How are pixels and centimeters related to each other in Photoshop?

Question

Design

I need to store it on the server in a jpg file in order to show the user a picture on the screen that he can choose for himself as a cover for a postcard. Then I need to embed the selected jpg file into pdf for further printing in a printing house. On the server, you need to store only one file, which the php script can reduce on the fly to the desired size.
The question is how to properly prepare a file for the required task? As far as I understand, I must store the file at a resolution of 72 dpi. But what is the right way to reformat a jpg if it was originally given to me at 300 dpi?
I open my file in Photoshop, go to "Image - Image Size" and try to adjust the size that suits my needs. I have the original jpg file at 300 dpi and 1240 by 1748 px. There are two ways to change the size:
1. When I set the file resolution to 72 dpi, the size in pixels does not change, but the size in centimeters increases greatly.
2. If I check the "Resamling" box and set the same 72 dpi, then the size in pixels decreases significantly, but the size in centimeters remains the same.
It seems obvious that you need to use the first option without reducing the file size in pixels. But at the same time, it seems suspicious to me that when the resolution is reduced from 300 to 72 dpi, the size in pixels does not increase. Why? On the other hand, if this is normal, won't the observed increase in size in centimeters damage the print?

Answer 1

Pixels and centimeters are connected just through dpi. Dpi does not affect the display of the image on the monitor screen, since 1 px of the image always occupies 1 px of the monitor. This parameter is solely responsible for the size of the printed image, since, roughly speaking, a different number of dots (pixels) can be applied to one inch of paper. When you change the dpi from 300 to 72, you are setting a lower print resolution (fewer pixels per inch). With a constant image resolution in pixels, of course, the value in centimeters increases, since the "density" of pixels during printing decreases.
For printing, you need to operate only with the size of the image when printing (in centimeters), changing the dpi of the image, but without touching the size in pixels. Let's say you need to print an image on paper 21x15 cm. For high-quality printing at 300 dpi, the image size must be at least 2480x1772 px. If the picture is 2 times smaller, then be prepared for the quality as at 150 dpi, even if the image itself says 300 dpi. The printer is not a fool and will enlarge the image to fit the required print size. There are all sorts of converters that allow you to find out the required image size in pixels for a specific print resolution and paper sizes.
I hope I helped you with this question.

Answer 2

If you want more details about dpi and ppi, here is a huge post on the subject .