DPI changes in Lightroom CL to Photoshop workflow

Perfect explanation from @dj_paige 

Consider also-

If you are scanning 'small' photos, eg. contact prints 2x3inches, then your scans at 600ppi will produce 'digital' files that are 1200x1800pixels., and that may not be enough pixels to make 'larger restored' images.

Another example- 35mm transparencies scanned at 600ppi will produce files of about 600x900pixels, and a scan at 2400ppi will produce 2400x3600pixels.

Small photos may require your scanner to scan at a larger ppi setting in the scanner.

ie. You must consider the size of the photo you are scanning, and not just stick to your 'fixed' "600ppi".

Awesome! Thanks you two.

LrC always sends the full resolution file to Photoshop, that is, all the pixels in the image.

The PPI value (which is optional metadata) determines how these pixels will be distributed in a print.
Pixel dimensions divided by the PPi value = Printed dimensions in inches.

The PPI value you choose when scanning is somewhat different, it determines how many pixels will be recorded per inch of original (film or paper). This PPI value is usually saved as metadata to the file by the scanner.

You can set the PPI for images sent to Photoshop for editing in Preferences > External editing. (it is set to 240 by default)

And to answer your question, there is no quality loss, no matter what value you choose here.

DPI is meaningless for digital images. Photos are not measured in inches, they are measured in pixels. DPI only has meaning for actual photos like the ones you scanned or the ones you print on paper.

So, however many pixels you scanned in from an actual physical photo at 600 dpi, all those pixels are all there in LrC and they are all there in Photoshop. That's right, every single pixel that came out of your scanner is still there in Photoshop and LrC. For digital images, you need to think in terms of pixels, not DPI or PPI.