Does DPI mean anything?

June 24, 2018 at 16:19:15
Specs: several
Image viewers report the number of dots per inch an image has.
Looking at images from many sources, some I created or modified
myself, some have DPI values in them and some do not. 72 x 72
and 600 x 600 DPI are common, but they can be anything, and there
is no connection to the image's dimensions. An image created
recently by a friend has a DPI value of 9 x 9. I see no effect
of the DPI value on the appearance of images.

Does the DPI value have any significance at all?

How about when an image is embedded in a PDF?

-- Jeff, in Minneapolis


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#1
June 24, 2018 at 18:34:08
Which will show more clarity & detail, 72 x 72 (5184 dots per sq.in.) or 600 x 600 (360,000 dots per sq.in.)?

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#2
June 25, 2018 at 03:55:05
riider,

It doesn't seem to make any difference. The 9 x 9 DPI image is clear and detailed.

-- Jeff, in Minneapolis

message edited by Jeff Root


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#3
June 25, 2018 at 05:09:51
If you can see no difference between a 600x600 dpi image and a 9x9dpi one then I would respectfully suggest that you visit an optician. If your eyesight is anywhere near normal you will be able to count the individual dots in the latter image.

Alternatively, you are mistaken about the resolution - I have never heard of an image with such a coarse resolution.


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#4
June 25, 2018 at 07:18:15
I'm not saying that any image has the resolution it says it has.
To repeat, some images contain DPI values, and others do not.
Whether an image contains a DPI value or not, and what that
value is if it does contain one, seems to have no effect on the
size or appearance of the image.

My graphics creation program calls it "pixels per inch". Here is
the program's help info about the setting:

> Image resolution is the number of pixels per inch (ppi) or per
> centimeter in the image. You create bitmapped images at a
> specific resolution that is based on how you will use the image.
> An image for the web usually has a resolution of about 72 pixels
> per inch. An image for print usually has its resolution determined
> by the printing device. In general, creating an image at about
> 1.5 to 2 times the resolution (measured in dots per inch, or dpi)
> of the printer produces the best results.

I have never noticed that the value of this setting does anything.

-- Jeff, in Minneapolis


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#5
June 25, 2018 at 07:38:05
I put the 9 DPI image on File Convoy:

http://www.fileconvoy.com/dfl.php?i...

The file(s) that can be retrieved with the above link is: fullloon.jpg (574.889 KB)
It will be available on the server for the next 5 days.

IrfanView reports this image as having 9 x 9 DPI.
My graphics editor/creation program reports it as having 9 pixels per inch.

-- Jeff, in Minneapolis


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#6
June 25, 2018 at 08:13:33
The dpi setting embedded in a jpeg file is just a number. You could put anything that you want in there.

With that dpi setting the image should be 177 inches wide. I wouldn't recommend that you try printing it.


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#7
June 26, 2018 at 00:03:32
The reason I ask is that I am editing and laying out the text and
illustrations of a book for my neighbor. The book will be about
100 pages long and contain about 80 illustrations. The printing
file will be a PDF with the pictures embedded. The author knows
little or nothing about the technical aspects of printing. He also
does not have funding for a professional book designer for this
particular book, as he had for his previous books. That's why he
asked someone with no expertise and no experience to do it.

The pictures he provided have various DPI values in them. I am
cropping some of them, so changing the dimensions. In order to
avoid degrading the quality, I have not resized any of them. The
dimensions of the pictures usually have no relation to the printed
size on the page, although one picture I've dealt with so far has
such small dimensions that I'm using it at 100% of the pixel size.
Almost all of them are much larger.

I need to know if the DPI settings could affect how the pictures
will be printed from the PDF. Making a mistake about this could
double the printing costs, because the printer will charge the full
cost of the printing job to make any changes after the proof has
been printed.

-- Jeff, in Minneapolis


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#8
July 8, 2018 at 01:58:54
Interesting article here explaining the issue you are having.

http://www.rideau-info.com/photos/m...

"I've always been mad, I know I've been mad, like the most of us..." Pink Floyd


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#9
July 8, 2018 at 05:11:55
Thanks. That covers the general question. I still don't know if
the DPI values have any significance when the (human/corporate)
printer prints the PDF file I send to them. They shouldn't, but ...

-- Jeff, in Minneapolis


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