# I thought that I understood about dpi



## edgley (Nov 15, 2010)

But it seems I don't!
I use LR for my photos, but export certain ones to iPhoto.I have been exporting at HD resolutions, at 1''DPI, as I thought the largest I would be displaying would be on a TV.
I have now just discovered the Order Print feature in iPhoto, and want to make the most of it.Currently the largest they do is 12x8, but I want to export my photos for a max size of A3, and hope they offer printing of that size at some point.
So, I need to re-export my pictures at 3''DPI thought I. Did some reading, and came across the view that the DPI chosen for exporting is irreverent. So I exported a JPG from a NEF at 1'' and 3'' DPI, and the file size is the same.
Would the simplest thing be just to re-export the photos as full sized JPGs? As the file size is 5mb a pic, its not too large.
Thanks,Simon


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## dj_paige (Nov 15, 2010)

First, its PPI, not DPI.

The important issue is how many pixels are available in the photo, not the PPI number that is selected for the export. If you want to print at 12x8, and a quality print requires 3'' pixels per inch, your photo needs to be in the 12x8 (or 3x2) aspect ratio, and so the exported file needs to be 36''x24'' (or larger). It doesn't matter if the exported file is selected to be 1'' PPI or 3'' PPI or any other number.


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## djaef (Nov 15, 2010)

It's like Paige says. PPI is pixels per inch and is 'resolution'. At 3''ppi an image file that is 24''px x 36''px will print at 8 x 12". (do the math) If you change the reslolution without changing the number of pixels, then the print will be a different size. So at 24'ppi, that same image will print as a 1' x 15" print. DPI is a term for dots per inch which is how many dots an inkjet printer puts down on the paper and not relevant to the discussion at all. 

The important thing is the pixels. x number of pixels is x number of pixels regardless of resolution (ppi). If I were you I'd export your full size jpgs and if they are 54'' x 36''px, then you can rest assured they'll print without interpolation @ 12 x 18" (about A3).

Complex topic in many ways, but in other ways it's very simple. Hope that helps.


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## edgley (Nov 15, 2010)

Thanks, I will just export at full size then; 2848x4288 (which will get me 9x14)


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## dj_paige (Nov 16, 2010)

But ... 2848x4288 is not 9x14 aspect ratio!

If you want a certain size print, you have to first crop to that aspect ratio, then export (and have enough pixels)


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## edgley (Nov 21, 2010)

Okay, so I do need to export full size, then make copies in the right crop ratio, depending on what size I need to print.


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## erro (Nov 21, 2010)

No, you crop in LR to the desired aspect ratio. Then you export without resizing. Now you have a JPG with the maximum number of pixels.


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