Illustrator Document size / DPI (newbie)

[Juc1]

Hi all,

I think I read somewhere that documents such as .ai and .pdf do not have a DPI (dots per inch) or a DPI setting but rather DPI is determined by the printer when printing. On the other hand I have read that documents need to be at least 300 DPI to for good quality printing - or does this 300 DPI only apply to particular documents (eg documents that include bitmap images)?

Thanks...

 

[hawkeye]

DPI is dots per inch which is determined by the printer. PPI is pixels per inch which applies to raster images and raster effects. In Illustrator if you are using either images or raster effects then setting a higher PPI will be meaningful, if it's strictly vectors then it is not.

 

[MrToM]

LOL....no offence but this is a hottly disputed topic and you'll get a billion and 1 answers all of which are correct.

It really is a can of worms and has been discussed many many times before......and never a verdict is reached.

For the record, this is how I see it:

Digital images have no resolution. Period.
The 'size' of a digital image is measured in pixels, as in X x Y px. They cannot be measured in terms of linear measurement.
Digital images 'inherit' the resolution of the medium on which they are being viewed.
For an LCD monitor this will be the width or height divided by the pixel 'pitch', this can found from the screen specs. This is PPI.
For a piece of paper from a printer this will be whatever the printer prints at, be that 100 dpi, 300 dpi or 1200 dpi, for eg.
PPI and DPI should not be confused as being the same, they are not.
Pixels per inch relates to LCD screens.....or something with physical, measureable, (pitch), pixels.
Dots per inch relates to 2 things:
1. How many 'dots' of INK per inch the printer uses when printing.
2. A ratio saved in the meta-data of an image which the printer uses to calculate how BIG an image should be on the paper.

The latter is probably what you are referring to in your question.
Illustrator is 'vector' based editing so I dunno if it depends on dpi or not, but if you print from it then the following will probably apply to it as well.

Regards Photoshop, and only Photoshop as I don't have Illustrator, this is what the dpi means:
The DPI of an image in photoshop has nothing to do with how many pixels it has.
It is purely a value, used to by a printer to calculate the relationship between the number of pixels in the image and physical distance on the paper. How much INK it uses to do that is the printers own DPI and nothing to do with the DPI found in Photoshop.

For example:
You want a 10" x 6" image when printed out on a piece of paper.
You want to print @300dpi...
...therefore the image dimensions must be 300px x 10" = 3000px wide, and 300px x 6" = 1800px high.

This is where a problem could occur as the image may not have that many pixels in it. If it's bigger it can be cropped, but smaller...that's 'can of worms 2'.

Working the other way you may have an image which is 3072 x 2048px. @300dpi the biggest image you could print would be 3072 / 300 = 10.24" wide, and 2048 / 300 = 6.8" high.

Most people tend to insist on printing @300dpi but have a 640x480px image and want a 10"x6" print.....do the calcs, it won't work.

I just thought of this:
Think of it like an old 'Polaroid' photo, remember that blank space at the bottom were you could write stuff?
Digital images also have this 'space'...and applications like Photoshop also write info to it....like time taken, camera make, the date, focal length, did the flash fire, width in pixels, file size........AND........the image resolution!

As with a 'Polaroid' photo, you could write ANYTHING in that space, for an image of a cat you could write 'Dog', it doesn't have to be related to the image and it doesn't change the contents of the image neither. Similarly anything written to meta-data by Photoshop is not directly linked to the file contents.......meaning? Meaning you can write any dpi there you like without changing the image.

Once you've got your head around that lot consider using different computers for the same purpose....MS, back in 19?? decided that every monitor will only ever have 96ppi, and set their OS to use just that. Every piece of software was then written with 96ppi in mind, but time moves on and well, has it changed? Has it bolderdash.
Mac users now have 'retina' disply which is just 4 times as many pixels. Completely useless to begin with as nothing was written for 144ppi (twice the usual 72ppi). The latest Photoshop does now have a '200%' option (LOL), to accommodate the larger ppi but thats just one piece of software...out of how many?

Just to add even more confusion Photoshop defaults to writing a dpi of 72 to a new document.....why 72? Old mac days I guess.

The finale:
So, unless you plan to print your work from Photoshop forget all about dpi.
If you are going to print stuff out, do your calculations first....know how many pixels you need for the size of print you want BEFORE File > new....etc.

Resolution recap:
My monitor: 102ppi
Windows 7: 96ppi
Mac OS: 72ppi
Retina: 144ppi
Photoshop: 72dpi
Document: ??? Calculated from pixels and required print size.

I'm going now.

Regards.
MrTom.

 

[Tom Mann]

hawkeye - Excellent, to the point answer.

-----------

@Mr.Tom - Don't forget that you posted very similar comments on the meaning of the term, "resolution", just a few months ago in my thread from 2013:
https://www.photoshopgurus.com/forum/photography/45029-meaning-term-resolution-photoshop.html

I agree with almost all of your operational suggestions, but but we do have somewhat different takes on the nuances and explanations of some aspects of this subject. For example, here are a couple of your statements with which I would differ, followed by my take on each:

"this is a hottly disputed topic and you'll get a billion and 1 answers all of which are correct...It ... has been discussed many many times before and never a verdict is reached..."

- - - IMHO, like many subjects, it's hotly disputed only by people who don't really understand it. I don't think there is any difference of opinion whatsoever among people who truly understand it.

-----------

"...Digital images have no resolution. Period...."

- - - IMHO, this is a very misleading statement because it acts as if the only valid use of the term, "resolution", is how Adobe uses it in their dialog boxes, ie, synonymous with "dpi". Unfortunately, the statement completely ignores the most common usage of the term, "resolution". For example, suppose I take a photo of a street scene and find that I can make out cars and people but can't read their license plates. This tells me that the resolution of that image is somewhere around 1 foot. Technically, this meaning is described as "resolution referred to the input image". This lack of appreciation of the several different, common meanings/usages of the term, "resolution", is what causes many, many misunderstandings, and is exactly why I started the thread I cited above.

------------

"...Digital images 'inherit' the resolution of the medium on which they are being viewed...."

- - - I think I know what concept you are trying to get across, but unfortunately, the most precise way to say this uses some advanced concepts and terminology from optical engineering: "The point spread function of the overall imaging chain can be found by convolving the point spread functions of the individual components in the chain, eg, lens, sensor, data transmission (including file format issues), image processing, and finally, the display."

------------

Once past these opening statements in your post and get into the meat of the matter (ie, how these things impact one's image making decisions) we are in closer agreement.

Cheers,

Tom M

 

[Juc1]

Thanks @ hawkeye, MrTom and Tom Mann

This is how I see it so far - have I got this right?

An image taken by a digital camera has a resolution determined by the camera sensor's setting for example at a setting of 6MP the image will contain 6 million dots of detail. As Tom Mann said in the other thread:

Resolution #1 tells you the amount of information contained in an image.... #1 can't be increased without asking your software to guess at the new information (RGB values of pixels) that didn't formerly exist

Too much adding pixels by "guessing" results in poor quality.

Although an image file might contain 6 million dots of detail, the number of dots actually displayed will depend on the screen which is displaying the image. A High Definition screen is only 1920 x 1080 = 2073600 ie about 2 millions pixels so can only display about one third of 6 millions pixels. (I think this is something like MrTom's "Digital images 'inherit' the resolution of the medium on which they are being viewed." ??) So the number of pixels that an image file contains is one thing and the number of an image's pixels that are displayed is something else.

A vector only .ai file has no resolution. We might say that a particular .ai file is printed at 300 DPI or at 600 DPI etc but this is determined by the printer's setting (number of dots of ink squirted per inch) and is not part of the .ai file itself.

 

[MrToM]

At the risk of confusing things further.....LOL (Honestly, I'm trying my best not to.)

An image taken by a digital camera has a resolution determined by the camera sensor's setting for example at a setting of 6MP the image will contain 6 million dots of detail.

Well this is where the confusion comes into play:

Tom Mann has quite correctly said that the more 'common' understanding of the word 'resolution' defines how many pixels are in the image...as you've demonstrated in your above statement.

Now, I agree wholeheartedly with that, but there is another meaning of the word 'resolution' which is the one I stand by and believe to be the [less misleading?] definition, namely that 'resolution' is a ratio of the amount of pixels per a linear measurement, hence the term 'Pixels Per Inch'...the amount of pixels is irrelevant.

Both a 1x1 px image and a 1000x1000 px image could have a 300ppi or a 1200ppi resolution written to their meta-data....or even 1ppi.

6MP is just the total number of pixels, from just this info you cannot even determine the dimensions of an image....it could be 1px high by 6Mpx wide....who knows? This is why, along with other information, that the real dimensions are 'written' into the meta-data, it is not calculated. 'Resolution' is another piece of written info, it bears no direct relationship to the image itself...it could be anything.

(Just to add, the actual dimensions of a digital image are determined by the image data but any software that shows you the 'image dimensions' will use the info written in the meta-data.....and yes, you can write anything for this too...3 x 2 pigs or 3072 x 2048 plum puddings, for instance).

There are many applications capable of writing this information, Photoshop is just one of them. When you 'set' the resolution in PS all you are doing is writing that information to the meta-data, it has no effect on the image. (See later about upsampling and downsampling).*

My argument, (And I really hate to call it that), is that a DIGITAL image cannot have linear measurement, you cannot get a ruler and physically measure how big a digital image is....its just a grid of squares. Based on that, without a linear measurement a DIGITAL image cannot have a 'resolution', (you need both pixels and length), but it can inherit the 'resolution' of the medium on which it is being viewed.....99% of the time this will probably be an LCD screen of some sort.

Every LCD screen does have a 'resolution', you CAN physically measure it. Therefore, any image you view on it 'inherits' that 'resolution'.

Although an image file might contain 6 million dots of detail, the number of dots actually displayed will depend on the screen which is displaying the image. A High Definition screen is only 1920 x 1080 = 2073600 ie about 2 millions pixels so can only display about one third of 6 millions pixels.

@100% zoom, yes.
Obviously if you want to view all the image you have to zoom out, in your example that would be to around 33%.

I would say that to view an image at its best you would ideally need 1px of your screen to denote 1px of the image....sound logical?
You would, therefore, assume that 100% zoom would give you that 1px per 1px relationship, but, as I said before, Windows OS has a default 'dpi' setting of 96dpi which is probably not the same resolution as your screen.

One way you can, and I stress, can, overcome this is by changing the default dpi in Windows to match that of your screen, but as most software is written for the default 96dpi many will 'break' if you change it.....so you're pretty much stuck with it....so...

...Photoshop has a handy 'Screen Resolution' setting under Preferences > Units and Rulers.
Here you can set your actual screen 'Resolution' to give you a more accurate 'Actual Size' image on your screen.

As a test you can try thisWith PS Screen Resolution @ 96dpi)
Check the 'resolution' of your OS. For windows 7, right click the desktop, select Personalize > Display > Set Custom Text Size (DPI).
In the resulting window the 'dpi' for the OS should be displayed, 100% = 96 pixels per inch.(Unless you've already changed it)
Cancel out of this.
Measure the Width of the visible area of your screen, pref in inches. Divide your screen width in pixels by your screen width in inches....this will give you an approximate PPI for your screen.
In photoshop create two rectangles, one 960px wide (OS dpi x 10) and one (<your screen PPI> x 10)
View at 100%....('Actual pixels' for pre CC) and position so you can see the first rectangle.
Mark off 10 inches on a piece of paper, hold that up to the rectangle to compare widths......no where near 10" right?
Now check the other rectangle.....a lot nearer, yes?

This shows that its the OUTPUT device (in this case, your screen) that denotes the resolution of an image and changing the 'resolution' of that image via software does not have an effect on how you view it.

So if you want to view images on your screen at the same size as when printed out, change your Screen Resolution in PS and use View > Print Size.
For this to work you would then need to change the 'resolution' of the image to whatever your screen resolution is. (Yes I know I said it had no effect but 'Print Size' is determined by the dpi and pixel dims....so despite it being displayed on-screen it needs to be treated as if it was going to a printer).

I don't have a mac but I assume it's default dpi is 72, and retina is 144.....this is based on a screen shot posted here from someone with a retina display mac....its resolution was written as 144dpi.

Were you to use a PRINTER as the output device then you would have to tell the printer the relationship between the amount of pixels in the image and how much of the paper those pixels cover. A printer does not have its own 'grid' of pixels like a screen, so you need to supply it with that info. Just supplying the amount of pixels or the pixel dimensions is not enough information for the printer to determine how big to print the image....hence it needs that relationship....or ratio...or DPI.

As I said before, if you are NOT sending the image to a printer, or to be printed elsewhere and you don't want to view using 'Print Size' , you can ignore the dpi of an image.
*If you intend to upsampling or downsampling an image though then dpi does have an effect but as this is generally regarded as bad practice I'll not go into it.

I dunno if that's going to clear things up, and like I said this is just MY interpretation of how things work, you can make up your own mind as to whether you agree or not.

Regards.
MrTom.

 

[Tom Mann]

I think that both of us are frustrated with the problems surrounding the use the word, "resolution", and, as I said, most this arises because of the many different meanings of that single word. I feel that 90% of the problems would simply vanish if people would consistently use a more accurate phrase instead of always using the single word, "resolution", and then spending the next few sentences (or paragraphs) of their post describing, over and over, which meaning they meant in that particular instance.

Below is a list of phrases that succinctly describe the different meanings of the term, "resolution", that are commonly encountered in photography. Each is followed by an example or two of typical usage of that term.

1. Pixel dimensions
- - - - e.g., "The pixel dimensions of the sensor in my camera is 4000 x 3000."
- - - - e.g., "The pixel dimensions of the file that I started with is 4000 x 3000."

2. Number of pixels
- - - - e.g., "The number of pixels on the sensor of my camera is 12,000,000 (ie, 12 Megapixels)."

3. Pixels per inch
- - - - e.g., "To achieve 300 ppi on my 20x10 inch wide print, I calculated that the pixel dimensions of my image had to be 6000 x 3000 pixels."
- - - - e.g., "My monitor has 96 ppi."

4. Scene referred linear resolution
- - - - e.g., "For the police camera to read the characters on the license plate, it needs a scene referred linear resolution of better than 1/4 inch."

5. Scene referred angular resolution
- - - - "For the police camera to read the characters on the license plate of a car 100 feet away, it needs a scene referred resolution of better than 0.1 arc minute."

6. Pixel size / pixel spacing
- - - - "The size of each pixel on my camera is 5 micrometers by 5 microns."
- - - - "The pixels on my monitor are spaced approximately 0.01" apart."



So, Mr.Tom, I think it should be fairly self-evident how the consistent and correct use of the above phrases could clarify much of the discussion that goes on about this topic. Here is one example based on your last post:

MrTom: "Now, I agree wholeheartedly with that, but there is another meaning of the word 'resolution' which is the one I stand by and believe to be the [less misleading?] definition, namely that 'resolution' is a ratio of the amount of pixels per a linear measurement, hence the term 'Pixels Per Inch'...the amount of pixels is irrelevant.

Both a 1x1 px image and a 1000x1000 px image could have a 300ppi or a 1200ppi resolution written to their meta-data....or even 1ppi.

6MP is just the total number of pixels, from just this info you cannot even determine the dimensions of an image....it could be 1px high by 6Mpx wide....who knows? "

The concept actually being expressed above is simply that the pixel dimensions, the total number of pixels, and the number of PPI for an image file are almost completely independent of each other. They describe different things, they can be set almost independently (some by the hardware, some by writing to the metadata), and, depending on the topic under discussion, zero, one, two, or all three of them could be relevant to that discussion.

HTH,

Tom M