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Author Topic: Need further clarity on Native resolution 180-480 PPI  (Read 10692 times)
Gemmtech
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« Reply #20 on: March 03, 2010, 03:15:23 PM »
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Quote from: MichaelHCothran
I will buy Photokit Sharpener. Never having used LR, though, I don't understand the need for it. ACR in conjunction with PS makes logical sense in my mind. But where does LR fit in? What you state above about printing from LR does whet my interest, as I do print in a variety of sizes.

One more thing referencing the 180-480ppi - I get it if I'm over 480 - just leave the native rez alone. BUT...I still would like a more specific response to files less than 180. You state "upsample." I know that much. But how much do I upsample?? Do I go up to 180, then let the Epson take over, do I go to 360, or where? The answer to this question was the base intent of my original inquiry here. Thanks.
(No funny remarks on where else I can go please!)  

Michael H. Cothran
Nashville, Tennessee

"Ignorance is bliss"  I rarely use Photoshop anymore because of Lightroom and V3.0 (Beta) is even better.  Lightroom just makes everything easier.  I'm not a huge fan of overly processed images and LR is simple, intuitive and does a great job with the basics (cloning not being one of them).  Printing from LR is much easier than from Photoshop, just make the presets and you are off to the races.  
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Schewe
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« Reply #21 on: March 03, 2010, 03:25:23 PM »
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Quote from: MichaelHCothran
BUT...I still would like a more specific response to files less than 180. You state "upsample." I know that much. But how much do I upsample?? Do I go up to 180, then let the Epson take over, do I go to 360, or where? The answer to this question was the base intent of my original inquiry here.

As Wayne said, I would simply upsample 200% using Bicubic Smoother and add additional sharpening and perhaps even a touch of grain. See this article: The Art Of The Up-Res
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Michael H. Cothran
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« Reply #22 on: March 03, 2010, 07:02:25 PM »
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Wayne and Jeff -
Thanks for your replies. Jeff, I went back and watched the episode on resolution in the CtoP video again, and everything you and Wayne stated corroborate. But I'm really having a senior moment here, maybe a senior "day," as my mind is drawing a blank on how to calculate 200%.
If you have a 150ppi file, and increase it 200%, how big will the resulting file be? I know that sounds dumb, but if I multipy 150ppi times 200%, I get 300ppi, and if I add it to the original 150ppi, I get 450ppi, so I'm not sure what the correct answer is. Hmmm...
Forgive my stupidity here, but if you, or anyone, can just tell me how big a file you would have if you increase a 150ppi by 200%, then I can backtrack the math.
I promise this will be my last question on this matter.
Thanks.
Michael H. Cothran
« Last Edit: March 03, 2010, 07:14:01 PM by MichaelHCothran » Logged
Wayne Fox
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« Reply #23 on: March 03, 2010, 07:24:05 PM »
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Quote from: MichaelHCothran
Wayne and Jeff -
Thanks for your replies. Jeff, I went back and watched the episode on resolution in the CtoP video again, and everything you and Wayne stated corroborate. But I'm really having a senior moment here, maybe a senior "day," as my mind is drawing a blank on how to calculate 200%.
If you have a 150ppi file, and increase it 200%, how big will the resulting file be? I know that sounds dumb, but if I multipy 150ppi times 200%, I get 300ppi, and if I add it to the original 150ppi, I get 450ppi, so I'm not sure what the correct answer is. Hmmm...
Forgive my stupidity here, but if you, or anyone, can just tell me how big a file you would have if you increase a 150ppi by 200%, then I can backtrack the math.
I promise this will be my last question on this matter.
Thanks.
Michael H. Cothran
Just use the percentage option in the image size dialog box, and plug in 200.  This will actually calculate the pixel dimensions at the top to accomplish that.  For the DPI box, put in any number you want, it doesn't affect the upsample.   This is now your new "native" file size, when printing in photoshop you should be well over the 180dpi lower limit.  And yes, this will result in a file that is 4 times larger than your original file in size.  The reason you use 200% is it will use the optimum resample algorithms which only happen when you do even sampling, such as 12.5%, 25%,50%, 200%, 400%.

One last thing ... 180dpi is not a hard fast rule.  Depending on file quality and subject matter, using a number lower than that may yield a perfectly acceptable image.  Something with a lot of edge detail and sharp contrasting elements probably not, however a soft focus portrait of a smooth skinned child would probably look just fine at a lower dpi setting.

Here's a screenshot

[attachment=20633:DIALOG.jpg]
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BartvanderWolf
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« Reply #24 on: March 04, 2010, 03:52:41 AM »
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Quote from: bjanes
Printing the Qimage test chart on my lowly Epson 2200 at maximum resolution does show a difference between 720 and 360 dpi as shown here. Sorry for the typo on the right.

[attachment=20609:Qimage2.png]

Hi Bill,

The more advanced printers also vary droplet size <4 pico litre, and will show even more difference (paper quality also makes a difference). Also don't forget that the human eye (or rather the brain) can resolve much more detail than the sample would suggest. The phenomenon is called "Vernier acuity". Besides, sharpening the output at native printer resolution gives cleaner results than sharpening with larger radii.

Cheers,
Bart
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bjanes
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« Reply #25 on: March 04, 2010, 08:02:34 AM »
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Quote from: BartvanderWolf
Hi Bill,

The more advanced printers also vary droplet size <4 pico litre, and will show even more difference (paper quality also makes a difference). Also don't forget that the human eye (or rather the brain) can resolve much more detail than the sample would suggest. The phenomenon is called "Vernier acuity". Besides, sharpening the output at native printer resolution gives cleaner results than sharpening with larger radii.

Cheers,
Bart
Bart,

I wasn't familiar with vernier acuity even though I have used it many times when taking readings from instruments. A quick google search shows a couple of interesting articles: here and here. The essential points are that vernier acuity can be as high as 4-5 arcseconds rather than the commonly reported value of one arcminute, but that this type of acuity is contrast dependent. I'm not certain how this type of acuity would apply to most photographs which do not have high contrast line patterns.

Human vision is astounding in that it uses a relatively crude sensor (the eye) and then processes the information in the neural pathways and brain to achieve very good perceived results. If only we could to this with our cameras!

Although the eye can resolve down to one arcminute (giving a resolution of 30 cycles per degree), the contrast function peaks at about 6 cycles per degree and this is the basis for SQF (subjective quality factor) as explained by Bob Atkins in an essay for the intelligent lay person. MTF in this range is most important for the perceived quality of a print and this value is considerably below the limits of visual resolution.

Although most photographers want their images to be as sharp as possible, most observers would not view an 16 by 24 inch print at nose tip distances and good results can be obtained with resolution that is lower than the limits of visual acuity, since the eye is most sensitive to 6 cycles per degree.

Theoretical considerations aside, I think that the suggestions of Bruce Fraser and Jeff Schewe as outlined in the Real World Sharpening are reasonable for practical photography of most scenes and that is what I use in my own work. Most of the time we do not have as much resolution as we would like, but as the megapixel count of cameras increase, downsizing is necessary either in one's editing software or in the printer driver. As per recent discussions on this site, we probably should give more attention to aliasing and down sampling our sharpening halos out of existence in the process. Also, more attention should be given to image restoration algorithms that go beyond the early 20th century unsharp mask.

I would value your opinion in these matters.

Regards,

Bill
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Michael H. Cothran
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« Reply #26 on: March 04, 2010, 08:30:01 AM »
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Quote from: Wayne Fox
Just use the percentage option in the image size dialog box, and plug in 200.  This will actually calculate the pixel dimensions at the top to accomplish that.  For the DPI box, put in any number you want, it doesn't affect the upsample.   This is now your new "native" file size, when printing in photoshop you should be well over the 180dpi lower limit.  And yes, this will result in a file that is 4 times larger than your original file in size.  The reason you use 200% is it will use the optimum resample algorithms which only happen when you do even sampling, such as 12.5%, 25%,50%, 200%, 400%.

One last thing ... 180dpi is not a hard fast rule.  Depending on file quality and subject matter, using a number lower than that may yield a perfectly acceptable image.  Something with a lot of edge detail and sharp contrasting elements probably not, however a soft focus portrait of a smooth skinned child would probably look just fine at a lower dpi setting.

Here's a screenshot

[attachment=20633:DIALOG.jpg]

Thanks Wayne. And especially for the screenshot. Things like that really help sort out confusion. In my further ignorance, I did not realize one could change from inches to percentage in the image size dialog box. What a revelation!
I just purchased Photokit Sharpener last night, so tonight I'm in for a treat trying it, along with the resolution practices you and others have shown me here.
I do have pretty good files - from a Hasselblad V camera with Zeiss lenses, from scanned film (producing roughly 8900x8900 pixels) and a CFV 16 mpix digital back (4080x4080 pixels). My digital back will produce a 24" print @ 170 ppi. I feel pretty confident in leaving it alone, but I plan to sample a file tonight with 200% uprezing, using the Photokit Sharpener's Creative Sharpening as described by Jeff in the CtoP video, then the Output sharpening, and see what happens.
I also understand the concept of "even sampling." I currently use the same concept in monitor viewing, utilizing only even sizes in which to view and work. Due to the size of my files, I normally use 12.5% and 6.25% for full image view, and the larger even sizes for local work. You mention smaller even upsampling amounts like 100%, 50%, 25%, etc. Is there ever a time when one might want to use these smaller amounts to uprez, or is it best to stick with 200%? Jeff made mention of 400% uprez, but only on the very best quality of files.
Thanks again,
Michael H. Cothran
Nashville, Tennessee
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Doombrain
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« Reply #27 on: March 04, 2010, 10:45:43 AM »
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Quote from: BartvanderWolf
You seem to suggest that the resolution is limited to 360PPI. If so, then how would you explain that a resolution difference in output (upto 720PPI) can be demonstrated?

The printer driver also feeds back info to the calling program that the native resolution is 720PPI with glossy paper. This allows to apply sharpening at the interpolated data at 720PPI, which allows to apply more sharpening without visible artifacts.

Cheers,
Bart

i'm talking about the mechanical rez of the heads, as in how many nozzles per row.
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Schewe
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« Reply #28 on: March 04, 2010, 11:52:39 AM »
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Quote from: BartvanderWolf
The more advanced printers also vary droplet size <4 pico litre, and will show even more difference (paper quality also makes a difference).

Just to be clear, Epson pro printers do not use variable droplet size when the output is set to 2880-it ONLY outputs the smallest droplet. Depending on the paper they may also not use variable droplets at 1440. They will use variable droplet sizes when set to 720.
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BartvanderWolf
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« Reply #29 on: March 04, 2010, 09:20:47 PM »
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Quote from: bjanes
Bart,

I wasn't familiar with vernier acuity even though I have used it many times when taking readings from instruments. A quick google search shows a couple of interesting articles: here and here. The essential points are that vernier acuity can be as high as 4-5 arcseconds rather than the commonly reported value of one arcminute, but that this type of acuity is contrast dependent. I'm not certain how this type of acuity would apply to most photographs which do not have high contrast line patterns.

Human vision (eye+brain) is mostly sensitive to edges. Edges may be of high spatial frequency and, if at adequate luminance level, they will be resolved at higher than perhaps anticipated levels. Edges are also easier to discern than gratings. Also, human vision doesn't mind at which angle the edges are presented (due to the random placement of the (mostly) rods and (less important for resolution) cones. Digital images however, in a common rectangular X/Y grid configuration, have a (sqrt(2)) higher resolution in the diagonal direction.

Quote
Although the eye can resolve down to one arcminute (giving a resolution of 30 cycles per degree), the contrast function peaks at about 6 cycles per degree and this is the basis for SQF (subjective quality factor) as explained by Bob Atkins in an essay for the intelligent lay person. MTF in this range is most important for the perceived quality of a print and this value is considerably below the limits of visual resolution.

Yes, but it is important to understand that boosting the image resolution at a given (e.g. limiting) resolution, will also boost the resolution of slightly lower spatial frequencies (there is no sharp cut-off, it's gradual).

Quote
Although most photographers want their images to be as sharp as possible, most observers would not view an 16 by 24 inch print at nose tip distances and good results can be obtained with resolution that is lower than the limits of visual acuity, since the eye is most sensitive to 6 cycles per degree.

At common resolution, yes. That doesn't mean that at optimal luminance levels, and for slightly displaced features, we cannot see a difference between optimal an sub-optimal resolution. How important that distinction is, remains open for debate. However, when the higher quality level can be achieved with only an investment in technique, I don't see why we shouldn't use it (unless 'cost' is an issue). 'Cost' can be expressed as a monetary effort, but also as a computational (or intellectual ...) one. It also has something to do with risk management, the willingness to revisit 'some' percentage of the results (if we're given a chance), to promote them from 'good enough' to excellent. From a commercial standpoint 'good enough' is not competition resistant, especially when the 'cost' is relatively minor.

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I would value your opinion in these matters.

It depends on one's goals, (hopefully) good enough versus (as good as it reasonably gets) excellent.

Cheers,
Bart
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Wayne Fox
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« Reply #30 on: March 05, 2010, 02:27:06 PM »
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Quote from: Doombrain
i'm talking about the mechanical rez of the heads, as in how many nozzles per row.
guess I'm confused as well ... how is that relevant to output resolution?  Lower mechanical resolution just means more passes to get the same dots on the paper.
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deanwork
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« Reply #31 on: March 05, 2010, 06:41:14 PM »
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Exactly. Works great with the Z3100, esp with dslr files.







Quote from: Ernst Dinkla
Or just use Qimage today and select the right extrapolation algorithm for the image content, the 5 degrees of upsampling, the anti-aliasing slider on downsampling and the smart print sharpening slider. All done on the fly at print processing time. Enough ways in Qimage to make a small proof print when in doubt.


met vriendelijke groeten, Ernst Dinkla

Try: http://groups.yahoo.com/group/Wide_Inkjet_Printers/
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