Export PPI does not effect print quality????

[Mr. McGalla]

Hello so I am trying to teach my students about resolution and wanted to print a photo at 10 PPI but when I export and change the PPI to 10 nothing changes.  When I right click>properties on the image from my desktop (on a PC obviously) it shows that the PPI is 10, but when I printed it is not.  So what is happening here?  No matter what PPI I pick the photo never looks different on the screen or in print.  What am I missing?

[dj_paige]

If your settings in the Export dialog box have the "resize to fit" box unchecked, then the PPI does not affect anything.

If your settings in the Export dialog box have "resize to fit" checked, and you choose a resizing method that does not use inches or centimeters, you choose a resizing method that involves pixels, then the PPI does not affect anything.

Why? Because digital images cannot be measured in inches, they can only be measured in pixels.

So, what you are seeing sounds perfectly normal.

[Mr. McGalla]

Thanks that makes sense, but what is the point of being able to change the PPI?  With resize checked or unchecked it does change the way it looks.  When I change to 10 PPI and then print the photo it looks normal.  How can I print from LR so that the printed image is actually 10 PPI?

[dj_paige]

You have to check "Resize to Fit" and then select a resizing method that is in inches. Then PPI makes a difference. If you ask to export a photo that has a dimension of 6 inches x 4 inches, and you ask for 300 PPI, then the exported photo is 1800x1200 pixels. If you ask for 10 PPI, then the exported photos is 60x40 pixels. 

[richardplondon]

How can I print from LR so that the printed image is actually 10 PPI?

You can set a 10ppi resolution override (or any other number) in the Print dialog. This allows the same (original, imported) picture information to be resampled on the fly, during printing, to differing "picture resolutions" (by which I mean: with more or less fineness of detail represented within the photo, when that photo is sized to consistent dimensions on the page).

That is equivalent in practice, to exporting different versions of the same image with different settings for Resolution (PPI), and with Resizing checked, and with constant inch / centimetre dimensions specified for the output.

These different versions must contain different numbers of pixels, because that's what you get when the same number of inches contain differing numbers of pixels per inch.

If resizing is un-checked in Export - it is the natural picture information as-imported, and as-cropped, which is output - however many pixels that may consist of. In this case, whatever Resolution figure is set will NOT alter how many pixels / what fineness of representation is shown. It will only alter how large the output notionally becomes.

To explain notional: say a photo has got 2000 actual pixels across its width, coming from the camera, and not thereafter changed. If considered at 200 ppi, these 2000 pixels work out to 10 inches wide notionally. If considered at 20 ppi, they work out to 100 inches wide notionally. Both scenarios involve the same picture content, subdivided into the same 2000 pixels across the width.

And the 20ppi version could be later set to print at 10 inches wide instead of 100 inches wide, which would in effect "condense down" its notional 20ppi to an effective 200ppi, at which point these two output versions will become effectively identical - having always been actually identical so far as their content.