PPI vs. DPI

Ok, I'm sure I'm going to sound amateurish by asking this and looked

upon with scorn (just kidding), but I need to know the difference

between DPI and PPI. I've Googled it but can't really make sense of

the explanation, so if someone can break it down for me in more

understandable terms, I'd appreciate it.

 

I recently did my first commercial shoot and they are asking for

photographs of approximately 4 by 5 in size at at least 300 dpi.

When I resize the original image (they were shot very large) to fit

the approx. 4 by 5 size, the PPI in photoshop is around 70. What

does this mean? Is there a corresponding DPI number or are they

completely separate? To me, it seems like they're two different

things, and that the printing department that's asking for these

specifications might be confused. However, I'd gander that it's

actually me that's confused.

 

If anyone can help me make sense of this, I'd greatly appreciate it.

Thanks in advance for your help!

 

Kristi

In the case you're referring to, people are usuing dpi and ppi interchangeably. It may not be technically accurate, but it is very common to use the term dpi when discussing file resolution.

 

The distinction people usually make here is that dpi referrs to the actual number of dots laid down on the paper. In other words, dpi is really an output setting that isn't a reflection of the resolution in the image itself. For example, inkjets often use dpi settings of 1440, 2880, or 5760. These don't necessarily correspond to the file's ppi, but rather are a reflection of the many small dots laid down to make up each pixel of color.

 

PPI, in turn, is the number of discrete pixels of (possibly) different color in the file. This is the image's resolution. What they are looking for is a file that is at least 1200x1500 pixels across. If you are resizing the image to 4x5 and it is 70ppi in PS, your file is 280x350 pixels across and will not print well at 4x5. If you shot it large, then only having 280 pixels across seems unlikely. Hope that helps a little.

These two terms basically mean the same thing. Dots or Pixels per inch. If they require a photograph to be 4x5 at 300dpi then they are asking for images that are at least 1200x1500 pixels.

They definitely do not mean the same thing. Listen to Marshall.

Without getting overly technical about this, think of it this way. Electronic devices like cameras and LCD displays have discrete, individual, picture elements (pixels) that either receive (cameras) or display (LCDs) light at each pixel.

 

Printers (output devices) display images by making the images into dots. The more dots per inch (DPI) the finer the image is rendered.

 

It's really a bit more complicated with computer output devices as the original definition of DPI relates to printing presses and how finely a continuous tone image is screened. You really had xxx dots per inch that you could count if you used a magnifying loupe on a printing plate or in the final printed image. You can see this on a laserjet printed photo image, as those printers use physical dots per inch.

 

Today, inkjets and LighJet type printers really don't print in "dpi" but, it's a convenient way to summarize the amount of data being sent to the output device for it to interpolate into the final image.

 

As to your 70 dpi question versus 300 dpi required output, you don't give any information as to the original image size in pixels per inch (2000 x 3200 for example) other than saying "very large" - which is meaningless.

 

Your controls in the resize image menu should allow you to set the dpi, and the image size. Uncheck the resample box, and resize the image to 4x5 inches and see how many dpi it shows. If it's not 300 then you need to resample the image. Check the "resample" box - use "bicubic sharper" for the image if it needs to be reduced in dpi -from 600 -> 300 for example. Use "bicubic smoother" if it needs to be resampled upwards 180 -> 300, for example.

<I>These two terms basically mean the same thing. Dots or Pixels per inch. If they require

a photograph to be 4x5 at 300dpi then they are asking for images that are at least

1200x1500 pixels.</I><P>Technically dpi and ppi are not the same thing,

But yes they are asking for an image that is 1200 x 1500 pixels. (4x300 pixels = 1200

pixels, 5 x 300 = 1500pixels.<P> What camera are you using? Go back to your original

image. In Photoshop, Go: Image> Image Size > and uncheck Resample Image. this will

link resolution (PPI) and height and width. enter 300 as resolution. What size in inches d

oyou get as height and width?

<I>"Electronic devices like cameras.............."</I><P>

 

LOL! Don't you mean "Digital cameras?" Can't find the battries in my Leica M3.

take your original image unresized and do the following:<br>

first using the image resize tool, uncheck the resample box at the bottom. <br>Then, and only after unchecking the resample box, set the resolution box to 300 pixels/inch and press OK. <br>Next save your image. <br>Next open it again and using the image resize tool do the following: <br>check the resample box <br>set the resample type to bicubic. <br>check the constrain proportions box <br>

 

Next depends on whether original image was portrait or landscape:<br>

If image is landscape then:<br>

set image width to 1500 pixels (height will change automatically). <br>if height is now less than 1200 then set height to 1200 (width will change automatically). <br>press OK.

<br><br>

If image is portrait then:<br>

set image width to 1200 pixels (height will change automatically). <br>if height is now less than 1500 then set height to 1500 (width will change automatically). <br>press OK.

<br><br>

 

you will now have an image which needs either width or height cropping(using the crop tool) depending on the original image width:height ratios. i.e. if original w:h ratio wasn't 5:4 or 4:5 then you will need to crop to that ratio.

<br><br>

after this use the image resize tool to check all is OK. just change the document size to read in inches and it should say 5x4 inches with the dpi setting at 300dpi.

Thanks for everyone's input. I'm shooting with a Canon 20D set at Large/Fine, so if I'm thinking correctly, that's a 3504 x 2336 pixels image, right? If that info helps out any more towards a solution, let me know.

 

When I get home tonight (at the day job right now), I'll open up Photoshop and try out the solutions a couple of you have provided. I really appreciate everyone's help with this and for not making me feel like a complete gomer (dork).

 

:) Kristi

"My red dog can read and read a red book yesterday". A perfectly clear sentence to a native English speaker but hard to decode for someone else. Context is the key. If someone is talking about image files in, or being made by, cameras or computers or scanners then ppi is what is meant even if dpi is used. Inkjet printers create images with dots of ink, up to thousands per inch. Epson printers (mine at least) prefer files at 360 ppi. So I might say, if I'm being a bit casual, "my Epson prints at 2880 dpi and likes 360 dpi images." What I mean in the second instance is ppi. Each pixel is printed with many many dots thus creating the impression of a continuous-tone photograph.

 

Two other points: pixels have no size at all; they are just data. The physical size of an image (e.g., 4x5 inches) is just a instruction to the printer like telling the printer that a landscape (horizontal) image is on its way. But print size and ppi definitely interrelate. Open any image and play with the Photoshop Image Size dialog. Leave the "constrain proportions" box checked and play with the "resample image" checked and unchecked. You'll see what's happening.

 

If resampling is off and you double the document size, the ppi is cut in half. That is, you're stretching x number of pixels to twice one length of the document, there will half as many per inch as there were. That is OK only if there is still high enough ppi for your printer--360 ppi in my case. Conversely, with resampling on if you double the document size the ppi stays the same but the file size grows four times larger. Where is that extra data coming from? Photoshop is creating more pixels as best it can. That too may be OK depending on the print size. In your example of 4x5, that is so small that up sampling may not be noticeable. But with your example of printing to 4x5 inches with only 70 ppi--well, there is not much hope for such an image. So I am not sure what you mean in saying "they were shot very large." What happens if you up sample? What are the original file pixel dimensions?

"Gomer" is a funny word. And it's even funnier to see it used and translated...

I can't think of an easier and more concise way to illustrate the difference between ppi and dpi.

 

If you have an image that consists of only 1 green pixel. That's it. The whole image is one pixel big. Then you print it on a printer that doesn't have a green ink cartridge, but it has blue and yellow. The printer will then print one blue dot, and one yellow dot in order to get a green dot. So, your file was 1 ppi, but the printer had to print 2 dpi to render it correctly.

 

Marshall gave you a very good answer to your question.

Hi, Marshall--

 

Were you aware that "Gomer" is also a highly technical medical term? It refers to those hypochondriacally inclined folks who employ the E.R. to treat their imaginary illnesses. It stands for "Get outta my emergency room." As in, "Hey, we got a gomer in number 3. Give him a placebo and get rid of him, stat."

 

Sorry I don't have anything constructive to add to this thread, but my esteemed colleagues above have addressed it quite thoroughly.

Ok- so if I did this right, my 4.2 by 5.125 image at 300 dpi/ppi should be 1260 by 1538 pixels, correct? Using the advice above, that's what I got :)

 

Thank you, thank you, thank you everyone!

Bernard - Actually, I had heard that. Married to a doctor. But it's funny to include it here anyway... Glad we helped, Kristi.

"LOL! Don't you mean "Digital cameras?" Can't find the battries in my Leica M3."

 

My, haven't you demonstrated a keen sense of the obvious...LOL back at 'ya

I may have missed, but I don't think the origin of "dpi" was explained:

 

"dpi" was used long before there was any sort of electronic involvement in print-making. It referred to the size of dots of ink used in lithography (and photo silkscreen).

 

Lithographers traditionally used halftone screens (essentially optical masks) to convert continuous tone photographic images into dots of various density (the halftone screens determined the density of dots), simulating the tone using only one (in the simplest case, as in newspapers) very black ink...or only four (in the case of full "four color" as in typical magazine ads).

 

In magazines the highest "resolution" used to be 300dpi, and lower quality reproduction, such as cheaper catalogs, was typically 75-100dpi..big dots.

 

Black dots were all literally the same color, but would create a impression of dense black when they were clustered together closely, and an impression of nearly-paper-white when they were sparse...this was the magic of the halftone screens.

Kristi Zike seems to have resolved her immediate problem, but John Hennessy wrote:

 

"... pixels have no size at all; they are just data."

 

This isn't exactly correct. Check your favorite dictionary and you will find pixel definitions like " The smallest image-forming unit of a video display", "n : (computer science) the smallest discrete component of an image or picture on a CRT screen (usually a colored dot)", and so forth. There is nothing imaginary about a pixel and they actually have a size and they also can have a shape.

 

The thing is, however, that the pixel size and shape can vary even for the same image because they depend on something outside the image file. For example, my screen is currently set at 1280 x 1024 pixels but that can be varied. I am no computer monitor gear head so I don't know how these things work exactly, but I seem to recall that the CRT required three electron guns to produce separate red, green, and blue dots that combine to make a color on your CRT or TV screen. I am even less sure how the LCD screens work, but somehow light is transmitted through glowing colored elements to produce the screen I actually see.

 

So here is my summary of all of this:

 

1. A pixel is the smallest element of a digital image and may be fully described without reference to size or any dimensions at all.

 

2. A pixel rendered on a CRT, LCD or other computer device is rendered as a number of pixels horizontally x the number of pixels vertically always without reference to inches or other linear dimensions. The characteristics of the CRT, LCD, etc will determine the actual, physical dimensions of the resulting image.

 

3. An image printed on a hard copy device requires physical dimensions. These dimensions are not provided by the pixel information, so they must be supplied by the user prior to printing. The convention is to supply a measure of pixel density i.e. the number of pixels to be rendered per inch (or centimeter, etc). This flexibility results in nearly limitless variability in the precise size (width & height) the image will be rendered. Due to the peculiar history of printing this measure is usually called "dots per inch" or DPI, but of course it is really refering to "pixels per inch" or PPI.

 

4. The printing world has a second meaning for "dots per inch" and this other definition refers to the number of dots of actual, real colored ink needed to render a pixel of a specified color and intensity (e.g. light green) taking into account the colors and intensities of adjacent colored dots. Due to marketing strategies grounded in a "more is better" philosophy, printer manufacturers routinely refer to printer resolutions of 1440, 2880, or 5760 dpi. These resolutions do not refer to the image resolution, but the the fineness of the printing mechanism.

...actually, 4000ppi or 5400ppi doesn't refer to the fineness of the printer, it refers to the purported resolution of the scan. Which isn't the same as sharpness, even if true.

 

There's a 4800ppi scanner that is less photograpically sharp than 4000ppi scanners , there's a 5400ppi scanner that resolves a measured 4000ppi but builds much bigger files than its advertised and measured 4000ppi rival. Photographic sharpness (detail rendering), advertised ppi, measured ppi, and file size are not necessarily related.

What matters is you know what the other chap is meaning. It is abit like argueing over formal definitions for poop and dung. If you step in either; and say I just stepped in dog poop or dung; most folks know what you meant. Scanners have used "dpi" in their specs for many many decades; going back before some so called Photoshop experts were even born. Early greyscale scanners for photoshop were often built with fax machine scan bars; and built built the same folks too. Every scanner I have owned or used accept one has the resolution units in the software as "dpi". This really seems to bother new folks to digital; who want to rename old terms; used in scanners; in the patent literature; in formal government specs for scanning. Almost all scanners today usually have "dpi" setting in the software; and never "ppi"; this has been true since before Photoshop was invented. If you want to be clear; jsut say "I used a 2700 dpi setting" when I scanned the 35mm slide. This is clear to all. <BR><BR>Files in photoshop are sent to printers with a "X by Y inch size; at ABC pixels per inch" This is very clear to all; and it is what Photoshop shows too. This might be a 8x10 inch image at 300 pixels per inch. <BR><BR>With a printers specs; a giant poster printer might be a "600dpi device; or a 300dpi device if older" Usually these will accept about half these values for the max sharpness; ie a 300 ppi image is sent to the 600 dpi device; a 150ppi image is sent to the 300 dpi device. <BR><BR>In older fax machines; they used dpi for the scan bar resolution; and lpi/lines per inch; for the paper feed direction. this goes back at least 3 decades and more. When photoshop scanners first appeared; some used lpi for the light bar travel direction; and dpi for the scan bar pitch. Marketing later made both axes "dpi"; when scanner were being sold more to amateurs. <BR><BR>A pixel can be square; a rectangle; a hex; a triangle. Many older scan bars have sensor elements that are round; they "math model" of the input is then a square in photoshop. <BR><BR>In many service bureaus; dpi is used as a scanner resolution setting; ppi is used for files sent to a printer. Folks use what has been used before photoshop was invented. <BR><BR>make sure you know what the other person is talking about; many folks are new; and seem confused.