Change what PS?
PS uses UI scaling
Your operating system determines DPI settings
do i have to restart PS in order for change to kick in, because i changed it and it still comes up with 72DPI
ok, then is that changeable? the interface?
Digital images don't have a DPI setting. All they have is pixel dimensions. Changing the DPI (Really, it's PPI or pixels per inch) setting has no impact whatsoever on image quality in Photoshop. The only time it comes in to play is when you are trying to set up a print job. And then the setting is relative to the printer. It has no impact on image display on your monitor.
300 ppi is stored as Metadata, which is information about the file separate from the image.
PSD and Tiff will always carry the 300 ppi, but using Save for Web on Jpeg and PNG can remove that 300 ppi if Metadata is set to "None". The file then defaults to 72 ppi.
So if you save those kinds of files, you either have to set Metadata to "All" in Save for Web, or use File > Save As... to create a jpeg or png file with the 300 ppi metadata.
Yes - restart after changing
As JimHess points out :
Print Resolution affects the image when it is sent to print - and calculates the image size at that resolution.
Screen resolution affects the rulers and print when you click on View - Print Size. If the screen resolution is set correctly the rulers and print will appear on screen at the actual physical dimensions.
It does not affect 100% zoom or other views at all
Most changes require a restart of PS
Sent from my iPhone
When opening raw camera files through Adobe Camera Raw, one can also set the PPI value in the “workflow” text at the bottom centre of the ACR interface (click on the hypertext):
As previously mentioned, the PPI value is stored in metadata. If you wish to change this value on saved images it could be performed using an batch action or image processor script+action to resize to 300ppi with resampling turned off – however I would not recommend doing this for JPEG files as it will decompress/recompress them which is not best practice for a lossy compressed file (even if the quality loss is next to invisible). I would use ExifTool to batch update the PPI metadata in the files without affecting the actual image data.
P.S. For images, it is PPI or pixels per inch, not DPI or dots per inch.