Is a common misconception that digital images have a DPI setting embedded in them. This is not true. The only value that counts in the digital image is the pixel dimensions. The only value a DPI (or really PPI) setting has is to enable a program such as Photoshop or Photoshop Elements to calculate how large the image will be at a given setting. Regardless of what you set it to in one of those programs the pixel dimensions do not change. And changing that setting will have no impact on the quality of the image or a print from that image.
DPI values refer to dots per inch printed by a printer. PPI refers to pixels per inch in an image. The size of the print is calculated by dividing the number of pixels in the image by the pixels per inch you want printed. And that little math exercise will give you an indication of how large the print can be. But programs like Lightroom, as well as many commercial printing services, are capable of interpolating and printing larger images from smaller files. So you need to do some experimenting with different print settings to discover what you are willing to tolerate in your prints.
All images are imported into Lightroom in their original aspect ratio. The digital SLRs that I use take photos in the 2:3 aspect ratio. You can choose other aspect ratios and create custom ratios using the crop tool. And if you want to be able to identify different aspect ratios I suppose you could use color labels or star ratings to indicate.
Jim's answer is not to the point.
I may have any kind of images from outside in Lightroom, for instance panoramas. And i would like to see which print size will be possible with which resolution - like, if i print this 90 centimeters wide, will it be above 180 dpi or not - and i need the aspect ratio for instance to order prints.
Still hope to get more to-the-point info. Thanks!
Jim's answer is right on the point: Lr doesn't provide this information because it's irrelevant in an Lr context.
To find out what size an image will print at a given ppi, you will have to do some math.
Select the image in the library, and you will see the pixel dimensions in the Metadata panel.
Say you want to print this (cropped) image at 180 ppi.
Divide 6956 with 180, and you'll get 38.6 inches.
If you want centimeters, multily 38.6 with 2.54, which is 98 cm.
And 180 ppi equals 70.87 ppcm, so 6956:70.87 also equals 98 cm.
Per, thanks for explaining including a screenshot.
I'd love my computer to do this math that you explain automatically. I also know how to calculate it.
Using the File Size box in Photoshop or Photoshop Elements does this math automatically and allows me to check a variety of print scenarios and also let's me check on the aspect ratio (i don't have only images of one kind).
It seems LR isn't that goo to get down to the technical information.
I think the reason LR does not have the Image size dialog like in PS, is that it is not a pixel editor, and never touches the original.
It's only during exporting and printing that you get the option to resample an image, and the resampling will only be applied to the exported/printed file, not to the original.
I suppose it would be possible to implement a PS-like Image size dialog in the export and print modules, but the developers have chosen not to.