how come need so high dpi for printing photos?

A 15" monitor at 1024x768 resolution is about 85 pixels per inch.
Viewing a photo on the monitor at that resolution looks great, right?

So how come if you print out a photo with the same resolution, size,
density (1024x768, 12"x9", 85 dpi), the print will look very poor? Even
if you hold the print at the same viewing distance as the monitor.

What do you mean by dpi? The dpi value of an inkjet printer is something
totally different from a monitor's dpi. Every image pixel has to be
assembled from a number of droplets of ink, while you have three brightness
values (R,G,B) assigned to each monitor pixel. With 24 bit color on an
inkjet printer you need 16 times the dpi of the monitor (256 spaces for
droplets).

"Bucky" schrieb im Newsbeitrag
oups.com...
A 15" monitor at 1024x768 resolution is about 85 pixels per inch.
Viewing a photo on the monitor at that resolution looks great, right?

So how come if you print out a photo with the same resolution, size,
density (1024x768, 12"x9", 85 dpi), the print will look very poor? Even
if you hold the print at the same viewing distance as the monitor.

"Bucky" wrote in message
oups.com...
A 15" monitor at 1024x768 resolution is about 85 pixels per inch.
Viewing a photo on the monitor at that resolution looks great, right?

So how come if you print out a photo with the same resolution, size,
density (1024x768, 12"x9", 85 dpi), the print will look very poor? Even
if you hold the print at the same viewing distance as the monitor.

There are several reasons I can think of - others may add to this. The
first is that, compared to a piece of paper, the monitor is brighter, and
has a much larger dynamic range, so the loss in resolution more than made up
for by the overall impact of the image Second, we are used to the
underlying texture of TV images, and tend to filter it out or even treat it
as if it were part of the image's native resolution. No so with paper
prints, where we take the glossy sharpness of even 4x6 prints for granted,
and are very sensitive to unwanted texture and jaggies. Third, we do tend
to hold even an 8x10 print very close to our eyes.

There are some practical ideas that follow from this. It should be possible
to make an acceptable print at the approximately 72 ppi resolution of a
monitor. To do so, make sure you use the full dynamic range of the image,
and use sharpening as much as possible. Photographers have long known that
softer images can be made to appear sharper by increasing the grain, and
printing on a textured paper. So add texture to the image, so that the eye
always has something to lock on to, even at close examination.
--
Mike Russell
www.curvemeister.com/forum/

Cgiorgio wrote:
Every image pixel has to be
assembled from a number of droplets of ink, while you have three brightness
values (R,G,B) assigned to each monitor pixel.

I see now. Wikipedia says something similar too (should have looked
there first!):

"The DPI measurement of a printer often needs to be considerably higher
than the pixels per inch (PPI) measurement of a video display in order
to produce similar-quality output. This is due to the limited range of
colours for each dot typically available on a printer. At each dot
position, the simplest type of colour printer can only either print no
dot, or a dot consisting of a fixed volume of ink in each of four
colour channels (typically CMYK with cyan, magenta, yellow and black
ink)."
http://en.wikipedia.org/wiki/Dots_per_inch

I didn't know that printers only had "1-bit depth" for each color
channel. I mistakenly thought that printers could vary the volume of
ink per dot, like a monitor. So if there was a printer that could vary
the volume of ink per dot by 256 different levels, then the 85 dpi
print would be comparable to the monitor, correct?

Cgiorgio wrote:

What do you mean by dpi? The dpi value of an inkjet printer is
something totally different from a monitor's dpi.

Bucky means PPI - Pixels Per Inch. When printing, these gets converted
to the printers higher resolution in order to reproduce the dots, as
you try to explain.

So Bucky has two images, each about 85 PPI, one of them also 85 DPI
(the one on the monitor), the other one 2400 DPI or whatever his
printer is capable of.

Every image pixel has to be assembled from a number of droplets of
ink, while you have three brightness values (R,G,B) assigned to each
monitor pixel. With 24 bit color on an inkjet printer you need 16
times the dpi of the monitor (256 spaces for droplets).

That only explains why inkjets have such high resolution, not why they
should need more pixels than a monitor.

Mike's explanation sounds right. I don't think the sharpening would
work, though, but it's easy to try out.
--
Toke Eskildsen - http://ekot.dk/

Toke Eskildsen wrote:

So Bucky has two images, each about 85 PPI, one of them also 85
DPI (the one on the monitor), the other one 2400 DPI or whatever
his printer is capable of.

Correction: The one on the monitor is 255 DPI, as the monitor uses
three colors with variable intensity for reproduction.
--
Toke Eskildsen - http://ekot.dk/

"Bucky" wrote in message
oups.com...
A 15" monitor at 1024x768 resolution is about 85 pixels per inch.
Viewing a photo on the monitor at that resolution looks great, right?

So how come if you print out a photo with the same resolution, size,
density (1024x768, 12"x9", 85 dpi), the print will look very poor? Even
if you hold the print at the same viewing distance as the monitor.


Does "Why" really matter ???

You can print at 72 Ppi or 96 Ppi if you want, and if you think the results
are Ok, fine.

If the results are not good enough, then print at a higher Ppi, until you
get the quality you want.

Life is too short.

Roy G

In article .com,
"Bucky" wrote:

A 15" monitor at 1024x768 resolution is about 85 pixels per inch.
Viewing a photo on the monitor at that resolution looks great, right?

So how come if you print out a photo with the same resolution, size,
density (1024x768, 12"x9", 85 dpi), the print will look very poor? Even
if you hold the print at the same viewing distance as the monitor.

In very basic terms, my understanding is:

Even with the best monitors and the best software you never see on
screen the full resolution of a file that is very resolute. The monitor
down samples the image a lot for viewing in the case of the file you
have its perhaps up sampling it a bit. Monitors are around 72-96 DPI.


--
Would thou choose to meet a rat eating dragon, or
a dragon, eating rat? The answer of: I am somewhere
in the middle. "Me who is part taoist and part Christian".

In article ,
"Roy G" wrote:

If the results are not good enough, then print at a higher Ppi, until you
get the quality you want.

Uh huh, from a native file resolution of 85 dpi. *Sure you will*. :-/

--
Would thou choose to meet a rat eating dragon, or
a dragon, eating rat? The answer of: I am somewhere
in the middle. "Me who is part taoist and part Christian".

Little Green Eyed Dragon wrote:
Even with the best monitors and the best software you never see on
screen the full resolution of a file that is very resolute. The monitor
down samples the image a lot for viewing in the case of the file you
have its perhaps up sampling it a bit. Monitors are around 72-96 DPI.

I probably should have been clearer in my question. Let's say I have an
image that is 1024x768 pixels. When I view it on a 15" monitor, it
looks great (no downsampling).

Now I print the same 1024x768 image on a 12"x9" print (that is the
exact physical size as the monitor). But it doesn't look nearly as good
as the monitor.