LightRoom export

When you send a photo from LR out to another app, such as a noise removal app or even Photoshop, what would be the prime DPI/PPI? How do you choose a number, such as 300?

From what I understand, and someone please correct me if I'm wrong, DPI/PPI doesn't matter until/unless you get to a print phase for your image. As long as you're playing with it digitally, pixel density per inch doesn't come into play.

If you're exporting for further editing, you'd usually do that as a tiff or psd file. You'd never print from those formats. Once your editing is finished, you'd export that tiff or psd to a jpeg for final digital output. If that final output is meant for a printer, then DPI/PPI comes into play and you set it to your printer's specification.

In LR, I keep my external editing preferences set to 300 because you have to set it to something, but again, I don't believe it makes a difference at this point in the editing process.

This is my understanding also.

For Lightroom export function, specifying DPI does matter when you say you want a JPG export of a certain SIZE and you need a certain DPI to make the pixel count proper. For example, if you say you want dimensions of 40 x 60" at 300dpi, and your original image is from a Canon 30D, Lightroom will create a JPG with 12000x18000 pixels even though the original image only had 8Mpixels!

But dpi value embedded in as EXIF information within the JPG does NOT matter, relative to your sending the JPG to a home printer...no matter the embedded dpi value, the printed output looks no different
And embedded dpi value does not directly matter with regard to display of JPG on home computer monitors..no matter the embedded dpi value, the image looks no different on any monitor
It CAN matter if you are sending the JPG to a commercial offset printer (catalogs, brochures) services. And even for commercial inkjet printing, some services insist upon embedded '300dpi' in the EXIF, as if it really mattered (when the only thing that really mattered was a sufficiently high pixel count for the requested image size to be satisfactory to the eye of the viewer).

If your external app can be installed as a plugin to Lightroom you can configure LR to automatically send it to the app as a tiff (psd or jpeg) file at full resolution for you and then manage importing the completed file back into LR. That's the easiest way to keep things organized if your external application can work that way. Many do.

Wilt wrote in post #19386691
For Lightroom export function, specifying DPI does matter when you say you want a JPG export of a certain SIZE and you need a certain DPI to make the pixel count proper. For example, if you say you want dimensions of 40 x 60" at 300dpi, and your original image is from a Canon 30D, Lightroom will create a JPG with 12000x18000 pixels even though the original image only had 8Mpixels!

But dpi value embedded in as EXIF information within the JPG does NOT matter, relative to your sending the JPG to a home printer...no matter the embedded dpi value, the printed output looks no different
And embedded dpi value does not directly matter with regard to display of JPG on home computer monitors..no matter the embedded dpi value, the image looks no different on any monitor
It CAN matter if you are sending the JPG to a commercial offset printer (catalogs, brochures) services. And even for commercial inkjet printing, some services insist upon embedded '300dpi' in the EXIF, as if it really mattered (when the only thing that really mattered was a sufficiently high pixel count for the requested image size to be satisfactory to the eye of the viewer).

To boil down Wilt's response, DPI/PPI resolution is all about printing with ink. The general nature of the OP question makes me think printing isn't a main concern right now. When it becomes a concern, especially if he's making giant prints, there can be a lot to learn.

You're in pretty good shape if you leave things at 300 ppi. I'm pretty sure there's no penalty for doing that even if you don't need it.

drsilver wrote in post #19386742
To boil down Wilt's response, DPI/PPI resolution is all about printing with ink. The general nature of the OP question makes me think printing isn't a main concern right now. When it becomes a concern, especially if he's making giant prints, there can be a lot to learn.

You're in pretty good shape if you leave things at 300 ppi. I'm pretty sure there's no penalty for doing that even if you don't need it.

In the 'old days', photos were 'screened' in order to print images in magazines and newspapers. The daily news might only use a screen with 75dpi, while National Geographic might use 300dpi to print the identical image to higher reproduction quality.
Fast forward to inkjet printing...your Epson might have 1440 dpi. The EXIF value 'dpi' has NOTHING TO DO with how many inkjet dots the Epson puts on paper...it is always 1440 inkjet dots, no matter how many pixels are in the image and no matter what the EXIF 'dpi' value is.
Send that same JPG to your local newspaper, and they would always print your image at 75dpi, regardless of the EXIF value 'dpi' embedded within the file that you sent them.
Send the same file to be offset printed in a brochure, and their software could print a very different looking result using the same 3000 x 4500 pixels, when 75dip is in the EXIF vs when 300dpi is in the EXIF....'it depends'

drsilver wrote in post #19386742
To boil down Wilt's response, DPI/PPI resolution is all about printing with ink. The general nature of the OP question makes me think printing isn't a main concern right now. When it becomes a concern, especially if he's making giant prints, there can be a lot to learn.

You're in pretty good shape if you leave things at 300 ppi. I'm pretty sure there's no penalty for doing that even if you don't need it.

Yes, you are right. The DPI stuff doesn't matter. Noise removal, smoothing, and other math operations work on pixels and the algorithms involved don't care about the print or display resolution.