How do I check the DPI of an image for specifically A3 landscape print?

[Chris P. Bacon]

More specifically, I should check the print DPI for an A3 crop of the image.

[Chris P. Bacon]

InDesign calculates DPI from the image resolution and from the print size, I wanted to be able to do the same calculation using Photoshop. If I change those values in InDesign, the DPI changes. These 2 values are known to Photoshop too, the feature just seems to not exist?

[davescm]

@Chris P. Bacon

PPI is the correct term for making up a physical print from a pixel size. The PPI value is sent to the print driver in order for that driver to determine what physical size to print from those pixels. Hence pixels per inch.

DPI is the dots that a printer lays down in order to print those pixels. It varies by printer and is irrelevant in Photoshop (or InDesign) which use Pixels per inch . As Lumigraphics correctly states above, DPI is a setting in the printer driver.

As an example I can choose in Photoshop to make an A3 print at say:

180ppi which will measure 2977 x 2105 pixels and will print at A3

360ppi which will measure 5954 x 4210 pixels and will still print at A3

720ppi which will measure 11908 x 8420 pixels and still print at A3

Note all those values are ppi

When I print to my Epson printer (SC-P5000) I can print any of those 3 at different dpi settings in the printer driver. That has nothing to do with the ppi setting.

For example :
Fine 720 x 720 dpi

SuperFine 720 x 1440 dpi

SuperPhoto 2880 x 1440 dpi

None of those dpi settings in the printer driver affect the number of pixels printed (which is ppi) they just affect the fine dots that make up each pixel.

Dave

[Chris P. Bacon]

Then it seems that the term "PPI" is used to name both screen resolution and print resolution (how many pixels to print on an inch of paper).

[Chris P. Bacon]

The missing information for me it seems it was that printers always print the same PPI as much is defined in the image metadata. 

[Lumigraphics]

Lordy. PPI and DPI are not the same. Printers output in DPI, input in PPI.

And some things don't have PPI. For example, pure PostScript printed to a PostScript printer will simply render at whatever resolution is set in the printer driver. In fact, output resolution can vary, this is where you get into things like line screens. More advanced photo printers can change droplet size and density for better results or to match different papers.

Also remember that PPI can be HIGHER than output DPI. Say you have a 1" x 1" image at 3000 ppi. A printer can't do 3000 DPI, its hardware limited, so your output DPI might only be 300.

[Brad @ Roaring Mouse]

"If I change those values in InDesign, the DPI changes."

What values? What are you referring to?

Also, InDesign specifies resolution in PPI also like all programs do, so not sure where you are seeing DPI

[Chris P. Bacon]

2 values: resolution and print size.

[Chris P. Bacon]

Total ther are only 3 values that matter, right?

Resolution, print size and printer dot size.

So calculations account for these 3?

[jane-e]

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@Chris P. Bacon wrote:

InDesign calculates DPI from the image resolution and from the print size

---

InDesign also works in Pixels Per Inch (ppi). 

If you are in Photoshop and not printing, the numbers that matter are the width and height in pixels.

When you get to InDesign, there is one additional control. InDesign will show you both the Actual PPI and the Effective PPI in the Links panel. If you place the image at 100%, both numbers will be identical. If you drag-place to scale the image (or scale later), the Effective PPI is the one that matters.

Once again, the DPI is not controlled by InDesign. It's controlled by the output device — the printer.

Jane

[Test Screen Name]

"InDesign calculates DPI from the image resolution and from the print size"

No, it doesn't. But somethinh is making you feel that it is, because I think you've said this more than once. If you mean "effective ppi", yes, it does calculate that and we can help you do those calculations. But if you see a screen with DPI on it, and believe InDesign put it there, please show us a screen shot, so we can get on the same page.

[Chris P. Bacon]

I meannt effective PPI, yes the DPI was a wrong term I used. 

But since I cannot check for effective PPI in Photoshop, I want to establish a relation between the effective PPI and the pixel width (because my print sizes are fixed), and just look at the pixel width of the A3, A4 etc. crop of the image (which corresponds to a certain effective PPI / for each print size) because it's easier to check it than to open up InDesign and place the image only for that purpose.

I will just have to remmeber a minimum px width number for each quality threshold for each predefined print size.

[jane-e]

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@Chris P. Bacon wrote:

I am asking for DPI not for PPI and DPI is calculated for print area.

How do I see in Photoshop what will the DPI be if I crop my image to A3?

---

If you want to know the DPI, check the settings on the printing device that you will be using. Photoshop works in Pixels Per Inch (PPI). The terms are not interchangeable, although there are folks who get them wrong.

https://helpx.adobe.com/photoshop-elements/key-concepts/resolution.html

https://www.shutterstock.com/blog/ppi-vs-dpi-resolution-guide

https://www.sony.com/electronics/support/articles/00027623

https://photographylife.com/dpi-vs-ppi

Jane

[Brad @ Roaring Mouse]

There are a few ways to determine the PPI. One of the simpler ones, since you're in Photoshop already, is to look at Image > Image Size. On your cropped image, type in the dimensions you want and the PPI will reflect it's new essential resolution: e.g. in my example I'm using the width of a vertical A3 (29.7) to see what my original 300ppi image would be at that width = 218-ish.

 In your case, type in 42 cm for width (landscape)

[mick33700021lnpp]

you'll need to know the original image's dimensions and the desired print size. Once you have this information, you can use the following formula to calculate the DPI:
DPI = (Total pixels) / (Print width in inches) / (Print height in inches)

Keep in mind that 300 DPI is generally considered the standard for high-quality prints, so if you want to achieve the best possible results, you'll need to make sure that the original image has a high enough resolution.

[D Fosse]

@mick33700021lnpp 

Once again, an ink dot is not the same thing as an image pixel!

"Dots per inch" is incorrect and misleading. "Pixels per inch" is the correct term. This is not ink dots,  this is image pixels.

That so many use incorrect terms does not help understanding.

[Brad @ Roaring Mouse]

When it comes down to is that DPI and PPI are definitions each more accurate now for certain processes, but the intent is the similar. PPI has become the more contemporary term over te years to be a more accurate statement of what's being measured. i.e. in an image which is a collection of pixels. However, it's just as misleading to say that DPI is only for a dot of ink. (It's actually the output DEVICE that has a resolution, be it an imagesetter. In fact, DPI is still used today by scanner manufacturers to define their resolution. (Actually, in the early days of drum scanners, some used the termed SPI to define resolution, which is Samples Per Inch, but I digress). So yes, you can come down on people on which terms they use, but that's just picking nits. People who have been in the business a long time have DPI in our brains because it was the only term used for many years. Even I will still use it incorrectly without thinking beacusse as a prepress person, I'm more inclined to use DPI in scans and imagesetter resolution.

What IS incorrect in the post immediately above is that the resolution is NOT defined in two dimensions, it's either width or height, not BOTH, so it's PPI(DPI/SPI) = Total pixels(dots/samples) / width(or height) in inches. This of course changes if the image is scaled unproportionallyy, so you could have diffrent PPI depending on which direction you measure.

[jane-e]

People who have been in the business a long time have DPI in our brains because it was the only term used for many years.

By Brad @ Roaring Mouse

Thirty-five years for me, and from the very beginning I learned the proper terms and taught them to my students. PPI has always referred to image Pixels per Inch and DPI has always referred to Dots of Ink per Inch on paper.

Here's what Adobe Help says:

How does DPI differ from PPI?

PPI (Pixels per Inch) refers to the number of pixels that make up every inch of a digital image. It’s used to describe image resolution on a screen, rather than in print. DPI, meanwhile, refers to number of dots in every inch and is generally used for print purposes.

Jane

[D Fosse]

Right. The fact that these terms have "always" been used incorrectly, is no excuse to keep doing it.

[Trevor.Dennis]

you'll need to know the original image's dimensions and the desired print size.

By @mick33700021lnpp

I just use the New File panel and click on the Print tab.  If I only need the dimensions then commit to memory, but I usually do is create a new document and use it as a template.