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Which dpi is better?

Posted on 2005-03-14
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Is higher dpi better and what is optimised dpi as opposed to just dpi.

and would an optimisesed 4800 dpi printer produce photo quality prints (HP PSC 1215 All-In-One being the printer in qestion)
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Question by:wisemat
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hdhondt earned 500 total points
ID: 13539424
Higher dpi is generally better, at least when you are printing lines and text. When printing photos the "halftone screen frequency" is more important. This is usually expressed as "lpi" (lines per inch) and indicates the resolution in shades of colour. For most printers, the lpi is not indicated, but for a good printer it will be 150 to 200 lpi.

The lpi is related to the fact that most printers can only produce 1 drop size. For example, say we want to produce a 50% magenta shade. We can do that by putting a magenta drop, then a (white) space, another drop, etc. Hence 50% of the paper is covered in magenta and we have a 50% tint. To produce 16 million colours (256 for each ink) using this technique, we need to put our colours down in a 16 x 16 pixel cell (16 x 16 = 256, and 256 x 256 x 256 = 16M). To achieve 200 lpi this means a resolution of 200 x 16 = 3200 dpi is required. There is no printer with a "real" 3200 dpi resolution. However, using more complex halftoning techniques it is possible to achieve this lpi with lower dpi. This is what is meant by "optimised resolution": the photo quality is better than expected for the given dpi. Unfortunately, unless the lpi is specified, the term "optimised dpi" has no real meaning.

BTW, the lpi can be used to calculate the required resolution of an image: when printed its dpi should be about 1.5 to 2 times the lpi of the printer. In other words, a 4" x 6" photo on a 200 lpi printer should be between 1200 x 1800 and 1600 x 2400 pixels. Anything larger is a waste of time and disk space. Smaller images may look pixelated.

There is another factor that affects the quality of a photo: the number of ink colours used.  With more colours to start with, you can use a smaller halftone cell, and hence achieve higher lpi. For example, if you can use a 128 pixel cell instead of 256, your lpi goes up a factor of 2.

Another *very* important in photo quality is the colour accuracy. And that depends on so many factors that it is impossible to predict.

My recommendation? Select a number of your favourite high-res photos, and get them printed on a number of printers. Make sure the application used for the test is the same as the one you will use yourself (different apps may print the same colour differently). Then pick the printer you prefer.
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by:wisemat
ID: 13539617
So printer salesmen tell you a bunch of stuff and assume you will be impressed...

Thanks - excellent reply
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by:Watzman
Watzman earned 500 total points
ID: 13547267

DPI is not print quality, and it's not a measure of print quality, but it is a factor -- but only one of many -- in determining print quality.

The only way to actually measure print quality is to get actual sample prints from both printers (ideally of the same documents, and of the type of documents that you will be printing) and compare them, possibly under a magnifying glass.

Consider two printers, one 300 DPI, one 600 dpi.  BUT, the 600 dpi printer, while it can "center" it's "dots" at half the interval of the 300 dpi printer, produces dots that are twice the diameter of the dots from the 300 dpi printer.  In most cases, the 300 dpi printer will produce MUCH better print quality.

And that's only part of it:  The printers lay down pixels, and "dpi" is just a measure of the finness of the "grid" on which those dots can be laid down.  As mentioned above, it says nothing about the diameter of those dots (pixels).  However, even beyond that, there is the matter of the shape of those pixes, and the "edge" characteristics of those pixels (sharp edge, or a diffuse, fading edge).  Also, there's no gurantee that any of this willl be "uniform" acroos the page.  A laser printer, for exmaple, might produce "nice" dots near the center of the page, but elliptical, ragged and rough dots as you moved away from the center towards the edges.

The bottom line here is that there are too many variable to compare, and that the whole subject of "print quality" is not describeable in one or two or a dozen "specifications".  (and, to make matters worse, given three printers, one might produce the best text, another the best CAD drawings and the 3rd the best photographic prints -- the requirements for different types of output are different, and at times even conflicting).

So what you come back to (I've been in the printer industry) is that the only valid measurements are actual comparison of actual outputs of the type of documents that you want to print, from the different models of printers under consideration.
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by:hdhondt
ID: 13553313
Glad we agree: the only way to find out which printer is better is to run test prints on them.
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by:hdhondt
ID: 13884090
No comment has been added to this question in more than 21 days, so it is now classified as abandoned..
I will leave a recommendation in the Cleanup topic area that this question is:
Split hdhondt & Watzman
Any objections should be posted here in the next 4 days. After that time, the question will be closed.

PLEASE DO NOT ACCEPT THIS COMMENT AS AN ANSWER!

hdhondt
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