Trying to understand image resolution and pixel dimension and document size

[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!

 

[Benco]

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.

 

[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.

 

[christop]

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

 

[MiFleur]

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

 

[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?

 

[MiFleur]

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?