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I have not found anything using Google that actually contains a definition of this term. Most of the sites tell me that it affects the image quality, or something similar, and that more is better.

So when I see something like per inch, I expect it to be measuring something physical, since it is "per inch". I know it's not measuring anything related to a print (at least I hope not) since the digital image has little to do with the physical representation in a print. The only thing I can think of is that it is measuring the number of pixels per linear inch on my camera sensor. That would explain why it shows up in image software as an attribute of the photo.

Is that right?

If it is, then it seems to me that the number of pixels is more important than the PPI in determining how large a print you can produce from a given digital image. Is that right as well?

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You might find this answer useful: photo.stackexchange.com/a/26901/6508 –  Blrfl Sep 12 '12 at 17:23
    
Also, related: What does DPI mean? –  mattdm Sep 12 '12 at 18:57
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4 Answers

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Pixels per inch (PPI) is strictly concerned with the output resolution, and the value attached to a particular image file is only a suggestion.

The image file itself is composed of a set number of discrete pixels, which are to a picture what atoms are to molecules. Unless and until you change the number of pixels, the image quality when the image is displayed at the same size on the same output medium will remain the same. If you tell your printer to print a given image file at a certain fixed size (say 8x10 inches, for the sake of argument), it doesn't matter in the slightest whether the resolution tag in the image file says it's a 72PPI image or a 300PPI image; if the file is otherwise identical (that is, if it contains the same pixels in the same colour space), the prints will be identical. In the same way, a 1000 by 1500 pixel image will display at the same size on a web page (barring CSS or HTML scaling, which can stretch or squish the image, usually with a significant loss in quality).

Yes, you do generally want higher PPI values for output when you can get them (up to a limit -- if the resolution is higher than the output device can support, you'll actually lose quality), but that means printing larger image files at smaller physical sizes, or displaying the image on a screen with more tightly-packed pixels (which amounts to the same thing). You may need to calculate the appropriate number of pixels for output, which does involve PPI, but that is usually to match the output device's resolution, the desired print size, and the image file.

(If you want to print to a dye-sub or "light-jet" printer, you'll get the best quality if the pixels in your image match the printer's pixels. If the printer prints at 400PPI, and you want your image to print at 16"x20", it ought to be a 6400x8000 pixel image, otherwise the printer will have to scale it for you, and that doesn't always go well.)

The PPI value embedded in the file is used mostly in pre-print processing applications to determine the image size. That includes things like desktop publishing apps (e.g. Adobe InDesign), third-party raster image processors (RIPs), and multi-up gang printing applications that have to figure out how best to minimize paper waste. 300PPI is pretty much industry standard for prepress work, and image sizing is usually done in an image editing application to minimize scaling artifacts and optimize sharpening, etc.

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PPI is usually referring to your conversion size (output resolution you choose) not your sensor capability, DPI is usually the term for output (printing or screen) but same concept

Sensor pixel size, count, gain(ISO) (among other techno-porn specs as well as technique) determines absolute the capture quality. Never heard of PPI being used to express capture.

If I have 10 megapixel camera with x pixels by y pixels then import it to CS the PPI determines the native size of the image.

So 1000x1000 pixels native pixels converted to a 300 PPI resolution would mean a 3.3 by 3.3 inch image.

The definition and settings vary depending on native image resolution and desired quality and use (print vs. web)

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OK, so I guess I was completely wrong in my guess, which is fine. And what you say makes sense, I guess, except that I just don't understand why Photoshop and other tools tell me that a particular image has some PPI value. It sounds from what you say as though PPI is a value you can choose independently of the image (a viewpoint which is supported by the answer Birlf pointed me to in his comment to the original question). –  rogerl Sep 12 '12 at 17:27
    
When you import an image into CS and it asks you for a PPI (is this what you are asking about?) if so it's just letting you determine quality/size ahead of time and making a default suggestion. Typically it will set 240-300. You can also output(export) any PPI you wish for use on the web etc. –  Idistic Sep 12 '12 at 17:35
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DPI is not the same as PPI. DPI refers to dots per inch, which is only rarely the same as PPI (i.e. with dye sub printing). DPI generally counts the number of sub-pixel elements per inch, and in the case of screens and print is higher than PPI. For example, you can print at 150ppi, 300ppi, and 600ppi on a Canon ink jet, however the DPI is always 2400x1200 or possibly 4800x2400. When the LCD screens on a DSLR are rated at something like 921k "dots", those dots are not pixels, but the RGB subpixel elements. The interchanged use of PPI and DPI online only really leads to confusion. –  jrista Sep 12 '12 at 17:44
    
@jrista I said it's the same general CONCEPT but related to output devices, I also included a link to the definitions. –  Idistic Sep 12 '12 at 17:46
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Pixels Per Inch is the proper term to use when referring to how many pixels...whole elements that represent the smallest component of an image...per inch are rendered on any given device...be it a computer screen or print. The term DPI should only be used when discussing sub-pixel elements, such as the RGB emitters in a screen or the ink droplets produced by a printer. –  jrista Sep 12 '12 at 17:48
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If you look at your screen you'll notice (or not) that it is made of tiny points of light, each point is called a pixel, so Pixels per inch is how many points you have in a inch long line.

This affects the quality in a way that if you only have 4 or 5 ppi, you'll be looking at squares and not soft looking images.

iPhone and the latest smartphones have about 300 ppi, that's near what a human eye capacity, so it makes you feel like it is a sharp/smooth image without any strange squares or micro squares.

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Pixels Per Inch, or PPI, is a measure of density or resolution (resolution in the sense of fineness of detail, which again refers to the density of information, rather than dimensional resolution, or the physical parameters). Not all devices that can be used to view photos have the same pixel density. Computer screens tend to range from 72ppi to 109ppi in terms of screen pixel density. Relatively speaking, this is fairly low. Newer hand-held devices have screen densities anywhere from 135ppi to as high as 360ppi. Such devices would be the likes of smart phones, tablets like the iPad, Nexis Android tablets, and forthcoming Windows 8 tablets. Printers can range from as little as 100ppi to as high as 720ppi for ink jet printers.

The value of knowing PPI, or pixel density, is in knowing how large to export your processed photos for viewing on various devices. If you intend to produce images for viewing on a standard computer screen, you can generally assume around 72-100ppi, and exporing at 4x6 to 5x7 size from a physical screen space standpoint will usually produce photos that are easiest for potential online viewers to observe. If you intend to produce images for viewing on portable hand-held devices, you should target a higher resolution, around 150ppi would be better.

When it comes to print, knowing what PPI to print at can be extremely important, as it can greatly affect how large you can print. If you intend to print your photos at 8x10, you are probably interested in printing at least 300ppi or higher...even as high as 600ppi (Canon and HP printers) or 720ppi (Epson printers). Most photos, excepting some forms such as architectural photography (and even in some senses architectural photography) contain more detail than can be resolved in a 300ppi print at 8x10 for normal viewing distances, so printing at higher resolutions is often necessary to fully realize the detail you may have in a photograph. When printing at "native" print size...or the a size relatively close to the native image dimensions of your photographs...which tends to fall in the range of 11x16" to 17x22", you will probably want to print around 300ppi. At normal viewing distances, this should capture all the fine detail in your photo well enough to be observed by most viewers, and will usually require little to no scaling of your image to print (meaning you'll get the best results at the largest native size.) Finally, if you intend to enlarge your photos, you will probably want to print at a lower resolution. Average enlargements, which might be up to around 24x36" or so, can often be done at around 150ppi, however 200ppi tends to generate slightly better results for average viewing distances in normal papers, and in many cases even canvas. For significant enlargements, up to 40x60, you will want to print at 100ppi to 150ppi, with the expectation that such photos will be viewed at a distance of at least several feet if not more.

Understanding pixel density, or PPI, and how it relates to the physical dimensions of the images as observed...in either print or on screen, is important to producing the best quality "final output." Exporting or printing an image at an improper pixel density will produce less than ideal results, often softening detail that your viewers could otherwise readily observe.

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