# Thread: Long discussion on DPI

Even if ur Screen reso is big, but ur print reso is low when u go to printers to print the output is different.

just give the answer assuming the output paper size is the same for both as stated. I'm really trying to help you understand the concept. if not I would have given up like the others. To simplify the problem, assume both images will be fitted and printed borderless on a 4 inch by 6 inch index card using exactly the same printer.

2. Pixel dimension(e.g 1800)/ output dimension (6inches) = 300 pixels per inch.

Just think of what the term DPI means. The proper term for this is pixels per inch.

DPI is often used interchangeably, but is more appropriate in referring to the number of ink dots per inch for printers.

3. *give up*
u guys not seeing eye to eye, so disagreeing on the wrong things...

4. Are u here talking about photographic labs or offset printers?

B'cos where I'm coming from is saying that offset printers for eg, u don't submit a 72dpi image for print.

Originally posted by mpenza

just give the answer assuming the output paper size is the same for both as stated. I'm really trying to help you understand the concept. if not I would have given up like the others. To simplify the problem, assume both images will be printed borderless on a 4 inch by 6 inch index card.

5. Originally posted by denizenx
*give up*
u guys not seeing eye to eye, so disagreeing on the wrong things...
Think the point I'm trying to put across is to ignore and fancy stretching or interpolation algorithm and just consider the source pixels first.(As that's the amount of "real" info).

Taking into consideration software resampling just muddies everything.

6. Ya tt's what I been saying. On screen u can't tell the difference, but after printing is another story.
Originally posted by Zerstorer
Pixel dimension(e.g 1800)/ output dimension (6inches) = 300 pixels per inch.

Just think of what the term DPI means. The proper term for this is pixels per inch.

DPI is often used interchangeably, but is more appropriate in referring to the number of ink dots per inch for printers.

Are u here talking about photographic labs or offset printers?

B'cos where I'm coming from is saying that offset printers for eg, u don't submit a 72dpi image for print.

I get what you mean and that's probably where the source of the problem is. TVs, monitors and usual printers are not offset printers which you're familiar with.

For the question, assume you use normal inkjet printers or send the image to a photolab to print.

which pic has a higher resolution when printed on the same paper size? (using your definitions)

a. 1600x1200 72DPI
b. 640x480 300DPI <== amended to 480 instead of 280.

Ya tt's what I been saying. On screen u can't tell the difference, but after printing is another story.
On screen you CAN see a difference!

Viewing the same resolution image at different display resolutions gives a different look, simply because the pixel density of the display varies at different resolutions.

Just think, the 640x480 image filled your whole screen at 640x480 screen resolution.

At 1600x1200 screen resolution, the same image only takes up a tiny area in the centre of the screen.

This is what I mean by saying that the actual dpi(ppi) of monitors is not the standard quoted 72dpi at all times.

9. Ok. I don't know wat the print dimensions to ur image are.

In the case, the 1600x1200 72dpi image has probably a larger print size then the 640x480 300dpi image. (But not necessarily)

If I print it on a small piece of paper we can't see the difference, but if we blow it up there is a difference.

Originally posted by mpenza

I get what you mean.

For the question, assume you use normal inkjet printers or send the image to a photolab to print.

10. Yes I concur with u. But say u don't change ur OWN screen reso from 640x480 to 1600x1200. U cannot tell the difference between a 100x100 pixel at 72dpi and a 100x100 300dpi image.

Originally posted by Zerstorer

On screen you CAN see a difference!

Viewing the same resolution image at different display resolutions gives a different look, simply because the pixel density of the display varies at different resolutions.

Just think, the 640x480 image filled your whole screen at 640x480 screen resolution.

At 1600x1200 screen resolution, the same image only takes up a tiny area in the centre of the screen.

This is what I mean by saying that the actual dpi(ppi) of monitors is not the standard quoted 72dpi at all times.

11. Oh look what I've done...

Ok. I don't know wat the print dimensions to ur image are.

In the case, the 1600x1200 72dpi image has probably a larger print size then the 640x480 300dpi image. (But not necessarily)

If I print it on a small piece of paper we can't see the difference, but if we blow it up there is a difference.

erm I mentioned 4R, 4 inches by 6 inches.

Anyway, we found the source of the problem. You didn't associate the image's "DPI" with printsize and this is where you're incorrect. A 1600x1200 output is only 72DPI when you print it 22.22 inches by 16.7 inches. Else, DPI does not make any sense. Corresponding, a 640x480 output is only 300DPI only when you print it 2.13 unches by 1.6 inches.

If you print both ~4R, the 1600x1200 pic will give rise to ~267DPI and the 640x480 pic will be ~107DPI.

The point I'm trying to make is that for any printers, offset or not, and output device, DPI only make sense when there's a output size. Even for offset printing, you won't send it a 2x2 "300DPI" pixel image for "A4" printing right? Or send a 300000x20000 "300DPI" for printing on a 4x6 inch size right?

13. I believe all of you are saying the correct stuff.

To me there are two ways for accessing the output of a D image.

1) To get an image on the monitor screen, "the Dpi does not matters!!". Since the monitor screen resolution is already a fixed value, therefore the primary measurement unit for a screen display would be the pixel count. 300*450 pixel at 300 dpi will appear to be the same size on screen as 300*450 pixel at 100 dpi. This is because dpi "IS NOT" a variable for the screen.

2) To get an image on a print, a combination of two variable is required to determine the "Physical Size" of the printed image. First is the pixel count, then the Dpi of the printer setting. The reason is that the Dpi setting of the printer is a variable. The physical size of the print cannot be determined unless this is determined.

Take for example,

To print a S8R picture on your home printer,

if you've set the printer printing option to 200 dpi, then you'll need a (8*200) * (12 *200) = 1600 * 2400 pixel.

if you've set the printer printing option to 300 dpi, then you'll need a (8*300) * (12 *300) = 2400 * 3600 pixel.

and so on....

Lastly, I agree with what Jed has mentioned eariler on the content of a news image.

IMHO, A meaningful but lousy image is a million times better than a technically excellent but irrelevant picture........

14. This must have been the most confusing topic around.

To re-iterate

1. There is NO SUCH THING as 1800 x 1200 @ 72ppi, 300ppi, 2880ppi, 1000000ppi or whatever. It just doesn't make sense. Note I used PPI. PPI and DPI is different. DPI only comes into the picture when you output the thing onto print, where the image is then made up of DOTs. IMAGE resolution and PRINT resolution is different.

2. Like mpenza illustrated, on-screen, as long as image dimensions are the same, a 1ppi image and a 300ppi will be exactly the same. There's exactly the same number of pixels in the file. But if you print the image at 300ppi, and print the same image at 150ppi and yet another at 72ppi, you will get:
a. 1 huge print, not so good quality (72ppi)
b. 1 print half the size of (a) at reasonable quality (150ppi)
c. 1 print half the size of (b) at good quality (300ppi)

3. "72 dpi for screen, 300 dpi for print" is a myth.

Regards
CK

15. I think Ian's post sums it up in the name of the topic title. I learned a new thing today.

I however, still differ about 72dpi vs 300dpi for print and web. I don't neccsarily think it's a myth. 72dpi just became like a unwritten 'standard' so as to standardise images on the net. That's wat I have been talking about all along. Even at one point I think denizenx agreed on wat I mentioned about downloading a low res pic from the net and expecting it to get printed high res.

mpenza even pointed out "Many people are still confused by image resolution vs print resolution". I had the impression u already understood what I was driving at: exactly wat u said in the 1st. Image (pixel reso) vs Print Reso is different.

In anycase, I think most of us OT to some extent. Well, u can execute us now.

I think Ian's post sums it up in the name of the topic title. I learned a new thing today.

I however, still differ about 72dpi vs 300dpi for print and web. I don't neccsarily think it's a myth. 72dpi just became like a unwritten 'standard' so as to standardise images on the net. That's wat I have been talking about all along. Even at one point I think denizenx agreed on wat I mentioned about downloading a low res pic from the net and expecting it to get printed high res.

mpenza even pointed out "Many people are still confused by image resolution vs print resolution". I had the impression u already understood what I was driving at: exactly wat u said in the 1st. Image (pixel reso) vs Print Reso is different.

In anycase, I think most of us OT to some extent. Well, u can execute us now.
You still don't get it, do you?

"72 dpi" was derived back in the early days of the Mac, and their smallish monitors. And 1 inch of the monitor has 72 pixels. We have since come a long way, and the 72dpi thing is no longer accurate.

There is NO NEED for you to set an image to "72dpi" before you have it on the web, coz like I said, it means nothing. The image has its real image dimensions/resolution as x pixels by y pixels. REGARDLESS of the PPI setting.

Try this experiment.
1. Open any image in PhotoShop.
2. Go to Image -> Size
3. Disable "Resample"
4. Change the "ppi" to any number.
You will find that the image dimensions remain the same. But print size changes.

Web images are not 72dpi or 96dpi or whatever. Your monitor is the one that has this "dpi" (or to be more correct ppi) thing. 6" across your monitor and 6" across another monitor has different number of pixels. But the image remains the same.

Regards
CK

17. lyrrad, we (at least roughly) knew what we're talking about just that there's some contention over mixing "DPI" with the pixel dimensions. To most of us, pixel dimensions should be separate from DPI when no output size is meant to be specified. i.e. a 1024x768 image is still a 1024x768 image whether you change the "DPI" field in the image header to 72DPI or 300DPI, and either will look as good or as bad on the same monitor. The digital image itself is not changed.

18. Aiya... I dun think I dun get it lah. I've been working in web for 8 years now, I'm no expert or guru but I feel I know where I stand.

72dpi for screen became like a standard during the smallish 640x480 displays. As much as we can discuss how it doesn't matter TODAY, it should still play a part when creating web graphics. How can we selfishly assume everyone is using 1024x768 reso on 16bit colour depth on 17" inch monitors?

When web browsers came out in '94 640x480 with only 256 colour displays were the norm. So somehow 72dpi for web and 216 colour has stuck since then, because we're always designing for backward compatability. Alot of us just follow the majority and forget the minority that's out there. Not everyone's on IE, some still even on Lynx.

mpenza - I agree. And I've said it how many times in this thread. On screen DPI doesn't look any different, but when u move to print it's something else. This is where we agree right? How come u guys still thinking I differ from that?

Aiya... I dun think I dun get it lah. I've been working in web for 8 years now, I'm no expert or guru but I feel I know where I stand.

72dpi for screen became like a standard during the smallish 640x480 displays. As much as we can discuss how it doesn't matter TODAY, it should still play a part when creating web graphics. How can we selfishly assume everyone is using 1024x768 reso on 16bit colour depth on 17" inch monitors?

When web browsers came out in '94 640x480 with only 256 colour displays were the norm. So somehow 72dpi for web and 216 colour has stuck since then, because we're always designing for backward compatability. Alot of us just follow the majority and forget the minority that's out there. Not everyone's on IE, some still even on Lynx.

mpenza - I agree. And I've said it how many times in this thread. On screen DPI doesn't look any different, but when u move to print it's something else. This is where we agree right? How come u guys still thinking I differ from that?
The Commandments of Web graphics:

1. All graphics can only use 216 colors.
2. All graphics should not be bigger than 640x480.
3. Web graphics should be viewable in "terminal emulation" (use text to form composite figure?).
4. All images must be viewable by Lynx (isit even possible?).
5. No one should forget "72dpi".

Back in time "When web browsers came out in '94 640x480 with only 256 colour displays were the norm", PPI never did play a role in deciding the dimensions of the web graphics.

"Since density refers to mass per unit of volume, then a dense person must be one of great substance."

20. and:
- all bandwidth shall not exceed 33.6kbps
- singtel STYX shall randomly lock up every 14 days.
- proper English should be used
- use of @ other than email is cool
- prefixing 'i' and 'e' monikers to any product makes it an instant bestseller.

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