# DPI does not matter?



## UUilliam (Jan 18, 2011)

Okay, here is the thing!
Today, In college we were to write an outcome on the correct DPI for print and web.

I already know it is 300 and 72 but decided to do a little research, I found a website that stated the DPI simply does not matter, when on screen atleast.

To test this theory, I changed a 96dpi image to 1dpi
True to form, still looked perfect!
I then printed the 1dpi image and compared it to the 96dpi image, no difference at all!
(i must add, scale to paper was checked.)
But at the end of the day, IF you are going to print an a4 print, you can scale it to fit no matter what DPI you use in that case!

This also leads me to question, If most people set their dpi to 72 for copyright reasons (not to have duplicates printed.)
you can simply go into photoshop and change it back to 300 (if you need 300 dpi to print.)

this confused me!
I have decided, for the sake of the college work, just to go with what I have been told, but for personal education, I would like to know, Does DPI REALLY matter?


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## clarinetJWD (Jan 18, 2011)

The DPI setting in Photoshop, no (at least not that I've found).  DPI, yes.


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## Josh66 (Jan 18, 2011)

Dots Per Inch.

Without the 'inches' part, it's meaningless.

It only matters if you're talking about a print, of a specific size that you can measure.


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## UUilliam (Jan 18, 2011)

^ I tried printing the documents too, they both come out the same
but it just seems that reducing the DPI, it just scales the image up?
so I just pressed "constrain to page" and it makes it the same size image...


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## Josh66 (Jan 18, 2011)

All that is changing is what the viewing program thinks the print size is.  The actual pixel dimensions of the image stays the same.

Therefore ... it doesn't matter.  If you want to reduce the resolution to prevent people from printing it, you have to change the actual pixel dimensions of the photo.


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## Big Mike (Jan 18, 2011)

I think you are confusing DPI with PPI.

DPI is a printer function, not a digital image setting.  Although sadly, the two are quite often used interchangeably, which leads to a lot of confusion.


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## OrionsByte (Jan 18, 2011)

Think of it this way.

Let's assume you have a photo that is 8x10 at 300dpi.  That's 2400x3000 pixels.  If you change the DPI to 72 _without changing anything else_, the pixel dimensions haven't changed, but Photoshop will say it's roughly 33" by 42".  All the information is still there.

If you want to downres the photo so that people don't yank it and print it on their own, you need to change the DPI to 72 and change the print dimensions back to 8x10 (or whatever you want to use).  Try _that_ and then try printing it.  You'll see the difference.


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## PASM (Jan 18, 2011)

The number comes from years ago when Macs had tiny b&w screens which were approx 72 PPI.


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## KmH (Jan 18, 2011)

Math. Math. Math. And. _*Dpi and ppi are not interchangable terms*_ because they are not the same.

*DIGITAL IMAGES DON'T HAVE DOTS*. Pixels are square, dots are round.

A 3000 pixel x 2000 pixel image displayed on a 1024 px x 768 px computer display cannot be seen all at once.
It's bigger than the display.
Not all computer displays are at 72 ppi. Many displays approach 100 ppi today.
To find out the exact ppi of a computer display, measure the display's width and divide it by the native horizontal resolution of the display.
My display is 18.625" wide and is at a horizontal resolution of 1600 pixels, so it is 86 ppi.
*Key point*: _Monitor pixels are not the same size as digital image pixels, unless you are viewing the digital image at 100% zoom_.

*DIGITAL IMAGES DON'T HAVE DOTS*. Pixels are square, dots are round.

The size an image will print at, is the pixel dimensions of the image divided by the *PIXELS-PER-INCH*. You vary the image ppi setting to alter the size of the print.
An image having pixel dimensions of 3000 px by 2000 px that is set to 100 ppi will print at 30 inches by 20 inches because,
3000 pixels divided by 100 pixels per inch = 30 inches (note: the pixels cancel leaving only the inches.) and 
2000 pixels divided by 100 pixels per inch = 20 inches.

The same 3000 px x 2000 px image set to 300 ppi will parint at
3000px / 300ppi = 10 inches and
2000px / 300ppi = 6.67 inches.

Inkjet and press printers print round dots to render square pixels. How many dots it takes to render a single pixel varies. Some inkjet print heads have as many as 175 nozzels peer print head and use six colors (six print heads) so it can take as many as 1050 dots to render a single pixel.

Dpi is a term used before there was digital images. Photoshop CS5 uses the term *pixels* in the Image and Canvas dialog boxes.


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