Clear print with 72-100 dpi…how?

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Prophet621
Nov 20, 2003
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1612
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I see lots of files that are created at some design or graphics company and they bring them in to print. The files are usually tiff and the resolution is 100 and sometimes 72 yet the print is perfectly clear.

Can someone give a quick (or long if you like) but informative explanation how this is achieved. I would love to be able to do this, this would really help with some of the enormous file sizes I often deal with.
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zippy2000
Nov 20, 2003
Assuming you are referring to actual linescreen (not monitor resolution PPI), set your photoshop resolution to 144ppi (which is a linescreen of 72dpi). This will produce a relatively clear image. For a cleaner, crisper image, set the PPI in photoshop to 266/300ppi.

Just remember, if the images come to you in that low resolution format, you won’t be able to add resolution without sacrificing quality or image size.

If you ARE referring to photoshop resolution (ppi), I can’t see how the output is printing clearly. The resolution you speak of is typically used for monitor display (ie. internet websites). Whenever I have printed a 72ppi graphic (from the web), it always comes out pixelated. How are these images being printed (ie. inkjet, thermal, offset, etc)?

PS> I am trying my hardest not to lead this thread into another "printing resolution / pixel debate".
B
BLUDVLZ
Nov 20, 2003
Well Prophet621,

(I’m making the assumption that the native image files are 72ppi, but are placed in a layout program for output).

When an image is placed in a layout program and reduced, it does, in effect, gain the necessary resolution needed to provide a clean output at the smaller size. You can check this by running the file through a preflight program. You’ll see that files created at a particular resolution will report as a completely different resolution when enlarged/reduced.

If this isn’t the case, perhaps you can shed some light as to how the files are being printed. There are differences between inkjet & laser printers and high-end output devices. The inkjet and laser printers may be more likely to hide quality shortcomings than the more expensive, accurate devices.
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Prophet621
Nov 21, 2003
Thanks for the replies, I’ll try to explain a little better. A few weeks ago I had someone from a designb company come in an needed an oversize print. He said he had a tiff file on the cd and it was already sized to 8’x3′. At this point I got a little concerned, I’m thinking huge file size because the biggest file I can send postscript is about 130mb.
I opened up the cd and saw that the file came in just under 100megs so I opened it up and it looked clear but we all know how deceiving that is considering monitor resolution is below 100 itself and it was fitting inside the window. So I check image size and it’s set to 96×36 with a resolution of 100. I zoom in on one of the photos and it didn’t begin to pixelate until I got to 200%. So I go ahead and send it to print on an HP 5000.
The print was clear, no pixelation anywhere, all the lines were smooth, the pictures were sharp and I just stood there trying to figure out how on earth they did it, I would have asked the guy but he was gone and I wasn’t there when they picked the print up.
Other times I’ve seen the resolution as low as 72. I spend good parts of my day explaining to people why they can’t get a good print from this pic they grabbed of the net because of the resolution and why there is no way they can blow it up 10 times it’s size. Then I see these created files come along that go against everything I know.

I thought maybe create it in Illustrator then export as tiff but still…72 dpi should still be too low. Then why not just give the the ai file or an eps? Why a tiff? Maybe I’ll give that a shot later.

By the way, I was under the impression that ppi and dpi were the same.. pixels per inch-dots per inch, what’s the difference between them?
PH
Photo_Help
Nov 21, 2003
Prophet,

100 PPI is fine for large prints. As for zooming on screen you would see the same pixelization on a 100 PPI image at 200% that you would see on a 300 PPI image at 200% zoom. The zoom is simply the ratio of screen pixels to image pixels.

The reason 72 PPI web images don’t print well is because they are highly compressed JPG’s that are usually under 640×480.

When you have a good uncompressed. TIF at a true (not upsampled) resolution of 100 PPI you will get very good results as you have seen.

I must say though that I am a bit shocked to see a question like this coming from someone that works in the print industry.
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zippy2000
Nov 21, 2003
By the way, I was under the impression that ppi and dpi were the same.. pixels per inch-dots per inch, what’s the difference between them?

This is only my understanding of PPI/DPI. (feel free to correct me if I am wrong or off base)

PPI refers to the amount of pixels withing a one inch square on the monitor. DPI (AKA: linescreen) refers to the amount of dots within a 1 inch square of the halftone (printed piece/final output). When an image is printed, the PPI is converted to DPI (pixels converted to halftone dots) via RIP server. Typically, the PPI is double the DPI (ie. 266ppi would be 133dpi – a pretty standard high res output setting). So even though there is a difference between PPI and DPI, there is a relation between the two.

Hope this helps.
ZIP
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Prophet621
Nov 21, 2003
Thank for the reply. I work as tech support and a general problem solver for troublesome files and I handle some of the oversize printing as needed. That printer has a few idiosyncrasies that I know better than anyone else that even HP can’t seem to figure why it does some of the things it does.
I had a decent grasp on digital imageing prior to this job but I had never printed anything.. except text and such on my home printer so I’m learning as I go along. This just seemed to go against everything I knew about resolution.
PH
Photo_Help
Nov 21, 2003
Prophet,

Cool, your question makes a lot more sense now.

The main concern is the quality of that data (hopefully saved with lossless compression) and making sure you print at a high enough resolution that you don’t see square pixels in the print. Obviously large solid color areas aren’t a big concern where images with small text and high detail could be a problem.

I hope this helps.
L
LenHewitt
Nov 21, 2003
Zippy,

No. You’ve got it wrong.

DPI is NOTHING WHATSOEVER to do with LPI which is the linescreen of a print and is only applicable for prints made with a conventional linescreen.

In fact for a linescreen of 150 LPI, the printing device needs a resolution of at least 2400 dpi to achieve the full possible range of 256 levels per channel.

You may find the following LPI/DPI ready reckoner of interest:

For any given printer dpi, as the linescreen frequency increases, the number of reproducible grey tones decreases. Here is a ready-reckoner:Screen
frequency (lpi) Output device resolution (dpi)
150 300 400 600 1200 2540
30 26 101 179 401 1601 7169
40 15 57 101 226 901 4033
50 10 37 65 145 577 2582
53 9 33 58 129 514 2298
60 7 26 45 101 401 1793
65 6 22 39 86 342 1528
85 4 13 23 51 200 894
105 3 9 16 34 132 586
120 3 7 12 26 101 449
130 2 6 10 22 86 383
150 2 5 8 17 65 288
200 2 3 5 10 37 162
220 1 3 4 8 31 134
300 1 2 3 5 17 73

Intermediate values can be obtained by using the following formula:

(dpi/lpi)^2 = No. of available tones.

This does not take into account dot gain, tone splatter, PostScript limitations etc.

If you have the LJ4 set to 300ppi and a default linescreen of 53lpi (plc mode) then you will only get 33 distinct tones, which is not enough for even shading. At 600 dpi that will increase to 129 tones which will be much better.

But for what is normally accepted as ‘continuous tone’ (256 tones) you would have to drop the linescreen down to less than 40 lpi which would give a very coarse screen
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zippy2000
Nov 21, 2003
Hi Len,

No. You’ve got it wrong.

Mmmm…I know I still don’t quite understand the DPI/PPI/LPI dynamics but to say I have it wrong isn’t really accurate. I have been using the "resolution is normally double the linescreen" method for the past four years and have never had problems when offset printing. In the three years of design college, our prepress instructor would drill that point home at the beginning of each lecture and is now something I have been programmed to believe. I think they take this approach because it is a fairly easy to explain to newb designers (I still remember cramming for exams and trying to grasp the concept of resolution and printing. If an instructor was to take your previous post (which is an excellent, well detailed explantion BTW) and try to explain it to first year design students, you’d probabaly get alot of blank expressions and a lot of clueless faces.

I would suggest you take a look at this link.

< http://www.ou.edu/class/digitalmedia/articles/Resolution_.ht m>

A fairly concise explantion of DPI/PPI/LPI. Scroll to the bottom and read the section "Image Resolution for Printing via Offset Printing Press" and I quote:

Once you know the screen frequency of the finished halftone, you can scan and otherwise prepare your images. There is a rule of thumb used in preparing images for printed output. Scan or size images at a resolution (PPI) of
1.5 to 2 times the screen frequency (LPI) of the finished halftone. For
example, a 2 inch by 3 inch photo would be scanned or sized to a 2 inch by 3 inch image at 225 to 300 PPI for a finished halftone to be printed at 150 LPI (150 line screen). Were the same photo to be printed using a 120 line screen halftone, it would be scanned or sized to 2 inches by 3 inches at 180 to 240 PPI. This is a very workable rule and is easy to remember.

That qoute is what I was getting at in my previous post. I think I may be getting confused with the difference between LPI and DPI. To correct myself, I should of said LPI instead of DPI. My bad!
L
LenHewitt
Nov 24, 2003
Zippy,

No you got it wrong.

You said:

"PPI refers to the amount of pixels withing a one inch square on the monitor. DPI (AKA: linescreen) refers to the amount of dots within a 1 inch square of the halftone (printed piece/final output). "

1) ppi refers to number of pixels in a LINEAR inch
2) No way is DPI also known as LPI – the two are VERY, VERY different and only connected by the formula I posted for you.
3) Linescreen frequency is agian measured in LINEAR inches.

Now here is a bit about the oft-repeated "2x the linescreen frequency":-

I’ve always heard the "rule-of-thumb" number as ranging from 1.5 to 2.0x
thelinescreen frequency of the output device. <<

This little bit of conventional wisdom is so firmly embedded in the consciousness of the industry that I sometimes despair of *ever* rooting it out. But I continue to try. <g>

The easiest place to find quick reference on this is in the January 1994 issue of Pre magazine in a column by Brian Lawler.
http://www.thelawlers.com/FTP/Resolution Essay.pdf.sit) [unfortunately, that link is no longer valid] But it has also be covered elsewhere, among other places, in Publish, in the July 1992 issue in an article written by Jane Hundertmark about the process the San Francisco Examiner went through in switching their production over to a digital process.

To keep it simple, it goes like this. When you halftone an image, the *best* resolution you can get is the frequency of the halftone screen. So if you use a 1:1 oversampling ratio, your image resolution, (at 1:1), should be that of the halftone screen. 100, 120, 130, 150 ppi. HOWEVER, this is only going to work really well if the halftone screen is aligned with the pixel grid. If it’s turned at an angle, you’ll get dropouts and really bad averaging. So you need to get between 1.16 and 1.18 pixels per dot as a minimum to get good averaging and to compensate for screen angles. You can improve the quality of output on any image to an oversamping ratio of about
1.25:1. Problem images, (herringbone tweed, for example), can require as
much as 1.4:1. Beyond that, nobody can tell the difference.

But don’t take my word for it. Try it for yourself.

Oh, and BTW, You mentioned that Bruce and David used the "standard" rule of thumb in RWPS3. You’re right, they did. But take a look at the images illustrating concepts in oversampling in Blattner and Roth’s _Real World Scanning and Halftones_. They are resolved at 186ppi. 1.4 times a 133 lpi screen. <very wide grin>

Egads,
Wiz
06-Feb-97 at 18:48:17
DM
dave_milbut
Nov 25, 2003
newsreader bump for len and zippy! found this on page 3!
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zippy2000
Nov 25, 2003
Thanks for the bump Dave, I would have missed Len’s reply.

Hi Len,

Yea, I’ve always known I havn’t quite figured out the dynamics of this resolution debate (if you can even call it that<g>). I look at it this way…if the methods I have been using since I started working in the industry have bared fruit everytime without any hitches, then I am doing alright (the old saying "Why fix something that isn’t broke"). I have saved your posts for further study. Clearly, I still have some work to do in this area as far as understanding goes.

I really do appreciate you (Len) taking the time and effort to clarify this issue. Your synopsis on resolution for offset printing is very informative and shows that you know what you are talking about. And I can understand your frustration over the industry "programming" in regards to resolution. Since entering this field (and more recently becoming a regular forum reader), I have become aware of many of the misconceptions in the industry. It is for this reason that I invest alot of time reading these forums as they are the best resource for all things Photoshop and other Adobe products.(and I mean that)Thanks for the clarification.

I think Len’s posts in this thread should be added the FAQ so that it may be referenced for future use in the user to user forum.

What can I learn from this: 1) I still have a ways to go before I can say I know all there is to know about printing resolution. 2) That I should not be so hastey in replying to resolultion threads as it is clear that I do more harm than good when it comes to giving advice. I will stick to creative threads and threads on digital imaging techniques as that is were my real knowledge and talent is. <vbg>

Battered but not broken,
ZIP
B)
DM
dave_milbut
Nov 25, 2003
I think Len’s posts in this thread should be added the FAQ so that it may be referenced for future use in the user to user forum.

do you mean like this?

LenHewitt "Image Resolution versus Line Screen Frequency" 5/29/02 2:29am </cgi-bin/webx?50>

🙂
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zippy2000
Nov 25, 2003
do you mean like this?

I knew that had to be in there somewhere…thanks Dave!
L
LenHewitt
Nov 26, 2003
Zippy,

Battered but not broken,<<

Don’t feel ‘battered’ – if I have helped in your understanding then that is all that matters. Would that everyone was as open to considering information that conflicts with such "industry standards"
Z
zippy2000
Nov 26, 2003
if I have helped in your understanding then that is all that matters.

That it is why the forums exist and why I come back again and again. Thanks again, Len. 🙂

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