Dpi: what happens when an image is re-saved from 240 to 300 dpi?

Question

Hi
When I open my images in Adobe Raw, the images are 240 ppi. What happens exactly when I change the setting to 300 ppi in the Adobe Raw. For printing many companies ask for 300 ppi. And what happen if I re-save old images again in Photoshop after processing with 300 ppi, will the picture become unsharper? I cannot see a difference. Thanks for your explanation. 

Answer 1

Interesting question and I hope I understand what you're asking
An image in and of itself has no size. It is built out of square pixels of indefinite size. They could be one micron, or the size of a city block.
An image only has a specific length and width once the pixel size is established.
To illustrate open any 72 PPI image. View: Rulers. Change the image to 300 PPI with resample disabled. The pixel data of the image is not altered, the only thing that changes is the ruler around the image.
My guess is that the change from 240 to 300 in your example does not change the pixel result. What does change is the size of the file. The higher the PPI, the smaller the physical file dimensions. A 240 PPI image that is 10" by 12.5" would equal a 300 PPI image that is 8" by 10". Either one of these qualifies as a 7.2 megapixel image (2400 x 3000 = 7,200,000 pixels in the image).