Lightroom PPI

Photography by Rob Kelley 4:03pm, 18 April 2011
I need to get some prints done and the print shop have asked that I send them as 305PPI (pixels per inch), I can see in the Export options a PPI and I enter 305 and that saves it.
Just as a test I saved it again as 10 PPI expecting the image to be a lot smaller, but it isn't it looks exactly the same and same file size.

If I open the same images in Photoshop and resize them using 305 PPI and 10 PPI there is a huge difference in the saved images.

What am I missing here?
Godfrey DiGiorgi PRO 6 years ago
The pixels per inch setting means nothing unless it is linked to a particular set of output dimensions for the output image, not pixel dimensions.

For example, a 2000x3000 pixel image output at 305 ppi or at 10 ppi has exactly the same number of pixels in it ... what's different is that if a printer takes the 10 ppi image and honors that resolution setting, it will print to 300 inches by 200 inchs, where a 305 ppi image with the same number of pixels will print to 9.8 inches by 6.6 inches.

If you open the images and hold the output dimensions to be the same (for the above image, let's say 9x6 inches) and then export to 10 and 305 ppi respectively, you'll see the size difference as you see in doing a Photoshop resize operation. The 10 ppi image will be 90x60 pixels in size where the 305 ppi image will be 2745x1830 pixels in size.
ok, thats a lot to take in....
I'll have a play at what you suggested.

andyscamera 6 years ago
The real question is how large do you want to print it? If you want to print it at 10 inches across at 305 ppi, for example, you'll need an image that's 3050 pixels across.

I expect the printer is just specifying that ppi setting so that you provide images with enough pixels to make a good print. They're probably used to having people ask for a 6 inch print from a 600 pixel image, which doesn't work well.
James Youngman PRO 6 years ago
The photo as saved contains image data. The image data is divided up into pixels. A file has a certain number of pixels, as saved. This is a fundamental property of the image file.

PPI means very little. A PPI value is a physical characteristic of a printing process. It has nothing really to do with the data representation. Until a photo is printed a PPI value means nothing at all.

However, if the printer is giving you a PPI value, it is no doubt because their systems are set up to give the best results when the data they get matches the physical characteristics of their printing equipment.

If they're telling you 305ppi, it's my guess that what they really mean - even if they can't express it that way - is that to get a good 10"x10" print, the best input file should be 3050x3050 pixels, as andyscamera said.

Image export works best when the output image is sharpened only at the final stage, at its exported resolution (in our example at 3050x3050 pixels). However, Lightroom takes care of that for you.

I've been careful here to not say any thing like "printer resolution". The resolution of the printer is a separate issue. For example the printer I have here has a "resolution" of something like 2000ppi. But that relates to the accuracy with which it positions dots of pure ink, and has nothing to do with the best way to size and sharpen an image intended to be sent to it. (Also, printer marketing favours stating inflated ppi values even if they are measuring an aspect of the printer's functioning that is not very relevant to image quality).
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