# Trying to understand image resolution and pixel dimension and document size



## MiFleur

I read this very useful information in another thread which leads me to my question.



KmH said:


> Digital images don't have dots (dpi), they have pixels (ppi).
> 
> 
> For electronic display, image resolution is the image pixel dimensions and ppi is meaningless. PPI only has meaning for prints and along with the image pixel dimensions is directly related to image size. *Pixels / PPI = Inches*
> Output device resolution 9dpi) has an effect on image quality, not on image size. A high quality 300 ppi image may be printed on a print device that has 6000 dpi resolution, which means each pixel is rendered using 20 dots.
> 
> It takes some number of dots to print a single pixel. The minimum number required is 3. One cyan, 1 magenta, and 1 yellow dot make black, though the 3 in equal parts don't make a deep black. So a lot of printing devices also have black ink/dye. Higher end inkjet printers use as many as 12 tones of ink/dye to make a print.




If I open one of my raw images into photoshop, it gives me an image dimension of 6016px x 4016 px  (69.1M)
Document size is roughly 25 x 16.5 inches at a resolution of 240 pixel/inch

I have always been told that to put an image on the web, I need to save it as 72 pixel/inch
to print an image I need to save it  as 300 pixel/inch

This week I was told that if I save it as a resolution of 600 pixels/inch, it would be better...

I understand that part where a printer that has more dots per inch will make a better image because of the density of the pixels.

If I let photoshop choose the resolution it gives me 133 lines/inch and I end up with an image of: 6668 px x 4451 px and a resolution of 266 ppi  (84.9M)
which is slightly above the original dimension of my image.

*What I don't understand is what photoshop does when you ask to save your image with a resolution higher than its original  ppi. how does it fill the void?
*
I see that the pixel dimension is changing to (7520 x 5020) for 300 ppi  pixel dimension becomes 108 M.

How does it make it my image better than my native format?


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## Benco

It's interpolating the image, the software looks at surrounding pixels to 'guess' as to what colour to make the new pixels it's inserting into the image. It cannot improve the image, just make the resolution higher. That said there are different algorithms for interpolation and some work better than others depending on the nature of the image.


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## MiFleur

Benco said:


> It's interpolating the image, the software looks at surrounding pixels to 'guess' as to what colour to make the new pixels it's inserting into the image._ It cannot improve the image, just make the resolution higher._ That said there are different algorithms for interpolation and some work better than others depending on the nature of the image.



Does this interpolation make my image look less natural, so far, it is very hard to tell the difference because I did not start printing my images in a larger format.

When you say different algorithms, do you mean within photoshop? or different programs?

But if I print my image and the resolution is higher.... will it make a better quality print? or allow me to print a bigger file? I intend to start printing on canvas.
Should I save my images with a higher resolution than the native format?


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## Benco

If you're not going to print your images large then you don't need to interpolate them. If you do then interpolating them will allow you to print them as large as you like at the ideal 300ppi resolution. The catch is that the software cannot create sharp detail that does not exist, with careful sharpening you can get a reasonable effect but it will not actually improve your image. Bear in mind though that 300ppi isn't really neccessary if your print is intended to be viewed from a distance (as large prints often are), unless people are going to be pixel peeping right up close to it then as low as 200ppi is supposed to be OK for a good print.

I've not really got into it but I understand that photoshop has a couple of different ways of interpolating, there are stand alone applications that can do it too. Someone more knowledgable could help you out more with that.


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## MiFleur

Thanks Benco, sharpening check... that is a good hint.
I want to print poster size, but nothing the size of a billboard.


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## Majeed Badizadegan

KMH?


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## MiFleur

Rotanimod said:


> KMH?



I took his quote from a different thread, just as stated above.


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## MiFleur

a link that I found on interpolation, I am just discovering a new world

Some questions about &#39;image interpolation&#39; (CS6)?: Open Talk Forum: Digital Photography Review


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## Majeed Badizadegan

MiFleur said:


> Rotanimod said:
> 
> 
> 
> KMH?
> 
> 
> 
> 
> I took his quote from a different thread, just as stated above.
Click to expand...


I was just wondering where he is. Usually he'd chime in with some usueful info by now


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## KmH

MiFleur said:


> *What I don't understand is what photoshop does when you ask to save your image with a resolution higher than its original  ppi. how does it fill the void?
> *


It conjures pixels from thin air, but it uses adjacent existing pixels to make an educated guess. Still, it is only a guess.

Using more than 300 PPI is essentially useless for a print, because of human eye performance limits.

Back in the day computer displays were 72 ppi. They haven't been for years now and*, for electronic display PPI is meaningless because there are no inches, only pixels*. 

Here are 2 photos of mine. Both images are 3807x2719 px, 610 kb files, but resized by TPF to 1024x731. One is set at _*1*_ (one) PPI and a quality setting of 5, while the other is 240 PPI, quality 5. Tell me which one is which:






2.


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## MiFleur

KmH said:


> MiFleur said:
> 
> 
> 
> *What I don't understand is what photoshop does when you ask to save your image with a resolution higher than its original  ppi. how does it fill the void?
> *
> 
> 
> 
> It conjures pixels from thin air, but it uses adjacent existing pixels to make an educated guess. Still, it is only a guess.
> 
> Using more than 300 PPI is essentially useless for a print, because of human eye performance limits.
> 
> Back in the day computer displays were 72 ppi. They haven't been for years now and*, for electronic display PPI is meaningless because there are no inches, only pixels*.
> 
> Here are 2 photos of mine. Both images are 3807x2719 px, 610 kb files, but resized by TPF to 1024x731. One is set at _*1*_ (one) PPI and a quality setting of 5, while the other is 240 PPI, quality 5. Tell me which one is which:
Click to expand...


I am trying to follow you, the screen resolution of my computer is 1920x1080  so to fill it, I need an image of 1920x1080 pixels and it does the trick. So resolution can be as small as 1 ppi on my photoshop window called image size.
 Do I get this part right? so it will occupy very little space on my hard drive...  like 2 mp  (2073kb)

But if I want to print that same image, I need a density of 300 pixels per inch so my image becomes  1920 x 1080 / 300 =  6"4 X 3.6" a very small image

and if I want a 24 x 30" image I need lots of pixels 7200 x 9000 pixels. But my camera does not give me that... 

So back to the question, do I just leave my image at 240 ppi with the original size or do I put in photoshop image size 7200 x 9000 pixel at 300 ppi even if my original image is only 6016 x 4016 pixels?

This is confusing...

thanks


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## Benco

That's right, for example on the computer an image can be 5000 inches wide at 1ppi or 25 inches wide at 200ppi or 1 inch wide at 5000ppi, it makes no difference on screen because you can zoom in or out at will with no loss to the image. 

For printing though the image size is going to be fixed (in your example 24 X 30") so yes, if you wanted the image to have 300 ppi it would need to be 7200 X 9000. I'd suggest that you do not need 300ppi though, 24 X 30 is pretty big and IMO 200ppi will be just fine for it, the only slight problem is that the proportions of your image are wrong for a 24 X 30 print, 6016 X 4016 would give you a 20 X 30" print @ 200ppi.

6016/30 = 200
4016/20 = 200

...or close enough.


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## MiFleur

Benco said:


> That's right, for example on the computer an image can be 5000 inches wide at 1ppi or 25 inches wide at 200ppi or 1 inch wide at 5000ppi, it makes no difference on screen because you can zoom in or out at will with no loss to the image.
> 
> For printing though the image size is going to be fixed (in your example 24 X 30") so yes, if you wanted the image to have 300 ppi it would need to be 7200 X 9000. I'd suggest that you do not need 300ppi though, 24 X 30 is pretty big and IMO 200ppi will be just fine for it, the only slight problem is that the proportions of your image are wrong for a 24 X 30 print, 6016 X 4016 would give you a 20 X 30" print @ 200ppi.
> 
> 6016/30 = 200
> 4016/20 = 200
> 
> ...or close enough.



Thanks, I think it is clear enough, and will forget about saving in a resolution of 600 ppi


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## Benco

Yes, I'm not sure what they were getting at suggesting 600ppi.


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## KmH

Resolution for electronic display = the pixel dimensions of the image.
Image size for electronic display = the zoom setting of the display or web site.
Again - PPI is meaningless for electronic display.
Your 1920 x 1080 resolution display cannot show all of a 3807 x 2719 image at once at a 100% (1:1) zoom setting.
In my above examples at a quality setting of 5, changing the PPI from 1 PPI to 240 PPI did not change the image file size.

Resolution for prints = the pixels-per-inch
Print size = pixel dimensions / ppi. (See below)

In fact, using some basic algebra we can determine 3 equations that will help us calculate various values needed for *prints* in advance:
*Pixels / PPI = Inches
Pixels / Inches = PPI
PPI x Inches = Pixels*

As prints get larger, they are viewed from further away, and require less PPI.

Online pro and consumer print labs stake their reputations on the quality of the prints they make, so they set minimum resolution requirements.
Most set that minimum at or close to 100 PPI.
Wonder why they don't have that minimum set way closer to the 300 PPI so many like to claim is needed to make a quality print.


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## MiFleur

Thanks Keith, this is concise and clear, love how you explain things, your have probably been doing that for quite a while, 
well articulated!


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## Benco

KmH said:


> Resolution for electronic display = the pixel dimensions of the image.
> Image size for electronic display = the zoom setting of the display or web site.
> Again - PPI is meaningless for electronic display.
> Your 1920 x 1080 resolution display cannot show all of a 3807 x 2719 image at once at a 100% (1:1) zoom setting.
> In my above examples at a quality setting of 5, changing the PPI from 1 PPI to 240 PPI did not change the image file size.
> 
> Resolution for prints = the pixels-per-inch
> Print size = pixel dimensions / ppi. (See below)
> 
> In fact, using some basic algebra we can determine 3 equations that will help us calculate various values needed for *prints* in advance:
> *Pixels / PPI = Inches
> Pixels / Inches = PPI
> PPI x Inches = Pixels*
> 
> As prints get larger, they are viewed from further away, and require less PPI.
> 
> Online pro and consumer print labs stake their reputations on the quality of the prints they make, so they set minimum resolution requirements.
> Most set that minimum at or close to 100 PPI.
> *Wonder why they don't have that minimum set way closer to the 300 PPI so many like to claim is needed to make a quality print*.



If I'm being cynical I'd say that it's because to print at 100ppi the image needs to be tack sharp and a lot of what they get given will be a bit ropey, if they specify 300 ppi that gives them plenty of wriggle room for editing.


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## Helen B

Remember that people's visual acuity varies quite a lot, as does their expectation of image quality; and the actual dpi that the printing machine will print at also varies. The various LED and laser printing machines that expose photographic paper typically have a native setting, or in some cases settings, of around 200 dpi; 300 dpi; 400 dpi; 600 dpi or even 1200 dpi now. (In this case a single 'dot' consists of three coincident exposures, one each in red, green and blue.) If you wonder why 300 ppi is used as a common standard when 100 ppi is enough, ask yourself why bother with printers capable of 400 dpi or 600 dpi. 

A file sent at 300 ppi may be printed at only 200 dpi on a machine with an exposure unti that can do both 200 dpi and 400 dpi because the machine runs faster at 200 dpi, and most people will be happy with the 200 dpi print. It should not be difficult to find out what dpi your file will actually be printed at (and you should also be able to get the printer profile for the machine your image will be printed on, of course).

Inkjet printers often work best at ppi values that are simple divisions of their dpi setting - eg if you have an Epson that prints at 2880 dpi it will probably be optimal at 288, 360 and 720 ppi, and some people will be able to see the difference between the 360 ppi print and the 720 ppi print, if the original image has real detail at the 720 ppi level. Whether you need that extra resolution is an entirely different matter.

Inkjet printer resolutions and LED/laser exposure unit resolutions can't be compared directly, because the image is made up in two very different ways.

Some people find that they do not require an absolute resolution (pixel dimensions) greater than the equivalent of 10 x 8 at 300 ppi - ie pixel dimensions of 3000 x 2400. As the file is printed larger, the ppi drops, but so does the likely viewing distance. On the other hand, some images invite the viewer in when they are printed large, and in that case you may wish to keep the true resolution high so that as the viewer is drawn into the image it does not fall apart, but reveals more.

Rather than work to a fixed quality used by someone else, it might be worth doing your own testing to find the print quality you prefer for your images at the size you want with the printer or printing service you use. You can also test the difference between doing your own interpolation and letting the printing service / printer driver do the interpolation - it may be acceptable and easiest to leave the interpolation to the printer.

In some cases you may be asked for images with specific resolution standards, particularly for magazines, brochures etc. Often that requirement will be 300 ppi even though the printing method may not be capable of resolving that level of detail - the oversampling makes sure that the final image is as good as it can be.


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## christop

KmH said:


> In fact, using some basic algebra we can determine 3 equations that will help us calculate various values needed for *prints* in advance:
> *Pixels / PPI = Inches
> Pixels x Inches = PPI
> PPI / Inches = Pixels*



Slight correction for the second and third equations:

*Pixels / Inches = PPI
PPI x Inches = Pixels*


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## MiFleur

Helen B said:


> Remember that people's visual acuity varies quite a lot, as does their expectation of image quality; and the actual dpi that the printing machine will print at also varies. The various LED and laser printing machines that expose photographic paper typically have a native setting, or in some cases settings, of around 200 dpi; 300 dpi; 400 dpi; 600 dpi or even 1200 dpi now. (In this case a single 'dot' consists of three coincident exposures, one each in red, green and blue.) If you wonder why 300 ppi is used as a common standard when 100 ppi is enough, ask yourself why bother with printers capable of 400 dpi or 600 dpi.
> 
> A file sent at 300 ppi may be printed at only 200 dpi on a machine with an exposure unti that can do both 200 dpi and 400 dpi because the machine runs faster at 200 dpi, and most people will be happy with the 200 dpi print. It should not be difficult to find out what dpi your file will actually be printed at (and you should also be able to get the printer profile for the machine your image will be printed on, of course).
> 
> Inkjet printers often work best at ppi values that are simple divisions of their dpi setting - eg if you have an Epson that prints at 2880 dpi it will probably be optimal at 288, 360 and 720 ppi, and some people will be able to see the difference between the 360 ppi print and the 720 ppi print, if the original image has real detail at the 720 ppi level. Whether you need that extra resolution is an entirely different matter.
> 
> Inkjet printer resolutions and LED/laser exposure unit resolutions can't be compared directly, because the image is made up in two very different ways.
> 
> Some people find that they do not require an absolute resolution (pixel dimensions) greater than the equivalent of 10 x 8 at 300 ppi - ie pixel dimensions of 3000 x 2400. As the file is printed larger, the ppi drops, but so does the likely viewing distance. On the other hand, some images invite the viewer in when they are printed large, and in that case you may wish to keep the true resolution high so that as the viewer is drawn into the image it does not fall apart, but reveals more.
> 
> Rather than work to a fixed quality used by someone else, it might be worth doing your own testing to find the print quality you prefer for your images at the size you want with the printer or printing service you use. You can also test the difference between doing your own interpolation and letting the printing service / printer driver do the interpolation - it may be acceptable and easiest to leave the interpolation to the printer.
> 
> In some cases you may be asked for images with specific resolution standards, particularly for magazines, brochures etc. Often that requirement will be 300 ppi even though the printing method may not be capable of resolving that level of detail - the oversampling makes sure that the final image is as good as it can be.



This is the printer that our company has, here is the link to the spec sheet

Epson Stylus Pro 7900 | 9900 - Epson Professional Imaging - Epson America, Inc.

for resolution it says

*Resolution*

2880 x 1440 dpi; 1440 x 1440 dpi; 1440 x 720 dpi;
720 x 720 dpi; 720 x 360 dpi; 360 x 360 dpi

but I am not sure how to compare this info with the way I should save my file


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## Helen B

That's an excellent printer. I haven't used that particular model, but it is likely to perform well at 144 ppi, 288 ppi, 360 ppi and 760 ppi. Other resolutions will also work, of course.

(1440/10; 1440/5 & 2880/10; 1440/4 & 2880/8; 1440/2 & 2880/4)


Your D600 produces files of 6016 x 4016, as you say. Without resampling these would be the print sizes at those resolutions:

144 ppi: 42 x 28
288 ppi: 21 x 14
360 ppi: 17 x 11
720 ppi: 8.3 x 5.5

The 360 ppi size would fit well on 19 x 13 paper, for example.

If, with the Resample Image box unchecked, you can get the image size in the Image Size dialog to what you want, with a resolution that lies between 144 and 760, I wouldn't worry too much about resampling, but do a test on a small sheet of paper (and crop the image accordingly, without changing the resolution) first. With large prints you might want to do that anyway to tune the sharpening and colour before printing the whole, expensive image.

Saving a file that hasn't been resampled, but has only had changes to the resolution (and hence size) is non-destructive - you haven't resampled. You can play around with the resolution or size as much as you like, as long as the image isn't being resampled. Final sharpening is a different matter - you may wish to save the file before final sharpening and as a separate fixed-size print-ready file after sharpening.

Does that make sense so far?


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## MiFleur

Helen B said:


> That's an excellent printer. I haven't used that particular model, but it is likely to perform well at 144 ppi, 288 ppi, 360 ppi and 760 ppi. Other resolutions will also work, of course.
> 
> (1440/10; 1440/5 & 2880/10; 1440/4 & 2880/8; 1440/2 & 2880/4)
> 
> 
> Your D600 produces files of 6016 x 4016, as you say. Without resampling these would be the print sizes at those resolutions:
> 
> 144 ppi: 42 x 28
> 288 ppi: 21 x 14
> 360 ppi: 17 x 11
> 720 ppi: 8.3 x 5.5
> 
> The 360 ppi size would fit well on 19 x 13 paper, for example.
> 
> If, with the Resample Image box unchecked, you can get the image size in the Image Size dialog to what you want, with a resolution that lies between 144 and 760, I wouldn't worry too much about resampling, but do a test on a small sheet of paper (and crop the image accordingly, without changing the resolution) first. With large prints you might want to do that anyway to tune the sharpening and colour before printing the whole, expensive image.
> 
> Saving a file that hasn't been resampled, but has only had changes to the resolution (and hence size) is non-destructive - you haven't resampled. You can play around with the resolution or size as much as you like, as long as the image isn't being resampled. Final sharpening is a different matter - you may wish to save the file before final sharpening and as a separate fixed-size print-ready file after sharpening.
> 
> Does that make sense so far?



Yes it does make sense for the file saving, thank you so much Helen, but what you said leaves me with a question about sharpening.
I had heard about sharpening at the very end, I usually apply some sharpening in camera raw before I open my image in photoshop, 
Do you suggest not to do that?
you are mentioning final sharpening...
I use the unsharp mask, or the smart sharpen at the end, this is the part I would leave out until I am ready to print.
Do I get this right?


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