Why 240 pixels/inch?

[ntiberius]

Silly question: Why do files out of my 300D have a default of

240p/in? Most web images are 72. Print images should be 300,

right? Where'd 240 come from? :)

[markci]

>> Most web images are 72.

 

Not particularly.

 

>> Print images should be 300, right?

 

Says who? Actually it works best if it's an integral divisor of the printer's resolution, which is usually 720 (or 1440). 720/3 = 240.

[Spearhead]

<i>Most web images are 72</i><p>

 

This is meaningless, screens display pixels, not pixels per inch. A 600x400 image is the same when displayed on the screen with 72 dpi, 400000000 dpi or 1 dpi. Try it sometime. The pixels/inch is for printing only, and the default setting is probably there because Photoshop requires something in that box. Set it to what you want when you are printing, otherwise ignore it.

[markci]

72 dpi (or actually ppi) is MacIntosh standard. The corresponding standard in Windows is 96 dpi. But this is only used for computing the size of text. It doesn't have anything to do with images, and the "inches" are virtual ones in any case, not real inches. (Your computer and video card don't know how big your screen is in any case).

 

The dpi values in images files are completely ignored, as I already pointed out once today.

[Spearhead]

<i>The corresponding standard in Windows is 96 dpi</i><p>

 

There is no standard in Windows for this. For example, some TabletPC products, which run Windows XP, have as much as 120dpi. <P>

 

But, as we have both pointed out, it's irrelevant for image display.

[markci]

Actually, Jeff, I assure you there is. There are obviously higher resolution devices, but nevertheless the system was originally based around text rendered at 96 dpi, and the bitmap fonts still only look good at that resolution. Fortunately with TrueType fonts it doesn't make any difference, but we're talking about historical development here. (Having been writing Windows software since 1989, my perspective is skewed in that direction anyway).

[PuppyDigs]

"72 dpi (or actually ppi) is Macintosh standard."

 

Not really. Anything goes now. The original 1984 Mac had 72dpi screen rez. However, late 80s color screens had 69dpi and recent screens are about double the rez. 72dpi was nice in the desktop publishing craze of the 80s and early 90s as 72dpi = 72 points = 1 inch. For a while screen images were lifesize as far as type and page layouts.

[markci]

Yes, REALLY.

 

For the love of Christ, I never said any device was actually, physically 72 dpi (except for the original Mac screens). In fact, I specifically said it wasn't.

 

Regardless of the actual ppi of the physical device, the software internally used 72 pixels per LOGICAL inch in its text rendering computation. This internal standard continued to exist long after screens were no longer physically 72 dpi. For all I know it's still used today, though I'm not a Mac developer and don't know anything about OS-X, so I can't really say.

[kelly_flanigan1]

Each peron's computer on the planet maybe set to different settings. This depends on what type of monitor they are using; what they are looking at; how good their eyes are; how screen burned their monitor is; or their eyes...It is late; so tonight I backed off the Screen settings to 800 by 600 pixels. My monitor here is a 19inch Viewsonic; in which the actual viewable width is 14inches......Ok now to long division 800/14 is 57.1; say 57 pixels per inch; with slide rule accuracy...Tommorrow; I will reset it to 1152 by 864 pixels; and will be at 1152/14=82 pixels per inch; after the first cup of coffee.

[kelly_flanigan1]

Notice spelling also is different at night too!<BR><BR> 240 pixels per inch is not a bad "first guess" for printing; over a wide range of printers. I bet they choose the "240" number as a decent "first guess" number; for the masses to try first.

[jbq]

'Coz people like to have numbers to rationalize purchasing new toys.

[arunas_salkauskas]

Heh, even better - I had a local photo shop offer to scane my film at 300 DPI or 600 DPI.

 

Puzzled, I asked: at what size? If they scan the 35mm negative at 300 DPI, then it's going to be pretty low-res result. If they estimate that they'd be doing the equivalent of 300 DPI on a 4x6 print then it's a little different. But that's why slide scanners do things like 3000 DPI.

 

So Ultimately, what you really need to know is how Big at what DPI - who cares if it's 240 DPI or 10 DPI.

 

But if you _know_ how many DPI your screen is displaying (not just guessing at 120 or 96) then you can display images life-size. We do this with digital X-rays, where it sort of matters.

[bart feliciano]

Line Screen, DPI, PPI info here

 

http://www.techcolor.com/help/resolution.html

 

300 is from magazine reproduction, 240 is probably a common inkjet printer appropriate res.

[ron c sunshine coast,qld,a]

Computer screen pixels per inch is the dumbest thing you could ever try to work out.NOTHING makes sence like it should.I did some solid reserch into it recently as i was about to buy a new monitor.Did i learn anything usefull?Not really.In the end i just kept my old monitor and decided to wait for abit

[chip]

Hi Mark,

 

The recomendation of using an even divisor of the printer's selected print resolution is an old wive's tale. Epson's recommendation is to use 300ppi for the output image file. This was after Epson's lab people viewed printed output at 240, 300 and 360 and inspected the resulting images both magnified and at various normal viewing distances. The 240ppi images were not as good as the 300ppi images and the difference between 300ppi and 340ppi was found to be nil.

 

HTH

[carl_weller]

You'll probably find it has absolutely nothing to do with screen resolutions, print settings for output, etc.

 

It is most likely set by the number of pixels and the physical size of the sensor in the 300D - thus giving a 'pixel per inch' figure. Just use the image resize feature in photoshop to set the ppi figure you need (200-300 ppi is fine) and the image will be displayed at the biggest size that allows use of that ppi resolution.

 

Many digital cams use 72 ppi as a base output resolution, which is why pics can be bloody huge (ca. 17 inches wide)when you open them in an imaging program.

 

carl

[catchiest_zero]

you can set the screen dpi in ms windows so that a physically-one-inch-wide-image on your screen will be exactly one-inch-wide when printed.

 

the following procedure involves measuring a ruler-image on the screen to a physcial ruler, until the two match. see the following article for detailed steps (it applies to windows 95,98,NT,2000,XP etc.)

 

http://www.wordstar.org/computing/pages/monitor_cal.htm

[theoldmoose]

Thank you. I was wondering if anyone would point that out. I find it amazing in a world of folks that are obsessed with calibrating the color of their monitors, that one could be so cavalier about calibrating the physical geometry of their displays, as well.

 

Windows has long had the capability of allowing you to tell it (or it can discover it from more recent monitor/video card combinations -- so the chap that asserted that computers don't know the physical size of the monitor hasn't been keeping up, I'm afraid) the size of your display, usually indirectly, by using the little ruler gadget that you can find in the advanced display properties page, under Other... font size.

 

Stick a ruler on the screen, and drag the ruler in the dialog box to match it. My display in 1280x960 mode comes out to about 89 dpi. If you find the Windows system fonts to be smallish then, it's because Windows uses fonts for low-res displays by default. You can select large system icon, etc text in the appearance tab (Windows classic (large), or Windows standard (large) -- XP has similar options, IIRC).

 

Now, not only can you see the system fonts, but applications that pay attention to the system font metrics will show documents in 'real size' (if zoomed to 100%).

 

Try it, you'll like it.

[david choo]

I believe inkjet printers optimum resolution in print is 240 pixels per inch... it somehow translates best to dots per inch.

[bart feliciano]

Just take a picture of a post it note and then adjust your screen til they match, if you are worried about that.

[rodney_gold]

You mentioned 300ppi as print size , Here's something to consider if you want to print really BIG.

PPI has a relation to print size (or scanning)- sort of. It depends on your printer. The common CMYK printers only print 7 colours + white(well it doesnt print white- but anyway) so it prints multiple dots with various spacing between them in a cell to simulate the pixel's colour it's mapping and the theory is that due to nyquist , the best resolution will be a pixel count of 2x the printers "cell" per inch capability(LPI). Ie if the printer can print 200 LPI then the graphic will print at the best resolution at 400 pixel per inch of printed output. IE a 400 pixel by 400 pixel file will print best at 1" x 1".

Theory and practice are not the same and one can actually get away with less than that , about 1-2-1.5x the LPI ability.

We aren't talking Dye sub stuff here , cos a dye sub printer at 300dpi will kill just about any inkjet as it maps 1 pixel per dot of output.

Most "art type" coffee table books are printed at about 175 LPI , some photo quality inkjets could go as high as 250+ lpi IN THEORY , however here theory is also debunked to some extent.The ability of a printer is highly dependant and various factors , like the inks , the RIP , the cell structure , the media and so on.

Typically I can use my large format inkjet printer , a 1440 dpi device (how many dots it can lay down in an inch - not how many cells of these) using a 6 colour ink and a variable dot technology and print a stunning 60" x 40" pic from a 3000 x 2000 pixel graphic. This equates to 50 lpi or ppi. I OFTEN print 30" x 20" too with exceptional results (On gloss or semi matt - but my printer prints on just about anything - stuff like art canvases cant do hihg LPI's at all cos of surfaces and coatings etc)

One has to remember that the viewing distance on a graphic like this is not 12" and there is a dot gain factor at work here too , as a dot splashes on the media , it spreads and thus the delineation between pixels is not they emphasised , the media and ink combo actually make the print "smoother". Most ppl step back with a print and view the overall picture anyway , very few inspect it with a loupe.

The only comment I have ever had regarding printing this size is the oohs and ahhs of folk seeing every little detail etc;) One can also resample using some sophisticated programs and print equally as well up to 2x the size. I have often used Irfanviews lanczos filter to print 80" x 54" (aprox) sized prints (my printer only prints 454" wide by however long)

So in essence , the PPI *is* related to output size but the common myth that one can ONLY print to a certain size with a X xY pixel graphic is often junk. I have no idea where 240 come into play here tho.

Rodney Gold