DPIs for the www
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I think scanning is different - it's like printing (just vice versa) so if you have a picture you want to scan and set 150 dpi, it will create 150 pixels of every inch - regardless of the original resolution of the printed picture - while you scan at 300 dpi, it will have twice as big resolution in the digital image (thus the "size on screen" will be twice as big, as well).
Scanning and printing is the two ends of the connection between digital ("virtual" and physical ("real") worlds. Just conversion of prints into digital and vice versa.
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when I scan in at 300dpi the image is then 300ppi
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Semi Interesting trivia: (I got my degree in graphic design)
The confusion arises because of the terms left over from the printing industry. Historically, artwork destined for the printed page was photographed through moire screens of various resolutions....like 300 DPI. The setup was huge; the process camera was actually built into the wall of a small room. The artwork would be on the outside, lit by two carbon arc lamps and you walked inside the room...hence inside the camera...to set up the half-tone screens and expose the negative that was later transferred to the printing plate. This did actually produce a plate (whether photo-litho or letterpress) that comprised actual dots...think old black and white photos in newspapers or the very obvious dots in old comic books, as reproduced large in a Roy Lichtenstein painting....because the half-tone screens had the optical effect of focusing the image as a pattern of different sized white dots. Hence the term Dots Per Inch. You could use other fresnel patterns that might break the image down into straight or wavy lines or concentric circle, for artistic effect, but by far the most common was the familiar dot pattern. When preparing colour material, the screens had to be rotated a precise number of degrees for each colour so that the four printing colours CMYK wouldn't all print in exactly the same place on the page http://www.phys.ufl.edu/~avery/course/3 ... or_dot.jpg
This term got carried over into computing, but is more properly termed Pixels Per Inch in a digitally scanned image destined for a computer screen. However it may still shake down to DPI if an actual printing plate is being produced for a commercial print run...depending on the type of printing involved. If you are producing stuff like a web page that doesn't relate to any hard printed output, you can just forget about it.
Using a desktop scanner, then obviously you'll get a higher resolution image (and hence a larger one in terms of pixels) the higher you set the resolution of the scan. Usually you'd like it as clear as possible, but setting it too high on source material like an image from a book or a low quality postcard will actually reproduce the printed half-tone dots...which is probably not what you want. Some scanners have pre-sets for printed matter which will introduce a certain amount of random dither in an attempt to break down the resulting moire pattern. If you set the resolution very high it will even reproduce the actual film grain from a non-digital photograph.
I've always scanned in my own illustrations at 300 DPI, because that's what the publishers want for reproduction in books or book covers. If I need the image to be smaller than the resulting scan...for instance for a web page or other on-screen use, then I just resample it smaller.
Apps like Photoshop can seem overly confusing, but that's only because most people that use them only use about 10% of the features. The people that it was actually designed for...the publishing industry...do actually need all that stuff.
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I like to pretend I know photoshop well (I am the PS and SU guru in the office, though thats not saying much) but when it comes to the crazy features that you need only every once in a while PS is great for those things!
I learned a lot from my buddy who did billboards in Costa Rica, he was telling me that they had to do the entire sign at 1200 DPI, I can't even imagine the amount of RAM that would kill!
EDIT:
200th post! huzzah! -
wow, guys. you are something. a simple question of mine had as a result a great discussion that made me less ignorant.
kris is so right: no questions go unanswered on this board. i thank you all.
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