Understanding and Calculating DPI/PPI

[Jason Dunn]

A few weekends ago I was doing some 13 x 19" prints on my Epson R1800 printer for my brother and the issue of resolution and PPI came up. He shot his photos with a Nikon D70, a six megapixel camera with a resolution of 3008 x 2000 pixels. I was using ACDSee Pro to print the images, and it offered the option to resample the DPI to higher values - 200 DPI, 300 DPI, 600 DPI - but I didn't know the DPI of the current image. DPI and PPI can be difficult to understand, but you can think of them this way: DPI is the dots per inch of a physically output product - a print - while PPI, pixels per inch, is the measurement of an on-screen image. <br /><br />I tend to think of pixels like paint when I print them out - the amount of pixels is like the amount of paint you have. If you have a small amount of paint (say, 2 megapixels), that's enough to make a small painting (4x6" print) - but it's not enough paint to cover a huge canvas (13x19" print). The most paint (pixels) you have, the greater detail you'll have in an image, though there's a point of diminishing returns where having more pixels won't make your 4x6" print look any better. By the same token, there's a certain amount of paint you'll need in order to achieve good results on a canvas. In most cases, that's around 300 dpi (though I've seen great results at 200 dpi).<!><br /><br />Digital photographer Matt Spinelli has a <a href="http://www.mattspinelli.com/ppidpi.html">great explanation about PPI/DPI on his site</a>, but more importantly he, has a <a href="http://www.mattspinelli.com/ppicalc.html">PPI Calculator</a> that can be used to figure out what PPI your image will be at. In the example above with my brother's images, the six megapixel image printed at 13 x 19" in size would only give us a DPI of 153. That's not quite high enough, so we used ACDSee Pro to up-sample the image to a higher DPI. There's no replacement for having more pixels to start with, but the results were still quite impressive when viewed at a realistic distance (no closer than two feet). Bookmark that PPI calculator and take the guesswork out of your printing - you can even use it if you're uploading to an online printing service (though they often will warn you if the image isn't high-resolution enough).

[Citezein]

I think your definitions are a bit misleading in this case. If your image is 3000 pixels by 2000 pixels, there isn't any sort of on-screen physical measurement. It is simply that many pixels. The "per inch" of either PPI or DPI only comes into being when printed.

So when you go to print an image, you need to figure out how many inches you can print, given your number of pixels, as you said. At 300 PPI, the aforementioned image will give you a 10" x 6.66" printed photograph.

Here's where DPI comes in. Printers will print more than one physical ink dot per pixel. My Epson R800 has a 1440 DPI output capability. Thus, assuming I am printing at 300 PPI, each linear inch would actually contain 1440 ink dots, representing the 300 pixels.

Back when printers printed one dot per pixel, DPI and PPI were essentially the same thing. DPI is almost never needed at this point, although many people use it to refer to PPI when printing. On screen images have a resolution such as "X pixels by Y pixels" but there really shouldn't be any "per inch" unit used.

Clear as mud, right?

[Jason Dunn]

It's funny, I didn't post this for weeks and weeks because I knew that someone was going to tell me I was somehow wrong. :lol:

There is on-screen physical measurement based on the size of the LCD panel and the resolution it's running at - every display has a PPI value:

http://www.raydreams.com/prog/dpi.aspx

My 24" LCD monitor running at 1920 x 1200 has a DPI of 94.3.

All of this really only matters when you're talking about printing though, so ultimately the DPI of one's monitor doesn't have any bearing on the quality of a print.

[Citezein]

I have the same 24" that you do. And yes, PPI does exist when measuring screen resolution density, but that's generally a pointless exercise. What the discussion needs is further information on why Photoshop and most editors will say that an image is 72 or 300 dpi. I think the link you provided does a decent job at that. But hopefully people get the point that there are two main principles to understand:

1) The resolution of a digital image has no intrinsic "per inch" associated with it.
2) PPI only comes into calculations when printing.

[hippydpi]

I think of it this way. DPI is a lot like Digital Zoom. Both terms are used by manufacturers to deceive customers into thinking they are getting more than they really are. It's not breaking the law, but it's not being honest. DPI has been used by scanner and printer manufacturers so often and for so long that people just take the deception for granted.

[Citezein]

My 24" LCD monitor running at 1920 x 1200 has a DPI of 94.3.


Technically, our lcd monitors have a PPI of 94.3 and a DPI of 282.9 since each pixel is represented by three sub-pixels on an LCD. But that's just splitting pixels... 8O

[marlof]

Thanks for trying to shed some light on a subject I can never grasp. Frankly, in my experience with a pixel challenged DSLR (5 mp), I don't calculate. I try to be as light on my post processing as I can (to avoid unnecessary bruising), and convert to 16 bit TIFF. I feed that file to Qimage (http://www.ddisoftware.com/qimage/), and have that one do all the hard work in upsampling and stuff like that. I love the images coming out of it, so I'm a happy camper. For many things, my 5mp give me enough to print nice 13x19, but images with tons of small detail (foliage etc) most certainly could use some more resolving power.

[hippydpi]

My 24" LCD monitor running at 1920 x 1200 has a DPI of 94.3.


Technically, our lcd monitors have a PPI of 94.3 and a DPI of 282.9 since each pixel is represented by three sub-pixels on an LCD. But that's just splitting pixels... 8O

Note that you are talking about maximum PPI. If you tell the OS to use a lower resolution than maximum then things get more complex.

There could be some interesting pixel splitting going on when OS X 10.5 Leopard is released. It's possible that the OS will be able to respect physical sizes in inches, points, mm, or what have you on the screen over pixels.

This would make screens more like printers, where image and print sizes that are specified in physical units print the same size no matter what size the paper is and what the dpi of the printer is.