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Author Topic: Dpi for printing  (Read 7766 times)
Jonathan Cross
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« on: February 03, 2009, 03:30:34 AM »
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At the risk of rehashing an old topic, can anyone help me?  

I am sometimes asked about the dpi sizing of images for printing, and I trot out '300dpi or thereabouts is a good idea'.  I can be asked from where this figure comes.  I know it is related to the ability of the eye to resolve line pairs, but know no more, despite looking in my old College optics books and doing an internet search.  Can anyone tell me where to find the actual scientific reasoning behind this, preferably in English!  For example, if it is the case that the eye can resolve 6 line pairs per mm, who said so, and when, and under what conditions?

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joergen geerds
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« Reply #1 on: February 03, 2009, 08:15:36 AM »
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the 300 ppi has a long history.
I think the most common reason was that 152 lpi was the most common screen for offset printing in the 80s and 90s, a kinda middle of the road print quality between news print and high-end magazines. the basic theory said that one should have 2x LPI as a source resolution for good results, or 304 PPI.... most designers can't do any math, so they rounded it down to 300 PPI, because it didn't make any difference in the RIPs (raster image processors that convert the image data into highrez B&W films with screens for printing).

when the first lightjets and similar photo printers came to market, they settled for 3 different resolutions, 200, 300 and 400 ppi, which are the physical resolutions those machines are capable of. and since the printing process doesn't have any screening process, it is considered good practice to feed data to the RIP that doesn't require any resizing (mostly due to volume&speed). recent developments now have "high resolution" digital c-printers with 450 ppi, but only for small prints, not for super large prints.

inkjet/pigment printers use a stochastic screening process, and like offset printing don't require precise resolutions.

but yes, 300 is a good number.
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Doombrain
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« Reply #2 on: February 03, 2009, 03:12:22 PM »
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If it's an epson stick to 360, 240 or 180 at the lowest at most printheads have a 180dpi head. the 11880 and new 7/9900 have a 360dpi head.
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Wayne Fox
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« Reply #3 on: February 03, 2009, 09:06:48 PM »
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Quote from: Doombrain
If it's an epson stick to 360, 240 or 180 at the lowest at most printheads have a 180dpi head. the 11880 and new 7/9900 have a 360dpi head.

Why does the density of the nozzle itself have anything to do with it? I believe all Epson printers are capable of full 2880 by 1440 DPI.  It just means those with lower density nozzles have to do more passes.  The denser head is one reason the new printers are substantially faster than the older ones .. they can lay down twice as many dots in a single pass.

The relevant "DPI" is how the printer driver handles the data ... which for Epson appears to be 720/360, and for Canon 600/300.

Since any given actual dot can only be one of a few colors, the screening process is far more complicated than just sending your pixels to the surface of the paper.  Personally I find I get terrific results if I send the native resolution to the printer and let the printer driver handle all of the sizing and screening together.  Certainly simplifies the workflow.  Side by side comparisons with prints I have interpolated in photoshop to the magic "360" number are virtually identical with those that I just send at native resolution, as long as I stay above 170-180 ppi.

(Of course this really isn't the OP's question ... think he was really asking how did 300dpi sort of end up being a standard of some type for minimum resolution, which I have read about at some point in the past, but cannot pull the answer nor the source to that out of my aging brain cells.)
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TylerB
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« Reply #4 on: February 03, 2009, 09:41:36 PM »
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I think the 300dpi rule of thumb came about for files going to prepress, that's my memory of it anyway. For anyone wanting to get into the nuts and bolts of the issue, this page and the test files are revealing, as Ernst Dinkla has repeatedly pointed out-
http://www.ddisoftware.com/qimage/quality/
Downloading the test files and printing at different resolutions and resizing and scaling methods can be very revealing. My experience is similar to Waynes, I let my RIP do the scaling, and a lot of testing has not revealed a visually superior method the vast majority of the time. But rules of thumb do a disservice, and image content file characteristcs can always provide exceptions. Qimages test files and info can show potential problems even when images don't, so there is more to this issue than commonly discussed.
If the question regards upresing small files beyond their real capability, 3rd party tools and photoshops tricks and methods may yield more pleasing results than a straight driver scaling. But in this situation it's the lipstick/pig scenario anyway, as they say...
Tyler
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Farmer
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« Reply #5 on: February 04, 2009, 01:25:30 AM »
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Quote from: Wayne Fox
Why does the density of the nozzle itself have anything to do with it? I believe all Epson printers are capable of full 2880 by 1440 DPI.  It just means those with lower density nozzles have to do more passes.  The denser head is one reason the new printers are substantially faster than the older ones .. they can lay down twice as many dots in a single pass.

The relevant "DPI" is how the printer driver handles the data ... which for Epson appears to be 720/360, and for Canon 600/300.

Yup.  In a word, yup :-)  I am often glad for your voice of reason and straight-talking, Wayne!

The density of the nozzles has very little to do with it.  The Epsons, for example, use variable dot sizes and extremely complex LUTs in addition to various halftone processing to determine an effective dot pattern to lay down.  The pro level devices can achieve a matrix of 2880x1440 as Wayne says and the consumer level devices can do 5760x1440 (though I dare anyone to pick the differences from 2880x1440 on the same device).

Quote from: Wayne Fox
Since any given actual dot can only be one of a few colors, the screening process is far more complicated than just sending your pixels to the surface of the paper.  Personally I find I get terrific results if I send the native resolution to the printer and let the printer driver handle all of the sizing and screening together.  Certainly simplifies the workflow.  Side by side comparisons with prints I have interpolated in photoshop to the magic "360" number are virtually identical with those that I just send at native resolution, as long as I stay above 170-180 ppi.

Exactly right again.  There is no direct correlation between your image pixels and the individual dots laid down by the printer.  It is only a combination of various dots, of various sizes, in various positions, viewed relative to and in combination with, other dots on variable substrates that provides the illusion of colour (is that a redundancy? ;p ) at a given point.

If you resample (particularly upres) you create data that does not exist to fill the gaps.  The printer then attempts to render this non-existent data as accurately as it can.  Whether that will provide a better or worse result than the printer "filling the gap" itself if you had not upressed will simply "depend".  Sometimes it will, sometimes it won't, most times you wouldn't pick it except in a direct comparison (and not even then many times).

It depends on all the factors involved, starting with the original image all the way through to the final physical printing and the inks and substrates involved and their method of deployment etc etc.

If you have a scene with very sharp, diagonal lines in high contrast to their surrounds, then you're far more likely to see the advantge of higher resolution images - in fact, in that case, if oyu had the data I'd turn on Finest Detail in an Epson driver and send it 720 data.  But in most other cases, you have to ask, "Do I want the printer to do its best rendition of real and fake pixels or do I just want it to do its best rendition of real pixels, even though there are less"?  And only doing test prints will really tell you.

I have one image, that is of the pages of a book, torn and tattered.  The crop from the original 6MP isn't that large so it's a relatively low res image.  Upressing and printing even at A4 shows pixelation from the upressing because the text on the pages is a sticking point.  Printing natively at the same size renders a far more acceptable and desirable image even though it's softer.  Yes, it's hiding a lacking of resolution in the softness, but it's still a better result than a sharp image clearly showing fake pixels.

Quote from: Wayne Fox
(Of course this really isn't the OP's question ... think he was really asking how did 300dpi sort of end up being a standard of some type for minimum resolution, which I have read about at some point in the past, but cannot pull the answer nor the source to that out of my aging brain cells.)

It relates to LPI numbers from pre-press, physical capability of earlier printing devices such as laser printers, an easy number to use and it being over the normal level of human vision to see line pairs (so images looked "solid").  All in all, it remains a good number, but it's not a holy grail and often times effort, time, money and quality are lost in chasing it.
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neil snape
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« Reply #6 on: February 04, 2009, 01:51:59 AM »
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For press

300 DPI is a line screen often used for ripping to a press frequency of 150 or 152 LPI or DPI if you prefer. The function of the rip is to make a downsampling averaging the information across multiple pixel values to plot a best guess at smooth lines, transitions, etc.

For inkjets

300 DPI falsely put rather being 300 PPI is a number used for setting the grid resolution for driver interpolation into a native format. The interpolation is done resolution wise upres or downsampling to the native operating screen resolution for the driver (or ASIC) performing the most important functions of ripping the files such as masking. The masking resolution and bit depths are the user control settings where in they are toggled for different precisions in the driver settings.
HP's are running mostly at 600 and 1200 ppi sampling from the source grid which being 300 will be a very good compromise. If you send 600 ppi source res, the ripping has a nul transform/conversion if things like max DPI are not user selected. IF so they are usually upressed whatever the source to 1200 for LFP, and 2400 for desktop printers. Inkjet printers perform as frequency modulation printers and or hybrid (Epson variable dot) so the upressing is done for better screening potential, not for diagonal lines as in the case of press AM dots.

Epson seem to run best at integers of 180, Canon and HP run at 300ppi. IF you test your res on MTF charts, you can find the sweet points. What to watch for in images with higher resolution is smoother flat colour zones such as grey areas, and better less fringe coloured high contrast edges. Smoothnes and colour saturation are possible with the printers capable of 5760 or 4800 ppi printing but you need good eyes to find it. Those days are over for me!
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Ernst Dinkla
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« Reply #7 on: February 04, 2009, 03:26:08 AM »
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Quote from: Jonathan Cross
At the risk of rehashing an old topic, ..............?

That's what I thought in the first place. Then I asked myself why this question repeats itself. The practical value is limited; printer systems exist and it will be hard to do it otherwise with a DIY printer and driver. If there's a want to know for theoretical sake the internet provides good places to get informed.
A google does the job. For example:
http://www.clarkvision.com/imagedetail/eye-resolution.html
http://www.normankoren.com/Tutorials/MTF.html

If it is about getting a print as good as possible from the image data you have: use a pro printer, pro media and a workflow that has good extrapolation algorithms for up and down sampling. Starting from quality pixels is a must too, enough 600MB MF film scans with void pixels as a result of noise in the film, scanner + inflated scanner resolution numbers.

In practice a shop has to work with any file it gets, the print size resolutions vary between 50 PPI and 1000 PPI. Depending on the media you print on, a range between 150 to 500 PPI could show harder to detect quality differences. With good extrapolation algorithms this becomes even more difficult. With your nose on the print and at a meter distance. So much for eye resolution.


Ernst Dinkla

Try: http://groups.yahoo.com/group/Wide_Inkjet_Printers/
« Last Edit: February 04, 2009, 04:55:20 AM by Ernst Dinkla » Logged
Jonathan Cross
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« Reply #8 on: February 04, 2009, 06:27:25 AM »
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Thanks for all these replies.  I asked the question as I was originally a physicist,  and like to know why things are as they are, and where science and technology quotes come from.  The web links in Ernst's reply have given me a good start.  Obviously the science has to be modified by the ability of the printer to lay down ink, and by the properties of the paper as well as the ability of the eye to resolve.  The replies clearly show this.  I am much wiser now!

Jonathan Cross

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Doombrain
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« Reply #9 on: February 04, 2009, 11:01:46 AM »
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Quote from: Wayne Fox
Why does the density of the nozzle itself have anything to do with it? I believe all Epson printers are capable of full 2880 by 1440 DPI.  It just means those with lower density nozzles have to do more passes.  The denser head is one reason the new printers are substantially faster than the older ones .. they can lay down twice as many dots in a single pass.

The relevant "DPI" is how the printer driver handles the data ... which for Epson appears to be 720/360, and for Canon 600/300.

Since any given actual dot can only be one of a few colors, the screening process is far more complicated than just sending your pixels to the surface of the paper.  Personally I find I get terrific results if I send the native resolution to the printer and let the printer driver handle all of the sizing and screening together.  Certainly simplifies the workflow.  Side by side comparisons with prints I have interpolated in photoshop to the magic "360" number are virtually identical with those that I just send at native resolution, as long as I stay above 170-180 ppi.

(Of course this really isn't the OP's question ... think he was really asking how did 300dpi sort of end up being a standard of some type for minimum resolution, which I have read about at some point in the past, but cannot pull the answer nor the source to that out of my aging brain cells.)

Because the Epson HTM was built on a FM screen of 360 which was designed to accommodate Epson heads.
« Last Edit: February 04, 2009, 11:06:24 AM by Doombrain » Logged
Wayne Fox
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« Reply #10 on: February 04, 2009, 10:02:40 PM »
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Quote from: Doombrain
Because the Epson HTM was built on a FM screen of 360 which was designed to accommodate Epson heads.

So I guess my question remains the same, what does this have to do with the density of the nozzles, which was the point in your original post.

The lower density of nozzles in the head just means that the older printers must move the media and make another pass to accomplish the same thing the newer printers can do in a single pass.  The literal DPI of the both printers is the same, maxing at 2880 x1440 dpi and has no bearing on the printers screening methodology. Epson could build a printer with a nozzle density of only 60 and produce identical results to the 7900 ... it would just take 6 times as many passes across the media.

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mrkahn
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« Reply #11 on: February 04, 2009, 10:41:17 PM »
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I asked this question at the Epson Print Acadamy.  The answer I got was to do as little to change the base resolution of the file as possible.  Iuse a 1DS3 and it produces a 240 DPI file.  The epson said that resolution was more than good enough and better than up sizing to 360 that I thought Epson required for optimal results.  They also said that one would be better off to let the pixels fall where they may as long as they don't get lower than 180 DPI.  I have used this new approach on my new 9900 and it works great.

Just my 2 cents.

Regards,
Malcolm
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Ernst Dinkla
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« Reply #12 on: February 05, 2009, 03:00:13 AM »
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Quote from: mrkahn
I asked this question at the Epson Print Acadamy.  The answer I got was to do as little to change the base resolution of the file as possible.  Iuse a 1DS3 and it produces a 240 DPI file.  The epson said that resolution was more than good enough and better than up sizing to 360 that I thought Epson required for optimal results.  They also said that one would be better off to let the pixels fall where they may as long as they don't get lower than 180 DPI.  I have used this new approach on my new 9900 and it works great.

Just my 2 cents.

Regards,
Malcolm

Your 1DS3 produces a PPI file and the 240 number goes along with a certain image size in inches.

http://www.steves-digicams.com/techcorner/January_2005.html

There are very good reasons not to resample an image file that will be archived for later use. In every workflow though there's a stage where the image file to be printed will be resampled to the input (native) PPI resolution requested by the driver. The driver can do that resampling but some applications you print from can do the same, they intercept calls from the driver (API Windows for example) and do the task of resampling to one of the usual 300-600 or 360-720 PPI native resolutions, sometimes even higher. All on the fly and not changing the image file itself. Depending on the respective extrapolation algorithms the different work flows can show different qualities in print. In the past driver extrapolation algorithms were usually bad but they have improved in time. There still are applications that can do a better job, some even on the fly just before printing . A neglected part in extrapolation is the anti-aliasing for down sampling, I have not seen drivers that cope with that. I'm using Qimage which offers all the elements discussed here in a fast and flexible way.

The rule that you can throw any file resolution in the printing work flow is too general, much depends on the application and/or the driver qualities.

More by Mike Chaney, developer of Qimage:

http://www.steves-digicams.com/techcorner/oct_2008.html
http://www.steves-digicams.com/techcorner/january_2006.html

By Bart van der Wolf, on aliasing:

http://www.xs4all.nl/~bvdwolf/main/foto/do...le/example1.htm
http://www.xs4all.nl/~bvdwolf/main/foto/do...down_sample.htm


Ernst Dinkla

Try: http://groups.yahoo.com/group/Wide_Inkjet_Printers/
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Farmer
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« Reply #13 on: February 05, 2009, 04:31:15 AM »
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And I still say, do the print and then compare.  There's a lot of talk by interested parties, but the proof (pun intended) is in the printing.
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Doombrain
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« Reply #14 on: February 05, 2009, 04:57:04 AM »
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Quote from: Wayne Fox
So I guess my question remains the same, what does this have to do with the density of the nozzles, which was the point in your original post.

The lower density of nozzles in the head just means that the older printers must move the media and make another pass to accomplish the same thing the newer printers can do in a single pass.  The literal DPI of the both printers is the same, maxing at 2880 x1440 dpi and has no bearing on the printers screening methodology. Epson could build a printer with a nozzle density of only 60 and produce identical results to the 7900 ... it would just take 6 times as many passes across the media.

yep, the 7/9600 range has heads with only 90dpi, then the x800/880 with 180 and now 360. the speed isn't doubled, it's more like x1.6 quicker.
in terms of screen epson devolved the screen algorithm based on a 90 x 6 nozzle head (for photo) so the driver will always interpolate data to 360 or 720 if you check finest detail. The nozzle pitch is everything when producing the screen.
« Last Edit: February 05, 2009, 05:01:48 AM by Doombrain » Logged
Farmer
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« Reply #15 on: February 05, 2009, 05:05:27 AM »
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Quote from: Doombrain
yep, the 7/9600 range has heads with only 90dpi, then the x800/880 with 180 and now 360. the speed isn't doubled, it's more like x1.6 quicker.
in terms of screen epson devolved the screen algorithm based on a 90 x 6 nozzle head (for photo) so the driver will always interpolate data to 360 or 720 if you check finest detail. The nozzle pitch is everything when producing the screen.

The heads aren't 90/180/360 dpi - the current TFP heads are 360 nozzles per colour channel, it's nothing to do with inches in that sense of DPI and the driver has been completely redesigned around new LUT technology.

And, yes, 360 and 720 remain important numbers, but I'll say this again - print some tests before you assume that creating pixels is better than the driver figuring it out.  To make a blanket statement one way or the other is, frankly, wrong.
« Last Edit: February 05, 2009, 05:09:09 AM by Farmer » Logged

Doombrain
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« Reply #16 on: February 05, 2009, 05:31:27 AM »
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Quote from: Farmer
The heads aren't 90/180/360 dpi - the current TFP heads are 360 nozzles per colour channel, it's nothing to do with inches in that sense of DPI and the driver has been completely redesigned around new LUT technology.

And, yes, 360 and 720 remain important numbers, but I'll say this again - print some tests before you assume that creating pixels is better than the driver figuring it out.  To make a blanket statement one way or the other is, frankly, wrong.

The LUT only determines the colour output combination, not the screen.
The FM screen (HTM) was updated when the Pro3800 came out, until then it remain the same for years. The update was to improve transition curves to make better use of VDS 1 to 3.

For the most part is doesn't really matter what PPI you use as long as you don't go below 180. If you use something like 369PPI there's a chance of introducing Moiré patterns.

I wasn't making a blanket statement, I was just responding to a question regarding best output PPI and i answered the question for Epson printers, which i know alot about. Just trying to help out.

I don't want to become the villain here, it's obvious you're all very passionate about PPI. I've said my bit.
« Last Edit: February 05, 2009, 05:33:41 AM by Doombrain » Logged
Farmer
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« Reply #17 on: February 05, 2009, 05:46:40 AM »
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I wasn't really directing anything at you :-)  I agree totally that you don't need to worry so much about the PPI put through.
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Charles Gast
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« Reply #18 on: February 05, 2009, 10:33:32 AM »
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In the tutorial Camera to Print Michael and Jeff strongly assert the advantage of printing with "native resolution". That means don't upsample or downsample. The engineers who designed the z3100 say the same thing. Just send it 180 to 400 dpi and viola!  Jeff also seemingly suggests that genuine fractals is snake oil = )    Here's the catch:  **The Output**  In most prints native resolution is ok, But I printed a photo of a cat. When I sent the z3100 an (approximately) 204 dpi image with zero up/downsampling the cat has several ziggy zaggy whiskers... The jaggies stand out in the print quite visibly with no need for a magnifying glass or even my reading glasses. The jaggies are not there on the monitor. I went back to the pre-sharpened image and used the snake oil application (genuine fractals) to upsample to 300 dpi, used photokit sharpener with a little extra sharpening following the upsample and printed. The result was sharp clear cat whiskers free of jaggies on the print. So according to what I see on prints many if not most prints are ok with native resolution but if there is very sharp contrasty detail it may at least at times be better to go with the old 300dpi to the hp.  The print driver does some resampling and I guess under certain circumstances it is best to send it a file with which it provides more desirable output.
« Last Edit: February 05, 2009, 10:34:00 AM by Charles Gast » Logged
Tklimek
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« Reply #19 on: February 05, 2009, 11:44:05 AM »
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Charles....

In the Lightroom 2 tutorial, Jeff has indicated that in SOME images (I believe high-frequency) testing has shown to produce better printed output by uprezzing.  Check out the LR 2 tutorial and see....did anyone else catch this in that video?

Todd in Chicago

Quote from: Charles Gast
In the tutorial Camera to Print Michael and Jeff strongly assert the advantage of printing with "native resolution". That means don't upsample or downsample. The engineers who designed the z3100 say the same thing. Just send it 180 to 400 dpi and viola!  Jeff also seemingly suggests that genuine fractals is snake oil = )    Here's the catch:  **The Output**  In most prints native resolution is ok, But I printed a photo of a cat. When I sent the z3100 an (approximately) 204 dpi image with zero up/downsampling the cat has several ziggy zaggy whiskers... The jaggies stand out in the print quite visibly with no need for a magnifying glass or even my reading glasses. The jaggies are not there on the monitor. I went back to the pre-sharpened image and used the snake oil application (genuine fractals) to upsample to 300 dpi, used photokit sharpener with a little extra sharpening following the upsample and printed. The result was sharp clear cat whiskers free of jaggies on the print. So according to what I see on prints many if not most prints are ok with native resolution but if there is very sharp contrasty detail it may at least at times be better to go with the old 300dpi to the hp.  The print driver does some resampling and I guess under certain circumstances it is best to send it a file with which it provides more desirable output.
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