Does a 720 dpi printer actually print 720 true dots per inch?

I have a 5 megapixel image that has 426 dpi of actual data on a
6"x4" print (2556 x 1704 actual pixels).

If I have an Epson printer that does 2880 x 720 dpi, will that
printer actually be able to print / resolve 426 actual dpi when
it prints out the original image onto high-gloss photo paper?

Or is it not able to actually print 426 dpi?

I ask because someone told me that they thought the Epson could
only resolve 360 dpi max, which is a divisor of the Epson print
resolutions and that it could not resolve anything above that in
actual printing.

Any help would be appreciated.

Thanks

 

You need to divide the number of dpi by the number of colours plus
black to get the raw ppi measure as each printer pixel is a composite of
the colours. Calculating the actual ppi measure is much more difficult
as dither and anti-alias algorithms are applied to each pixel.

Printers such as the Epson 1290 have a 2880 dpi setting but there is no
visisble difference between 2880 dpi and 1440dpi and 2880 does not
seem to be recommended by Epson. The maximum resolution of
the Epson 1290 is about 150 to 180 ppi.

Graham

 

The printer (like most printers) prints bigger dots than the inter-dot
spacing.

The ink droplet covers a circle around 1/300 radius (or maybe a bit
larger - a lot will depend on paper absorbance).

The printer can place circle centres at 1/720 in the direction of paper
motion and 1/2880 in the direction of head motion.

So you get lots of overlapping circles on the paper. If you print at
1/360 circle centres X and Y, then the ink droplets would cover the paper
totally (no gaps would be shown).

However, the edges of droplets can be positionned at the centre
resolution - you can actually resolve details using this fact and it
certainly helps when you're putting down multiple droplets for colour
rendition.

All this apart, the position choices are not infinitely variable,
especially in the direction of paper motion. So if you want to print an
image at an effective 426dpi it will need to be resampled to something
that is based on the printer's actual capability. You can either use the
ordinary printer driver to do this (the default if you print from a
programme like Photoshop) or use superior algorithms in a programme like
QImage.

--Sophie

 

Not really.

If it's a 4 color printer (CMYK), then 720/4 = 180 per inch, but at that
"resolution" you can't resolve any colors. It take a good number of dots to
create a single pixel. Most modern drivers use a stochastic dithering
method, which makes calculating ppi somewhat less than straightforward.

Bob

 

The advertised resolution is an addressable resolution. As others have said,
dithering to create the millions of colors uses up lots of this resolution.
Dot gain and other factors determine the actual printable resolution by a
printer on a given type of paper. I would estimate that somewhere between
200 (or less) to 400 dpi is the max res. on paper and that varies depending
on the colors printed.
bg

 

The printer driver will internally interpolate to 720 ppi, and then
dither that to the printer's positioning accuracy. It will use
smaller/variable droplet size and error diffusion dithering to mix
intermediate colors from the available inks.

No, the printer uses 720 ppi internally, regardless of what you offer,
and will interpolate to reach that. That information can be obtained
by software that interrogates the driver, which will then return that
info. A program like Qimage uses that to optimize the interpolation
and sharpening, and only then sends data to the driver which dithers
without change.

Bart

 

Make that 426 pixels per inch. Dots and pixels are not the same.
It is doubtful that you could tell the difference between 300 pixels per
inch and 426 pixesl per inch. I know that I cannot.

The 360 value is left over from the old days. Current Epsons start with 720
dots per inch.
Epson has never revealed how the printer driver works, but a simple
explanation is that it uses the input pixels per inch value to compute the
number of dots per pixel. In the quoted case, it merely uses more dots in
the horizontal direction than dots in the vertical direction.

But, I cannot see any visible difference between 1440 dpi and 2880 dpi.

Jim

 

The Epson printers certainly print the number of dots per inch they
claim. I can see them with adequate magnification. (In an area of
heavy ink loading they sometimes merge.)

Each dot is *not*, however, a pixel; it cannot, by itself, contain the
full range of colors and brightnesses. One of the reasons inkjet
prints look so good is that they don't consist of simple pixels.

 

It is, but you may not be able to see it.

There's a box in the printer driver called "finest detail" that
switches from 360 to 720 ppi.

Andrew.

 

A 720 dpi printer produces ink blobs 1/720 inch apart, but they overlap.
You probably cannot truly resolve more than 200 to 300 dpi on the picture,
depending on the kind of paper. Nonetheless the higher "resolution" of the
printer helps give a smoother image.

 

Yes the printer prints 720 dots per inch. However, you need to remember that
it takes more than one dot to make most colors. For example if you have a 7
color printer than you need to divide the 720 dpi by 7 to get the printers
effective resolution. This is also how I decide what DPI to have my images
to print at. My Epson R200 is 5700 dpi and has 6 colors so when I print
using it I try to have my images at 950 dpi. For a 720 I would aim for 120
dpi. Now this isn't always possible and one must use what one has access to.
For example when printing a 13x19" print on my Canon i9100 I am usually
lucky of I have 200 dpi to work with.

John

 

...and here's me thinking they would be printed on top of each other...

5760 x 1440 actually;

So you'll need to resize your images to be about 960 x 240 dpi.

....

My R200 does its 90 nozzle check pattern about 3/4 inch high, which would
equate to less then even 150 DPI, so how Epson get 1440 DPI is completely
beyond me....

 

It's not that simple. Epson (and I believe others) use variable
droplet technology, with (currently) 6 different droplet sizes from
each of the 6 different tanks onto the same piece of paper. However,
the largest 2 of those will definitely overlap the next pixel
somewhat.

It's roughly (6 + no droplet = 7) x 6 = 42 different colors for each
of the 720 pixels in a linear inch. (Really less, because the color
gamut of some of those inks overlap, as does the larger pixel overlap
with the next one)

Bill Hilton came up with the following link that has some samples for
you to print showing a 720 ppi print looks better than a 300 ppi, with
files to download and see for yourself:

http://www.ddisoftware.com/qimage/quality/


I can see what you are getting at, but this logic is bogus. If I had a
perfect printer with say 4 million color tanks, then by your
calculation 720/4,000,000 = 0.000018 dpi which is silly. My 'perfect'
printer would be much closer to 720ppi than your one that can only mix
7 tanks by 6 droplet sizes each.

It comes down to this. Printers need to dither, and their current
effective full-color resolution is around the 240-300ppi mark.
However, you should send the jobs at 720ppi (for Epson, 600ppi for
Canon) because this is the driver's native resolution and assuming you
have un-sharped the image, there will be high-contrast areas that the
printer may be able to achieve towards 720ppi for.

The nozzle check pattern is done on a single pass without moving the
paper. If the paper can be moved by 1/2 the distance between nozzles,
then starting from Epson's 180 dpi you can achieve 360 DPI. Do it by
1/4 and you get 720DPI. (Watch an Epson head build the solid colors in
no less than 4 passes).

Realistically, that's all there is. The Epson's 1440 dpi does not
relate to the paper feed direction, it's now offset timing the dots in
the direction of head movement to give you 1440dpi along that line.
There is a hell of a lot of overlapping dots at this rez.

A single pass with all black heads firing will result in a gray line.
To go Black, several more 'interlacing' passes are required.

Use 1440DPI where the medium (and in my experience the internals of
the printer) requires saturating with ink.

You may ask, how can you ever get 1440dpi when you only send it 720ppi
information? - or rather why try? Well, if they could make the dots
smaller, a 2880 dpi printer would be better and 5760dpi or 11520dpi
printer almost perfect for true 720ppi full color printing, giving
enough space for dithering hundreds of different color dots to form
each pixel.

 

Aren't you guys mistaking DPI for PPI when you sed the pics to the
printer?
There is no way to correlate DPI and PPI. It just can't be done.
Trying to do so is a fool's errand.
Why? Becasue there's no way you can determine how many dots will be
used to print a pixel. The driver will chose anywhere from 0 (zero) to
the maximum the printer can print per inch for a given (user chosen)
quality.
When you choose the PPI to send to the pruinter, what you're doing is
attempting to get a reasonable number of pixels per inch on the
printed image; 200 to 300 is usually considered adequate. The printer
driver will translate this into dots, depending on the color and
luminance of the pixels involved. The fewer pixels per inch the user
stipulates, the more dots the printer will use to print each pixel,
because that pixel will cover a larger area of the paper than would be
the case with a higher PPI.

 

I'm surprised how often this comes up. There's an image at
http://www.littlepinkcloud.com/restest-2.jpg that shows just how good
high-resolution printers really are. The image on the left is printed
on an Epson 7600, and the upper grid lines are at the highest 720 ppi
resolution -- i.e. 360 line *pairs* per inch.

Andrew.

 

wrote in @news.supernews.com:

In B&W, but only vertical.

Try to make light grey or tan lines on a 5 degree angle and you probably
drop to 1/3 to 1/2 of that...

Bob