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Old Mar 21, 2006, 5:37 PM   #11
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eric s wrote:
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What I think people are talking about is the use of 72DPI (not 75DPI) and how it relates to TV (I think its TV, not movies.)

That value has abosolutely no effect on how the picture is displayed on your monitor. It is flat-out ignored. You can change the DPI and nothing will happen to the image on screen.

Eric
Absolutely correct.

But to answer Nagasaki, an RGB screen DOES display 16 plus million colors. Yes, it too is fooling the eye, but in a completely different manner. It does this by projecting the light, whereas a printed page is reflecting the light.

CYMK "fools" the eye in a much cruder manner. This can be proved by simply looking at a magazine print with a decent magnifyng glass. Even a 3000 dpi linotype can only show 200,000 colors.

But this is off the track. Picture those 72 ppi images displayed on your screen and try to translate that on the printed page. You are now dividing each horizontal and vertical part of the page into squares which must be filled in by your home printer.

Try to keep this in mind. Photoshops default, unless changed, is 72 dpi - which matches the output of monitors. Note that even a small image, let us say 700 by 437 pixels, will print out at 9.7 inches by 6 at that resolution.

It wont look pretty. Now your printer (assuming a 720 dpi) will not be limited to the number 72. It will, using IT's full dpi to fill in the boxes, so to speak. But what is it filling in? Mighty big boxes.

Now the same image at 300 ppi, will print out at 2.3 by 1.4 inches. If you double the size, to 4.6 by 2.8, that's still 150 ppi. Better. Might even look decent, but not something you want to frame.

We are barely scratching the surface of a complex topic.

Dave
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Old Mar 21, 2006, 8:46 PM   #12
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me2 wrote:
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If an image looks really good on a computer screen at 75 DPI, then why does it need 300 DPI when printed ?

Maybe 75DPI is a little low, but what would be wrong with 100 or 150 DPI ?
A big part of the reason is that you see a print by reflected light, and a monitor by direct light. Another reason is that you are viewing the monitor from arms length or more, but you generally look at prints a lot closer. Take a print of the same size as your monitor image, printed at the same DPI and compare them. They probably won't look too much different. Now, turn down the monitor brightness, and compare both from a couple inches away.

If making large prints, which are usually viewed from more distance than small ones, you can get away with lower resolution.

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Old Mar 21, 2006, 9:09 PM   #13
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My printer is 5760 DPI so each of the 300 dots of my image is made up of a mix of up to 28 individual dots of ink.
8 and 10 picoliter printers used 16 dots to represent a pixel. The last I have a manufacturer number from was a HP printer with 5 picolitre dots that used 32 dots per pixel. I'm guessing that 32 dots per pixel was also used on 4 picolitre dot printers. When the 2 picolitre dot printers came out I read that they used 64 dots per pixel but that wasn't from a manufacturer. Whether 1 picolitre dot printers went to 128 dots or stayed with 64 I haven't a clue.

The number of dots required for a pixel has gone up at about the same rate as the total dots, so images have required about the same input as the printer DPI has increased. I think the number of dots per pixel is what the printer would use to mix colors and make black. But whatever it represents it is also used by the manufacturer to represent the printer DPI. It doesn't necessarily use all the pixels you feed it.

If you just divide the DPI by the number of dots used you come up with too low a PPI required for a print. The old commercial printing LPI required 2X the calculated LPI for highest quality output. It has something to do with the diagonal between the dots. Even then the numbers usually come out to around 200 PPI input and I think that is still a little low. Under magnification I can see a tad of difference between a photo printed from 200 PPI and one from 250. I can't see a difference above 250 on either of my current printers or any I had before these.

I've hoped Steve would give us some idea of where the point of diminishing returns is passed with some of the current printers since all of them pass through his testing. But I think you are safely above that with any current consumer printer using 300 PPI.

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Try to keep this in mind. Photoshops default, unless changed, is 72 dpi - which matches the output of monitors. Note that even a small image, let us say 700 by 437 pixels, will print out at 9.7 inches by 6 at that resolution.
The camera has to put a number in a JPG or TIFF file and many choose 72 PPI, which is the resolution of the original Mac screens. My Olympus used 150 PPI and my Nikon used 300 PPI. From posts I think all Nikons will display at 300 and I would guess most Olys at 150. Whatever the camera puts in the file is what Photoshop displays when you originally go to Image > Image Size.

Many screens display more than 72PPI. It is a function of screen size and the resolution the user sets. My 19 inch screen is 14.5 inches across. So if I displayed at the maximum it can use of 1600 X 1200 I would display at around 110 PPI.

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Old Mar 22, 2006, 2:36 AM   #14
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This is a great topic. I want to make some large prints and I have never done it.

First question... back to the 75 DPI. I realize the DPI in the EXIF info doesn't mean anything, but my computer monitor dimensions sure do. I've got a 20 inch Flat Panel Widescreen, 1050 x 1680 pixels. It measures ahbout 10.5 x 17-, so lets call its resolution 100 DPI. At anything more than 10 inches away, nice digital pictures tend to look fantastic on it. So, if 100 DPI works on my monitor, why doesn't it work on a printed image ?

Next, how does interpolation fit into this ? Lets say my D50 captures an image 2000x3000 pixels and I want to blow it up to 20x30 at 300 DPI. There are several image manipulation packages that will increase the pixel count of an image by interpolating pixels into it. Would that help ?

Thanks.
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Old Mar 22, 2006, 3:56 AM   #15
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Interpolation helps but it has it's limits. They create extra pixels by averaging the adjacent pixels. Just how they do this and how well depends on the algorythm used. When used they create a smooth image without the jagged edges you'd get without interpolation. What they can't do is create extra detail. If you took a shot at 6000 x 9000 pixels you would keep seeing more detail as you get closer to the print. taking at 2000 x 3000 and interpolating up to 6000 x 9000 there would be an optimum viewing distance and getting closeryou would not see any more detail. If you are printing to 20 x 30 inches you are probable going tobe looking at it for a reasonable distance not arms length so it works.

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Old Mar 22, 2006, 7:35 AM   #16
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I am impressed by the fact that the resident big boys have come out to play on this topic. And they have different information, and sometimes disagree with each other. It shows how complex this whole issue truly is. That Dashboardgyno kid can go, but the rest, I want to thank for the information, and being the go to guys!!
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Old Mar 22, 2006, 9:22 AM   #17
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DBB wrote:
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We are barely scratching the surface of a complex topic.
True, but for the purpose of simplicity its probably enough to note the difference between pixels and dots. I have 8x10 prints made at 180 PPI (pixels per inch) that look ok, but not great. Others I have at over 250ppi which look very good. In order to print 250 pixels in that inch, your printer must print many more 'dots', because of the color mixing and all that technical stuff. It will take several dots to make a pixel.

So you have to have enough pixels per inch to have a good looking photo, and then the printer must use enough dots per inch to present those pixels well.

Most of the time, your going to use one printer (the one you have, or a particular store) so that set of variables is not in question. Mostly, we talk about pixels per inch, since thats a result of the camera, cropping in photoshop, interpolation... in other words things we are in control of.

To answer the original question, a 6mp camera is perfectly capable of printing billboards tens of feet wide. No sweat. Just never get closer than a hundred or two feet, and it will look great!

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Old Mar 22, 2006, 11:11 AM   #18
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tmoreau wrote:
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DBB wrote:
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We are barely scratching the surface of a complex topic.
True, but for the purpose of simplicity its probably enough to note the difference between pixels and dots. I have 8x10 prints made at 180 PPI (pixels per inch) that look ok, but not great. Others I have at over 250ppi which look very good. In order to print 250 pixels in that inch, your printer must print many more 'dots', because of the color mixing and all that technical stuff. It will take several dots to make a pixel.

So you have to have enough pixels per inch to have a good looking photo, and then the printer must use enough dots per inch to present those pixels well.

Most of the time, your going to use one printer (the one you have, or a particular store) so that set of variables is not in question. Mostly, we talk about pixels per inch, since thats a result of the camera, cropping in photoshop, interpolation... in other words things we are in control of.

To answer the original question, a 6mp camera is perfectly capable of printing billboards tens of feet wide. No sweat. Just never get closer than a hundred or two feet, and it will look great!
LOL! I used to write explanations of this for an image processing magazine. This was a LONG time ago, and I've actually forgotten quite a bit of stuff. Just reading these posts is bringing it back.

I actually make a living making prints...:lol:

But I'm doing my own prints.

First, when an image is printed it is "screened." There are options for this in Photoshop, but the algorhythems used for home printing are very simple and by pass the professiona printing process.

The term "lines per inch" do not relate to the "stochastic" form of printing used at home.

If you examine a magazine image that is screened conventionally, you will see a distinct pattern of dots, whereas the output of a home printer, the pattern appears random.

But back on topic.

1. How could I print an image that was 1600 by 1200 at 16 x 20 and still have it come out looking breath taking?

This was a landscape image, without much detail. I interpolated it up to over a 100 Megs and printed it at 300 PPI. But Photoshop doesn't have the term ppi in its image dialoge box. This is a mistake. Because the "dot" on your monitor does NOT correspond to a dot of your printer.

This is where much confusion sets in. Your image consists of pixels and your printer puts down dots.

PPI, means pixels per inch - meaning each little square can be any one of 16 million colors, while a printer dot is merely one dot of the arbitrary printer colors.

If your nimage is only 75 PPI (or as PS would say, dpi) - they mean that literally. Each inch is divided into 72 individual color squares. How can this possibly look good?

2. The second aspect of this is screening and using this to figure out the correct size. Those of you printing at home need not worry about screening or any of it's implications.

Screening is done for professionl printing of books, magazines, newspapers, etc, where an image is printed FOUR times by a machine, each pass laying down a different color. Each image is divided by a grid (which can take many forms) and creates a distinct pattern. Only by using screening can these multipass prints align the colors properly. Newspapers print at 75 LPI, Magazines at around 100 to 133 and fine art books at 200 LPI. meaning a smaller sized screen.

There is a rule of thumb for sizing an image that is screened, but it differs from that of the home printer.

And thus all the advice to make your prints at 300 dpi (actually ppi). Whew!

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Old Mar 22, 2006, 12:37 PM   #19
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But Photoshop doesn't have the term ppi in its image dialoge box. This is a mistake. Because the "dot" on your monitor does NOT correspond to a dot of your printer.
I don't know how far you have to go back to find "DPI" as a resolution in Photoshop, but they have used pixels/inch for quite a while now. This is how an image from a little Nikon I got for my daughter a couple of years ago displays in the image size box in Photoshop CS:



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And thus all the advice to make your prints at 300 dpi (actually ppi).
I don't necessarily agree with that. Any resample involves internal interpolation of the pixels with a very small but real degradation of the image. If you have enough resolution that more isn't going to improve the image there isn't any reason to resample to 300 PPI. If an image came out to be around 250 PPI I wouldn't resample it. If I have to resample anyway I use 300 PPI as I know it is plenty and there is no advantage to using a lower PPI.


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Old Mar 22, 2006, 2:47 PM   #20
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slipe wrote:
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But Photoshop doesn't have the term ppi in its image dialoge box. This is a mistake. Because the "dot" on your monitor does NOT correspond to a dot of your printer.
I don't know how far you have to go back to find "DPI" as a resolution in Photoshop, but they have used pixels/inch for quite a while now. This is how an image from a little Nikon I got for my daughter a couple of years ago displays in the image size box in Photoshop CS:



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And thus all the advice to make your prints at 300 dpi (actually ppi).
I don't necessarily agree with that. Any resample involves internal interpolation of the pixels with a very small but real degradation of the image. If you have enough resolution that more isn't going to improve the image there isn't any reason to resample to 300 PPI. If an image came out to be around 250 PPI I wouldn't resample it. If I have to resample anyway I use 300 PPI as I know it is plenty and there is no advantage to using a lower PPI.

LOL - I stand corrected. But if you notice most people don't understand the concept.

The other aspect of this question is that you can resize without resampling. Meaning that often enough the Photoshop defaults are being used (72 ppi), thus giving a huge print size. Merely changing the ppi, without resampling shows the size printout in a more meaningful way.

Thanks for the correction (and yes, you do have to go back a long way) :lol:

dave
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