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dpi... ppi ...resolution confused

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Old 07-03-2005, 06:43 PM
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nebgranny nebgranny is offline
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dpi... ppi ...resolution confused

Well here goes another silly question. Is dpi and ppi the same? What about resolution. Obviously I am trying to learn scanning techniques and settings. My friend just scanned some pictures I want to use in a photo montage for her. She is scanning at 72. They will not print out well at that size will they? I will be printing the end result at a 13x17 size. I have a large format printer. I will be using one picture of the church as the bg and lower opacity , and place other pics on top.Can someone HELP Please ?? ...Neb
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Old 07-03-2005, 07:17 PM
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Doug Nelson Doug Nelson is offline
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Two answers:

Files are made up of pixels, prints are made up of dots. Pixels are square and touch, dots are round(ish) and may or may not touch. So if you're interested in the resolution per inch, it's pretty obvious which to use. A 72ppi file can (and should) be printed at 300dpi or more.


Specs, documentation, and even user interfaces are written by marketing folk who can be lazy (or simply ignorant). It's much easier for them to rationalize that pixels are kind of dotlike, but dots aren't very pixellike, so they slap DPI on the whole shebang and figure tech-support (or user forums) will fix it in the long run. Or, more charitably, it's a case of if you know the difference, interchanging the terms is harmless, but if you don't know the difference, it can be very confusing (as you know).
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Old 07-03-2005, 10:25 PM
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Craig Walters Craig Walters is offline
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doug has it right, but i'm going to add just a bit more here.

before digital, the resolution of something like a photograph was most simply defined as how much detail can you see in the photograph. and if you think of newspapers and especially older ones, you might remember that you could see the individual dots of an image in a newspaper. they were closely enough spaced that when viewed altogether you would see a picture. that's resolution. how many dots per inch were there in that newspaper picture was its resolution. a lot of dots gave you better or higher resolution because you would see more detail. fewer dots and it didnt look as good.

well, the same is true with digital. and there's a formula computer monitor manufacturers use to figure out what their monitor's maximum resolution is. i dont recall the exact formula, but it's along the lines of how many distinct dots can a human eye at 20/20 see from a certain distance away from the monitor. and that's the resolution. only, like doug says, when you're talking about computers and monitors, it's pixels. this is short for 'picture elements = pic els= pix els= pixels'.

ok, stop here if you dont want to go to sleep

but also like doug says, this can get a bit goofy. some folks will define the resolution by the picture dimensions, like 1024 x 768, or 800 x 600 and they'll call that the resolution. in fact, if you have a pc, that's how your desktop resolution is defined. and that just seems wrong, but it is also basically used, so it's become sort of right in that instance.

in photoshop and paint shop pro, there is a default resolution set at 72 pixels per inch. this has become a standard. but, you can change it in those programs. the reason i mention this is, you're going to hear folks say 'my resolution is 1024 x 768'. when they say that, they're usually referring to their desktop settings or their video card settings. when they talk pixel resolution they're usually referring to how many pixels per inch are in the image. so, you could have a picture dimension size of 1024 x 768 but not have it at 72 pixels per inch. what the graphic editors do is keep the pixels per inch stable (unless you force a change) and then alter the dimensions of the picture to fit that. but since your monitor ONLY wants to display a certain dimension, they use the picture dimensions as the stable factor and alter the pixels within.

sounds a bit nutty, doesnt it. but, you can try this for yourself. your desktop picture is nothing but a picture. you can actually load it into your paint program and watch the dimensions change when it gets loaded. and likewise, you can take a picture and load it as your desktop and watch those dimensions change. the reason i know this is, i took a HUGE satellite image i'd downloaded and made it my desktop and it ALL fit within the desktop. but, in my paint program it went way beyond the edges of the program window.

so, in effect, the desktop resolution IS defined by the length and width of the image, while in the paint programs it's defined by how many pixels per inch there are in the image. the bottom line, however, is still 'how much detail can you see in the image'.

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Old 08-11-2005, 08:13 AM
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Resolution: density of image elements (screen pixels or printer dots)
Higher resolution = higher detail (consider HD or High Definition)

when scanning prints, most paper is only capable of defining 450-600 pixels per inch via a scanner, therefore scan using the same resolution. - and NO, 72px/in was FAR from enough.

When printing, your pixel density should reflect the maximum that your PAPER can define if you were to re-scan your reprints. In most cases anywhere between 250 and 600 PIXELS per inch. (did you notice it's pretty much the same as scanning?)

When printing, your PRINTER resolution should always be maxed out, period; HIGHEST QUALITY or PREMIUM GLOSSY if you use photo paper. The more dots the better; most printers can do about 1200 or 2400 dots per inch.

By the way, i hope you already know document paper is not designed for printing pictures, the ink will always bleed just a TEEENY bit, causing blurryness, plus if you have lots of dark areas in the original, document paper can never achieve true black, even if you SOAK it with dots. Always stick with photo paper for photos. Now go do your homework!!!

Last edited by chiko321; 08-11-2005 at 08:23 AM. Reason: correction
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Old 08-23-2005, 09:44 AM
TheDoctah TheDoctah is offline
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Scanning at 72 DPI is not going to give you the quality you want unless the original image is huge and you want to print it small. No way would you want to scan at less than 300 DPI if you intend to print. More is generally better, but you can generate some really large files. 600 DPI is generally a pretty good place to start, but never go over the optical resolution of your scanner. (Some scanners claim 2400 DPI or something similarly high, but their optical resolution is less than that. They use software to interpolate (guess at the pixel values between the pixels that were actually scanned.))
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