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.

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?

 

[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.

 

[MiFleur]

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?

 

[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.

 

[MiFleur]

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

 

[Majeed Badizadegan]

KMH?

 

[MiFleur]

KMH?

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

 

[MiFleur]

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

Some questions about 'image interpolation' (CS6)?: Open Talk Forum: Digital Photography Review

 

[Majeed Badizadegan]

KMH?

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

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

 

[KmH]

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.

 

[MiFleur]

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:

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

 

[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.

 

[MiFleur]

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

 

[Benco]

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

 

[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.