Why is there a "ppi" box in Lightroom Export Dialog?

[PhotoEcosse]

This might seem like a pretty inane question - until you actually think about it. I am kind of hoping that Jeff Schewe might be able to answer it given his involvement with Adobe and Lightroom.

When I export an image file from Lightroom as a Jpeg (or a Tiff or anything else for that matter), in the "Image Sizing" section I can enter not only the image dimensions in pixels but also a "Resolution" in ppi. I am at a loss to know why, if I specify my image size in pixels, this ppi figure has any significance. Quite a few people have suggested it is a necessary parameter for printing, but I just don't see that.

To test my understanding of this, I did a wee experiment. I selected a Raw file in Lightroom and then exported two Jpegs. Both Jpegs were specified at 2000 pixels on the longest edge and all other parameters in the Export dialog were identical - except that I entered 10 in the ppi box for one and 1000 in the ppi box for the other.

Both files were identical in size (in Kb) and both displayed identically on my monitor and both printed identically on my printer.

I can see that if I specified the image size in inches or centimetres, then a ppi value would allow those physical dimensions to be converted to pixels. But when I specify the image size in pixels to start with, it seems to me that any figure entered in that ppi box has no bearing whatsoever on the file I export or how it displays or prints. Any conversion of pixels to inches when diplaying or printing seems to be done by the display driver or printer driver and is not related to the ppi I entered in that Export dialog box in Lightroom.

What am I missing?

[Rand47]

The difference is that each of the exported files is at a specific "distribution" of the total pixels, which in a sense specifies a print size.  This, of course can be further modified when actually printing, but if you were to leave the exported pixel distribution in place, it would drive what size paper you would need to use - and at some point (somewhere around < 180 PPI) the quality of that print would start falling apart because the "pixels are spread too thin" (so to speak).  

When you state that prints were identical it is because you redistributed the total pixels to be "the same again" when you printed prints of the same size from the two files that had the same total pixels.  

Rand

[digitaldog]

Quite a few people have suggested it is a necessary parameter for printing, but I just don't see that.

Start here:

http://www.digitaldog.net/files/Resolution.pdf

The resolution 'tag' is rather meaningless as it can be divided up using the more important stat's, the pixels along the two axis of the image. If you have 1000 pixels, that could be 1 inch if you output at 1000 dots per inch or 10 inches if you divide the pixels such you get 100 dots per inch. Yet the file in both cases still has 1000 pixels along one axis. These files have no size other than the space they take up on a disk (which can vary, just save one as a TIFF, the other as a JPEG). Only until you divide up the pixesl you have can you predict what size the print could be!

So the tag (and that's all it is, a means of calculating how large a print could be with the pixles you have) can be anything you wish: 72, 385, 300, 1000.

it seems to me that any figure entered in that ppi box has no bearing whatsoever on the file I export or how it displays or prints

Correct. You can alter the value without adding or removing pixels and alter the size of the output.

[dieter268]

I don't know how the situation at the american side of the atlantic is, but here in europe there are still a lot of agencies and printers that require a specific "dpi" setting for a picture delivered to them. Of course this is without knowing their stuff properly, that setting is meaningless without a specified size to print, but well, it's much easier to say: "I need that picture in 300 "dpi" then to think about what they really need...
So, if your client wants 300 "dpi" write that value in and he or she will be happy

Dieter

[digitaldog]

I don't know how the situation at the american side of the atlantic is, but here in europe there are still a lot of agencies and printers that require a specific "dpi" setting for a picture delivered to them.

And that's fine, IF they supply the other part of the equation to allow the request make sense. It's like asking for RGB. RGB what? Adobe RGB is a complete request. RGB or DPI alone are not. You're dealing with someone who doesn't understand basic imaging 101.

[dieter268]

And that's fine, IF they supply the other part of the equation to allow the request make sense. It's like asking for RGB. RGB what? Adobe RGB is a complete request. RGB or DPI alone are not. You're dealing with someone who doesn't understand basic imaging 101.

Well, I luckily as a amateur I don't have to deal with such people, but if you follow german photography forums you will find that such lack of basic understanding is still a common thing here. Sad but true ...

[Silver Halide UK]

As the OP suggests, the ppi box only has a use if you want to enter an image size in inches and have Lightroom convert that to pixels. The image file does not have any  size in inches or resolution in ppi. It simply has a size in pixels such as 4000 x 3000.

[PhotoEcosse]

As the OP suggests, the ppi box only has a use if you want to enter an image size in inches and have Lightroom convert that to pixels. The image file does not have any  size in inches or resolution in ppi. It simply has a size in pixels such as 4000 x 3000.

I am sure that is correct.

But why does my camera club want images for projection to be "not more than 1400 x 1050 pixels and saved at 72ppi"?

And I have just sent some prints to the exhibition selection stage of a Royal Photographic Society competition where I had to accompany the prints with a CD containing "Tiff files at 300ppi".

So you can understand why there is confusion around this issue.

[Bart_van_der_Wolf]

I am sure that is correct.

But why does my camera club want images for projection to be "not more than 1400 x 1050 pixels and saved at 72ppi"?

Why? Beats me (you should ask them!), because it makes no sense (unless they use a stupid output modality that relies on the PPI placeholder to drive a certain output size). More commonly the image is either output pixel for pixel, such as on a display, or the output size is an input, which then happens to result in a certain PPI. In both cases the PPI is useless information that will be ignored.

And I have just sent some prints to the exhibition selection stage of a Royal Photographic Society competition where I had to accompany the prints with a CD containing "Tiff files at 300ppi".

Again, useless unless they use a 300 PPI output modality with a simple output driver that then tells them how large the physical output becomes (1 inch for each 300 pixels input size).

PPI can be used as a clue as to how large the output is intended to become, but may make little sense if the clue is not used to calculate the intended output size.

Cheers,
Bart

[digitaldog]

But why does my camera club want images for projection to be "not more than 1400 x 1050 pixels and saved at 72ppi"?

Because they are confused, maybe need someone to come lecture them about resolution 

1400x1050 at (any value you fill for PPI) is the same file. The PPI is meaningless. Put anything in there you wish, the image remains the same, will project the same. Goes back to that old, incorrect urban myth that for screen output, you must have a resolution of 72ppi (it's some standard or necessity which isn't the case). Some to this day still believe all displays output 72ppi. Some believe all you need is sRGB. Some believe a histogram tells you if your image is good or not. Some believe ETTR is over exposing etc. Far too many of these silly imaging misconceptions still around.

[dieter268]

But why does my camera club want images for projection to be "not more than 1400 x 1050 pixels and saved at 72ppi"?
And I have just sent some prints to the exhibition selection stage of a Royal Photographic Society competition where I had to accompany the prints with a CD containing "Tiff files at 300ppi".

You have 2 possible ways to go: Either you try to convince them to learn thier stuff about resolution, printsize etc. (could be a hard and stony path if they didn't learn it now ...), or you just type in the ppi value they want and save yourself some nerves and energy.

Dieter

[Ellis Vener]

"But why does my camera club want images for projection to be "not more than 1400 x 1050 pixels and saved at 72ppi"?"

Maybe those are the specs for the projection system they use?

[kaelaria]

Because it's a useless metadata field probably left in through the versions based on some weird internal discussion.  Ignore it.

[PhotoEcosse]

I am perhaps a little more surprised to discover that, as far as my eye can tell, the "ppi" box in Lightroom's Print module also appears to have no effect on the print quality. That seems to be wholly determined by whether I select "Speed", "Quality", or "Max Quality" in the Epson Print Setup properties. (I guess that depends to some extent, also, upon my file having sufficient pixels to feed the chosen quality setting.)

However, thanks to those who replied to my original question, confirming that an image file such as a Jpeg or a Tiff has no absolute size in inches or any "native" resolution in ppi. It merely has dimensions in pixels that are then converted, by other software, to physical dimensions and resolution at the point of output to either screen or printer.

[digitaldog]

I am perhaps a little more surprised to discover that, as far as my eye can tell, the "ppi" box in Lightroom's Print module also appears to have no effect on the print quality.

It can, depending on the image and how you've setup the module. Jeff Schewer has written about this, I believe it's covered in one or more video tutorials offered here.

[D Fosse]

when I specify the image size in pixels to start with, it seems to me that any figure entered in that ppi box has no bearing whatsoever on the file I export or how it displays or prints.

For me, working mostly for offset print, an image usually ends up in an InDesign file. There the size is set and an effective ppi calculated from the pixels in the file. What the actual ppi is, as set in the file metadata, is completely irrelevant.

Similarly, in a printer driver (and Lr print module) the image is usually scaled according to paper size, so the ppi setting is simply ignored. Or so it seems to me (I rarely print myself).

In principle, it's a simple equation: resolution = pixels / size. And like any equation, it can be inverted and turned upside down, so that you can start with any two values and calculate the third. I just can't think of any situation where it actually matters...

For screen it's moot. There you already have a pixel grid, the screen pixels, so it's just a matter of mapping image pixels to screen pixels.

EDIT: I once got a request for banner size "at 300 dpi" (sic). For fun I calculated the pixel count for such a monster, and it clocked in at some 35 000 pixels long edge. I still had a hard time convincing them that 300 ppi wasn't really what they needed...

[PhotoEcosse]

Many thanks to all who contributed.

Your input has been greatly appreciated.