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PPI on-screen and its affect on DPI on print

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Like many people, I've struggled with PPI/DPI and how they affect print quality. The discussions I've seen here are usually actually about PPI and include the advice, "You don't need to worry about DPI unless you're talking about printing," and then they don't mention printing again. But printing is exactly what I'm interested in.

I think I have a grasp of the fact that AP says DPI when it means PPI, and that I don't need to worry about what DPI AP says my image has until I'm ready to print. But when I am ready to print, I don't see anyplace to specify what DPI I'd like in my print. It's not in the Canon printer driver and I don't see it in AP's print dialogue. 

When I was using Adobe, I saved every image I worked with at 300 (DPI? PPI?) and then when I printed I didn't even think about it. But all the discussions here (and the video tutorial Understanding DPI) make me feel really sure that I have absolutely no idea what DPI I'm printing at. (My prints look good, but I'd still like to be clear about this; maybe they could be better.)

So how does one properly print at 300 DPI?

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7 hours ago, ncJohn said:

 

So how does one properly print at 300 DPI?

If your image is going to be two inches x two inches on the design and the printed page, it needs to be 600 x 600 pixels. If four inches by four inches, 1200 x 1200 pixels. 300 pixels per inch :) which equates to dots per inch for your purpose.

IMO, PPI is a stupid measurement, at least for screens. A 27 inch monitor has got the same number of pixels in the 1920 width as a lowly 1920 x 1080 phone. So how many inches are 1920 pixels?

On my monitor 1920 pixels is about 24” on my phone about 5”. That’s quite a big difference in the length of an inch if you use PPI as a measurement.


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1 hour ago, toltec said:

On my monitor 1920 pixels is about 24” on my phone about 5”. That’s quite a big difference in the length of an inch if you use PPI as a measurement.

I'm not sure I understand that. Doesn't that just show that your monitor is displaying about 80 pixels per inch, and your phone is displaying about 380 pixels per inch?

Inches don't change, it's the pixel density that changes. And you don't use PPI to measure length :)


-- Walt

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50 minutes ago, toltec said:

IMO, PPI is a stupid measurement, at least for screens.

Huh. I have always thought that to be one of the few contexts where PPI has a single, unambiguous meaning. That's because there is nothing virtual about it. It is a physical property of a physical device that can be measured independently of any software virtualization. It specifies the device's pixel resolution using inches but it can easily be converted to any other units of length like mm using standard length conversion factors.

Like @walt.farrell just said, you don't use PPI to measure length, but you can use inches to measure PPI.


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33 minutes ago, walt.farrell said:

I'm not sure I understand that. Doesn't that just show that your monitor is displaying about 80 pixels per inch, and your phone is displaying about 380 pixels per inch?

Inches don't change, it's the pixel density that changes. And you don't use PPI to measure length :)

OK, if you have an image that is created for a device using 300 pixels per 'inch' how many inches will it be on the various device screens?

So what is the point of the 'inch' if you can't rely on it being an inch?

This is what I see confusing new (and some old) users time and time again. The only thing that matters is pixel quantity. This controls display size, depending on the device resolution or print quality.

Designing in pixels per inch is more relevant to printing than DPI. 

My imagesetter printed at 2540 dots per inch, each dot being one blast from the laser. This size could not vary. But the halftones were 150 lines per inch. Because the size of the LPI dots needs to vary to simulate grey, the smaller 2540 dot has to vary in quantity, not size. (see images) Also, the LPI dots never falls in exactly the right place (like pixels on a monitor) but the laser can't antialias, so again it comes down to dot quantity.

So,. to make up a 150 lpi 'dot' at different levels of grey, varying quantities of 2540 dots are used. See my very quick and crude image of 2540 dots per inch making a 150 lpi dot.

dots.png.adddf53c4389795dfffd25e2901d65a1.png

A 150  lpi 'dot' made from 2540 laser dots.

 

dot.png.7a6cd3878d52b9f5e71039008e18e2f1.png

Making a smaller halttone lpi 'dot'. 

 

Actually, the number of 'greys' that can be simulated by this method is limited by the LPI chosen. For example, you cannot produce 256 level of grey with too high a screen ruling, like 400 lpi. The 2540 dots are just too big.

So actually, PPI is best suited for the page than anything else. Your 300 Pixels Per Inch image will control a 2540 dots per inch device. I think that works out as about 8.5 dots per pixel? Of course, that gets far more complicated with colour.

 


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32 minutes ago, toltec said:

OK, if you have an image that is created for a device using 300 pixels per 'inch' how many inches will it be on the various device screens?

If the image (or object) size (actual measurement) on the screen is meaningful to the designer, the designer will presumably prepare several different versions of it. E.g., that's why the Export Persona lets you export slices at 1x, 2x, 3x, etc. pixel sizes. Your device that displays 72 ppi would use one version (1x), while your device that displays 288 ppi would use another (4x), and each would then display the object at the same measureable size.


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44 minutes ago, toltec said:

OK, if you have an image that is created for a device using 300 pixels per 'inch' how many inches will it be on the various device screens?

It can be any size. That is because it is a virtual property of a digital document that has no specific physical display size. But that in no way invalidates PPI as a property of the screen size of any device because that device can & must have exactly one physical PPI property.

It is not difficult to understand this if you keep in mind the distinction between what is virtual, which in computing contexts literally means that it has no physical existence as such, & what does have a distinct physical existence independent of context. Or to put it even more simply, if someone can understand the difference between the screen itself & what it displays, then this should be easy enough for them to figure out.


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2 hours ago, walt.farrell said:

If the image (or object) size (actual measurement) on the screen is meaningful to the designer, the designer will presumably prepare several different versions of it. E.g., that's why the Export Persona lets you export slices at 1x, 2x, 3x, etc. pixel sizes. Your device that displays 72 ppi would use one version (1x), while your device that displays 288 ppi would use another (4x), and each would then display the object at the same measureable size.

I agree totally, the problem is that new users (new to photo imaging or printing) have so many difficulties because of the DPI and PPI settings. I think using inches (especially in Europe where we discarded them decades ago) is confusing. Especially when an inch (on a monitor/phone is never an inch. Surely there’s a better way to explain this for new users?

 

 

 


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1 hour ago, R C-R said:

It can be any size. That is because it is a virtual property of a digital document that has no specific physical display size. But that in no way invalidates PPI as a property of the screen size of any device because that device can & must have exactly one physical PPI property.

It is not difficult to understand this if you keep in mind the distinction between what is virtual, which in computing contexts literally means that it has no physical existence as such, & what does have a distinct physical existence independent of context. Or to put it even more simply, if someone can understand the difference between the screen itself & what it displays, then this should be easy enough for them to figure out.

What is the point of using inches if they are not actually inches?

If a taxi driver charged by the mile, is it OK if the miles actually vary in length if he’s having a quiet day?

Remember that outside of America, inches were either discarded decades ago, or never used. So why must everybody use them when they have no value, or no fixed size? At least when it comes to monitor displays/pixels.


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26 minutes ago, toltec said:

I think using inches (especially in Europe where we discarded them decades ago) is confusing.

Except that we haven’t discarded them, at least not here in the UK! The Metrication Board was set up more than half a century ago to deal with all this kind of stuff, but it was disbanded before they finished the job. We still have motor car fuel efficiency quoted in miles per gallon instead of litres per 100 kilometres, and we still have speed limits in miles per hour; we also have things which are sold in 454g packets or 568ml cartons because they were originally one pound or one pint and nobody ever bothered to round the quantity up or down.


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6 hours ago, toltec said:

If your image is going to be two inches x two inches on the design and the printed page, it needs to be 600 x 600 pixels. If four inches by four inches, 1200 x 1200 pixels. 300 pixels per inch :) which equates to dots per inch for your purpose.

 

So you're saying that I was doing it right all along: Resizing my files to 300 PPI before saving. Right?

It's funny that EVERY thread I've seen about this subject does exactly what this one has done: it explodes with further questions and opinions, with the occasional unrelated tangent. :)

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So PPI is more about density than actual measurements, like a balloon expanding, same number of atoms just further apart, they are the expandable pants of the digital world :D


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2 hours ago, ncJohn said:

So you're saying that I was doing it right all along: Resizing my files to 300 PPI before saving. Right?

Right.

It's the only time inches actually are inches. 


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3 hours ago, toltec said:

What is the point of using inches if they are not actually inches?

Inches are always 'actual inches' for objects that exist in the physical world. The number of pixels per 'actual inch' of a screen of any particular device are always the same, just like the dimensions of its screen, or anything else about it that you can measure in inches. Inches are just a standardized unit of length, just like feet, millimeters, meters, miles, or kilometers.

The images you see on device screens are not physical world objects. In fact, they are not objects at all, other than in the virtual sense. They are nothing more than data, an ordered collection of numbers, that can be used to vary the amount light emitted from the different parts of that screen. That can be done in a variety of different ways, including forming light patterns that look like pictures of physical objects, or as decimal or hexadecimal text strings that don't look anything like pictures of actual physical objects.

Regardless, there is always a physical 'actual inch' property that can be used to describe how that data is currently being displayed on the screen. Just don't don't confuse that with any property of the document, be it DPI or anything else. They are not the same thing.


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I do a lot of printing from images edited in Affinity Photo. I send my processed images on-line to DS Colour Labs Ltd, Manchester UK. For the most part all the image sizes for on-line image printing is in UK inches, bar a few exceptions that have the option of A4 and A3. I get my images printed onto 16inch by 12inch paper size with a white border around the paper of varying sizes. My camera pixel resolution is 4928pxs by 3264pxs = 16Mpxs. approx. After using the develop persona I always check the document size because AP defaults to 96 DPI.  If the document size is indicating 96 DPI I then change the value to 300DPI without resampling (untick the resample box). From then on the image is saved as 300 DPI. The actual size of the image in inches can be viewed by using the crop tool. I will crop the image in inches to suit the DS Colour Labs Ltd paper sizes which are in inches. Sometimes I have cropped to 15 inch by 12 inch to suit 16 inch by 12 inch paper with a 1inch border all around the paper for subsequent mounting on cardboard mounts with a frame also cut from card using a LOGAN Mount cutter for presentation. All image processing is at 300 DPI and saved as 300 DPI at all times. DS Colour Labs requires the images that are sent on-line to be colour space SRGB, in jpg format and at 300 DPI (ppi). DPI and ppi are effectively the same for printing purposes as far as AP is concerned. Final image is exported as a jpg file at 300 DPI and in SRGB for on-line shipment to DS Colour Labs and generally the prints return the next working day if sent first thing in the morning. The only item to be wary of is the bug in the export and untick the metadata in more because when one brings an image back into AP that was previously exported as a jpg as a 300DPI image, will when opened later may then be indicated as 96DPI in the Document Size. which is very confusing. (AP reverts to its default 96 DPI).

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29 minutes ago, R C-R said:

Inches are always 'actual inches' for objects that exist in the physical world. The number of pixels per 'actual inch' of a screen of any particular device are always the same, just like the dimensions of its screen, or anything else about it that you can measure in inches. Inches are just a standardized unit of length, just like feet, millimeters, meters, miles, or kilometers.

The images you see on device screens are not physical world objects. In fact, they are not objects at all, other than in the virtual sense. They are nothing more than data, an ordered collection of numbers, that can be used to vary the amount light emitted from the different parts of that screen. That can be done in a variety of different ways, including forming light patterns that look like pictures of physical objects, or as decimal or hexadecimal text strings that don't look anything like pictures of actual physical objects.

Regardless, there is always a physical 'actual inch' property that can be used to describe how that data is currently being displayed on the screen. Just don't don't confuse that with any property of the document, be it DPI or anything else. They are not the same thing.

Yes but the monitors are physical objects, and it would be reasonable to assume that an inch on a monitor that is described as an inch, with 72 or 96 pixels or whatever pixels in it is actually a physical inch. 

At least that is my real world assessment. 


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4 hours ago, toltec said:
6 hours ago, ncJohn said:

So you're saying that I was doing it right all along: Resizing my files to 300 PPI before saving. Right?

It's funny that EVERY thread I've seen about this subject does exactly what this one has done: it explodes with further questions and opinions, with the occasional unrelated tangent. :)

Right.

It's the only time inches actually are inches. 

That depends. I am going to amend Toltec's answer here.

Toltec already explained a bit how image setters work, but for a more elaborate answer check out this article:

http://samcoprinters.com/choosing-the-right-image-resolution-for-your-print-job/.html

Now, there are three additional variables which may completely change the PPI you need for a physical print:

1) whether the image is a continuous tone image or a pure black and white one, and;

2) your print output device, and;

3) whether the printed result is going to be of (very) large dimensions and seen from a (large) distance or not.

So far the discussion has limited itself to regular print work, e.g. brochures, small pamphlets, magazines, books, small posters, and so on. When printing continuous tone images, i.e. a typical colour or grayscale photo, we calculate the required PPI by first knowing and/or deciding what the physical output size is. For example, suppose we needed our photo to be printed in a magazine half top page size.

Let's assume we'd be preparing a continuous tone image for WIRED, a well-known magazine that is printed at 8” x 10.875” Final Trim Size (inches) or 20.32cm by 27.6225cm. The top half page size will be 8" x 5.4375". Which means in Photo we set up a document at that size and 300ppi (dpi is the wrong unit in photo's New document dialog). When we check the pixel dimensions this yields 2400 x 1632px (unfortunately Photo allows for decimal pixels, and shows 1631.3px).

That is the exact resolution we need to print our image at 8" x 5.4375". When this image is saved, we must also ensure that the file's ppi value is set to 300. When the image is imported into InDesign, the image will be imported with exactly those dimensions, and placed at the correct relative calculated size in WIRED's page layout.

[ ! ] I did not calculate bleed into the final size of this image. When an image is printed half-page like described, three of its edges will touch the page edges exactly, and the person doing the layout will have to bleed the image off the page by at least 1/8 of an inch (depends on the publisher). Ideally you would add this 1/8" to the dimensions of your image on those three sides. However, often the image is scaled up a bit, which hardly affects the quality of the print.

[ !! ] it is always a good idea to work at twice the required resolution (or more depending) in case the client needs a full page version (or even a full spread, which means working at 4 times the resolution), and then downscaling the result at the very end.

Also, keep in mind that many home printers cannot reach the full 300ppi continuous tone quality, but many can. Depends on the desktop printer type. Dedicated photo printers do a very nice job, so keep it at a minimum of 300ppi.

Now, this works fine for continuous tone images such as colour and black-and-white (greyscale) photos. But what if we needed to create a pure black and white design to be printed with pure black ink?

Then we need to reconsider our resolution requirements. Let's take a black and white comic as an example (something I work with). As Toltec explained earlier, a typical image setter works at a dot resolution of 2400~2540dpi. Ideally you'd probably think that it's a fairly simple calculation: 1 black pixel ought to represent 1 black printed dot.

However, printing on paper is a messy business, and the way paper is saturated with ink means it will not be possible to arrive at that perfect print resolution in practice. For higher-quality glossy paper comic pages, ideally we provide 1200ppi pure black and white inks. It depends a bit on the paper, though: I'd say 800ppi is the minimum at which to provide the artwork at (NOT CONTINOUS TONE!!!).

For coloured comic prints, the colours (continuous tone artwork) should be again provided at 300ppi and the correct calculated minimum pixel resolution as explained above). The 1200ppi black and white artwork is then printed on top of this colour work. (this is not possible to pull off in Photo, btw - you will need a layout app that supports such layering and output - communicate with your printer how they want this).

(Coincidentally, that is also what happens when lettering is printed: a black ink letter is printed at the highest (practically) resolution. This is why lettering looks so much sharper on paper compared to a colour photo: a photo is always converted to rasters/halftones, while text is not. It is pure black. You should never allow black text to be rastered, otherwise you'll get fuzzy looking text!)

So, expanding on the WIRED example, if we'd be preparing a pure black and white half-page advert or image, then we calculate the required pixel resolution like this: 8" x 5.4375" and 1200ppi: 9600px X 6525px at 1bit (ideally with transparency for easy layout layering later). This seems like a huge resolution, but we are working at the physical limits of an image setter and paper print. This will allow for the best print quality, and look razor sharp when printed. But again, it depends on the paper quality. If you print on newspaper medium, you can go much lower, e.g 800ppi.

To complicate matters, if our black and white output would be printed on a typical laser printer, we would have to check its DPI capabilities. Nowadays most do 600dpi or 1200ppi. Which means you'll be fine with working at the aforementioned calculated resolution. And don't forget it is probably a good idea to work at a higher resolution just in case the client needs to output to a full page or spread later. At which point we begin to realize why vector applications exist since working at these ridiculous pixel dimensions isn't very helpful. Instead, we'd create our black and white half-page design in Designer or Illustrator. And import and bitmap art work at the size we would need (back to the drawing board and recalculate the minimum 1200ppi numbers!).

No-one said this was going to be easy or simple. This is why so many people are utterly confused how DPI and PPI work, and how they relate to printing.

BUT WAIT! IT GETS WORSE! Because everything that I've explained about so far, is for regular print work that is read from an arm's length distance only. What if our client wants that image to be plastered against the top five floors of a 8 story high building?

Do we output that twenty meter by 15 meter print at 300PPI like before? NO. That would result in a file that is unworkable to print. Instead, the image is prepared at a surprisingly low resolution. It depends on the viewing distance. Check out http://resources.printhandbook.com/pages/viewing-distance-dpi.php

We would prepare our file at a max PPI of 2. Yes, 2ppi. So, let's take that WIRED half-page example again: suppose we should create a 20 by 15 meters large image that will be viewed from 100 meters and more. 20m X 15m and 2 PPI should do it. Result: 1575px by 1181px. That's it. It means that you could just crop and scale down your original WIRED image to that resolution, and hand it over to your printer (this will be a digital print, btw: no image setters exist that print at those dimentions). The banner printer will take care of the practical output technicalities, but that resolution will suffice.

(PS digital billboards work at very low resolutions. Here you would provide an exact pixel-to-LED pixel conversion. A typical digital billboard resolution is 888px X 260px. Just like the old times when working with computer screen assets. Sigh, easy times!)

Anyway, just a brief overview. Preparing assets for screen-based technology is a whole different story, and I won't go into that right now. Suffice to say, you can't rely on 1-on-1 pixel conversions anymore, as we did in the past.  Nowadays it's actually worse than print, which is relatively easy to wrap your head around.

And: always contact your print service provider/printer what they require for the job. And make sure to work on a colour calibrated screen (that means getting some hardware: a Spyder, Munki, etc.! And no, there's no way around this if you value colour correct work. Really, just get one.).

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51 minutes ago, R C-R said:

Inches are always 'actual inches' for objects that exist in the physical world.

"Actual inches" don't really exist in the physical world either. When you measure an inch with your ruler in -20 degrees in Alberta, CA in Winter time and at +40 degrees in Summer time, those inches will not be of the same length. All human measures don't exist in the physical world, and are but a human invented tool to make sense of things.

:22_stuck_out_tongue_winking_eye:

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23 minutes ago, Medical Officer Bones said:

"Actual inches" don't really exist in the physical world either. When you measure an inch with your ruler in -20 degrees in Alberta, CA in Winter time and at +40 degrees in Summer time, those inches will not be of the same length.

Yeah, I figured someone would mention something like that but it does not mean those inches don't exist as a standardized unit of length ... which is why I was careful to include that phrase in my post. So :24_stuck_out_tongue: right back atcha. xD


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1 hour ago, Medical Officer Bones said:

That depends. I am going to amend Toltec's answer here.

 

Well, I appreciate your (very comprehensive:)) response, but in my original post, my question was how to print at 300 DPI. That's what Toltec was replying to.

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A lot of the responses here refer to there being different sizes of screens. So, just to clarify...

On any device with any size screen, if I display a 300PPI image at 1:1, could I hold a ruler up to the screen and count 300 dots in an inch (If my eyes were capable)?

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1 hour ago, ncJohn said:

A lot of the responses here refer to there being different sizes of screens. So, just to clarify...

On any device with any size screen, if I display a 300PPI image at 1:1, could I hold a ruler up to the screen and count 300 dots in an inch (If my eyes were capable)?

No.

The Appli iPad ‘retina’ display has a very high resolution of 2048 by 1536 which works out at 264 ppi.

Your 300 pixel image would occupy (1.136  ish) of an inch.

A typical 24 inch HD monitor (1920 x 1080) has around 92 pixels per inch, so 300 pixels would occupy about 3.26 of an inch.

A big screen 50 inch HD TV (also 1920 x 1080) has about 44 pixels per inch, on which your 300 pixels would measure 6.8 inches.

So, based on a 300 pixels per inch image, an inch could be anything from just over one inch, to just under seven inches. 

 


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27 minutes ago, toltec said:

No.

The Appli iPad ‘retina’ display has a very high resolution of 2048 by 1536 which works out at 264 ppi.

Your 300 pixel image would occupy (1.136  ish) of an inch.

A typical 24 inch HD monitor (1920 x 1080) has around 92 pixels per inch, so 300 pixels would occupy about 3.26 of an inch.

A big screen 50 inch HD TV (also 1920 x 1080) has about 44 pixels per inch, on which your 300 pixels would measure 6.8 inches.

So, based on a 300 pixels per inch image, an inch could be anything from just over one inch, to just under seven inches. 

 

Thanks... sigh

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6 hours ago, ncJohn said:

Well, I appreciate your (very comprehensive:)) response, but in my original post, my question was how to print at 300 DPI. That's what Toltec was replying to.

Well, you did mention that there's a whole lot of noise relating to ppi and dpi on the web. I just wanted to clarify things a bit more.

By the way, you wrote that you have a Canon printer? If it is a newer model (last few years) it may very well support 600ppi prints as well at high quality settings on quality glossy paper. In that case you might want to feed it 600ppi images instead of 300ppi ones. That is another point I made: know (the hardware limits of) you output device.

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