When I export a 15 MP jpg file to be sent to a printing lab (no image resizing), and the printer asks for a DPI resolution of 300, what do I do in Lightroom that only allows me to choose a PPI resolution value? Are they assumed to be the same? The problem I am having comes from knowing that DPI (dots per inch) is a value that makes "sense" in terms of an actual print and that PPI (pixels per inch) is essentially "senseless" because it's impossible to measure pixels per inch since they only physically occur on an image sensor (and on the computer screen, I suppose) and a full frame image sensor (as everyone knows) is approximately 36 x 24 mm. And finally, is there a difference noticed when comparing a 15 MP image that was exported from LightRoom at a PPI 100 resolution and a 15 MP image that was exported at a PPI 300 resolution? If my questions are unclear, they are the result of my trail of confusion, and I sincerely apologize. Whatever help you can extend is sincerely appreciated.
As a practical matter, if your printer is asking you for a DPI of 300, she almost certainly is asking for what others would call a PPI of 300. That is, if you submit a 5000 x 3000 pixel image, the printer will print it at 300 pixels per inch, resulting in a print that's 16 2/3" x 10" print (5000 pixels / 300 PPI x 3000 pixels / 300 PPI).
In LR, you can set the PPI of an image in the Image Sizing of the Export dialog. If you want to send the JPEG without any resizing but marked for 300 PPI, you'd do:
And finally, is there a difference noticed when comparing a 15 MP image that was exported from LightRoom at a PPI 100 resolution and a 15 MP image that was exported at a PPI 300 resolution?
No. They will contain the exact same number of pixels with the same pixel dimensions. The PPI recorded in the metadata by LR just provides a "hint" about the intended size at which you want the image to be printed. In general, printing software and word-processing software (generating documents to be printed) often use the PPI to decide the default for how big the image should appear on the printed page. But image editing software like LR generally ignores any PPI recorded in the image.
You are referring to what is a resolution tag and it's rather meaningless. It could be 72PPI or 180PPI but it doesn't have an inherent meaning, only what you could produce with the number of pixels you have at your disposal. Work with pixels! For example, let us say you have 1000x1000 pixels to keep the math simple. And to simplify this further, let's only consider the horizontal axis. If you have 1000 pixels and divide that by 72, that is, you provide 72 pixels per inch, you could end up with 13.8 inches using that division (1000/72=13.8). Let's now say you divide up your 1000 pixels using 180 instead. 1000/180=5.5. In both cases, you had 1000 total pixels. The document itself doesn't have a size, other than what space it takes up on your hard drive. The sizes above are examples of what could be produced if you divided up the total number of pixels you have, with some number of which is just a tag within the document. In Photoshop, if you use the Image Size dialog, turn resample OFF (do not allow it to create more or remove pixels), you can enter any value, 72, 180, 1000 into the resolution field and the resulting size is calculated for you. But you haven’t changed the document or the data at all. You just changed a theoretical 'size' if you output your 1000 pixels using that resolution. So again, it's meaningless until you output the data. At that point, lets say you print the image, you can decide how big you wish it to appear and/or how many pixels you want to devote to the output. You have 1000 pixels and someone tells you that you must use 300DPI (which isn't true but that's a different story). 1000/300 would produce a 3.3 inch print. You want a bigger print? Lower the DPI (within reason). You set the DPI for output to use 180 of your pixels to produce 180DPI? You get a 5.5 inch print (1000/180=5.5). Work with pixels. That's a fixed attribute of the data unless of course you resample that data (add or remove pixels).