files, so all my files have at least 3500 pixels on the small side. This is enough to print banners if I ever need to. Keeping the smaller side to at least 2500 pixels and you should be good for most printing demands.

As a rule of thumb, if you want to print an image and you’re having to use a 1 pixel brush for details, probably your image isn't big enough. So make sure you check this as soon as possible into the workflow - sharpen filter won’t save your ass :p

So!
This was a long winded explanation, and since it’s such an abstract subject, it may not have made any sense to you. I will leave this other links here, to other texts trying to explain DPI in different ways. Hope they help if I didn't manage!

http://qinni.deviantart.com/journal/Canv...-352124029

http://raeistic.deviantart.com/art/Detan...-364315038

http://rahll.deviantart.com/journal/How-...-220530764

If anyone have any questions, fire away. I’ll try my best to address them.

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#2
Great explanation! I've always had a bit of a block when it comes to understanding virtual stuff.

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#3
Good read Ursula! :)

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#4
Excellent work Ursula-!

Comprehensive and fun to read. :D

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#5
Thanks, Ursula. I've always wondered why graphic designers and print-oriented people were obsessed with the DPI, now I know why.
Great explanation.

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#6
Fantastic write up. You've simplified such a vauge topic (at least to me). Very easy to understand now, thanks for clearing that up! :)

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