LPI vs. DPI

I was told to double the LPI, which is usually not quite as nice as 175. 300 DPI though is a good size for most things... not sure how much of a difference it would make. I suppose if you had a very low LPI printer you would want a lower DPI to conserve file size, since the extra detail would just be lost in the print.

Here is a brief explaination on LPI vs DPI:

http://www.graphic-design.com/Photoshop/Tips/85-lpi.html

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nick
ndesign-studio.com

Apfhex & ndesign, thank you for your feedback..

To go with 175lpi I need to produce images as 350dpi (or 385dpi) to save the designers from going through the conversion step. That was, to a certain degree, understood. What is puzzling me, why lines per inch and not simply dpi?

I'm still reading into the matter, and once I get to the bottom of it, I'll post an understanding here (I have a feeling it was derived from TV as computer screens and personal printers did not exist when it all started)..

Thanks again

The whole dpi=2xdpi was derived from a formula called Nyquist Theorem. Wikipedia states it as thus: "When sampling a bandlimited signal (e.g., through analog to digital conversion) the sampling frequency must be greater than twice the signal's bandwidth in order to be able to reconstruct the original perfectly from the sampled version."

So when scanning (digitizing) a photo you want to sample it twice the frequency of the linescreen to ensure that it will reproduce faithfully.

So I took that as gospel for years and years. Then one day it was slow at the printshop and I decided to do a little experiment. I scanned an image at 200 dpi and output film of it at 100, 133, 150, 175, and 200 lpi. I stripped the film together and had the press guy run a test print of the images.
The result was there was no decernable difference between any of the images. Even at 200 lpi there was no stairstepping (jaggies).

So I tried it with an image scanned at 100 dpi. The results were the same for 100, and 133 but beginning at 150 stairstepping was becoming apparent.

So as a rule of thumb doubling the lpi to get the scan dpi is all fine and good, but it is a tad overkill. For really large images or if your running short on storage space 1.5x the lpi should be more than sufficent for most common linescreens (lpi).

When I have more time and energy I'll post an exhaustive and drawn out post of dpi, lpi and how they relate to each other.