why is there a ppi setting in export?

[bwana]

if i have an image that is 3000x2000 pixels and i export it to a jpeg file, it's still pixels. i did this and as a test export to 50 ppi as well as 500 dpi. Both jpgs look the same and are the same size when opened in preview. I understand the concept of ppi when it comes to printing. I even understand it in photoshop-when you change the ppi, the actual size of the image in the print preview changes. But in Lightroom3, changing the ppi in the export dialog does nothing. in fact, changing the ppi in the print module does not change the print preview either.

[BartvanderWolf]

if i have an image that is 3000x2000 pixels and i export it to a jpeg file, it's still pixels. i did this and as a test export to 50 ppi as well as 500 dpi. Both jpgs look the same and are the same size when opened in preview. I understand the concept of ppi when it comes to printing. I even understand it in photoshop-when you change the ppi, the actual size of the image in the print preview changes. But in Lightroom3, changing the ppi in the export dialog does nothing. in fact, changing the ppi in the print module does not change the print preview either.

Hi,

PPI is just a placeholder for some anticipated output size, it has no effect on the image data itself. It's nothing more than the number of pixels in the image divided by the anticipated output size (in this case expressed in inches). Some applications use the hint to produce output at that suggested size, others just ignore it and ask for new directions as to the required size of the output.

Therefore the setting allows you to give directions for those applications that care.

Cheers,
Bart

[Slobodan Blagojevic]

Because you might want to print at a specific ppi, to match your printer's native resolution, e.g., 360/720 for Epson and 300/600 for Canon, thus retaining control over interpolation algorithm and sharpening.

[neile]

in Lightroom3, changing the ppi in the export dialog does nothing.

This statement isn't entierly true. If you turn on the checkmark next to Resize to Fit then the ppi setting at export *does* matter. Try two exports at the following settings:

Resize to width & height, width and height at 4x3 in inches, and ppi to 160
Resize to width & height, width and height at 4x3 in inches, and ppi to 300

You'll get very different file sizes as Lightroom will scale on export to get the right number of final pixels in each case (640x480 and 1200x900).

Neil

[bwana]

ah yes...the checkmark.  now i see.

well, if the ppi is ineffective with that checkbox unchecked, then it should have been disabled (grayed out). i guess it's an oversight of the lightroom team.

thank you.

[Schewe]

well, if the ppi is ineffective with that checkbox unchecked, then it should have been disabled (grayed out). i guess it's an oversight of the lightroom team.

Uh no...if you are outputing a JPEG for the web, then the final PPI is irrelevant since the web doesn't see the PPI tags. But there are other reasons for outputing a JPEG like for a photo lab. There, PPI comes into play and impacts the print size. So, it would be useful if your critique of the Lightroom team was just a bit more precise...they kinda know what they are doing...maybe you not so much? You should be just a bit more careful when critiquing others? Just saying...stuff doesn't just "happen" by accident.

[elied]

Ppi is also important for page layout in publishing applications. Having all your graphic elements at the same ppi (generally 300) can save a lot of tedious resizing.