Please check the below links for brief information about cropping in Lightroom CC 2015
Hope it helps.
Thank you for these references but I had reviewed them earlier and found that they did not address my specific need. I want to crop to a specific aspect ratio (3:2) for all of my crops, and also maintain a constant magnification. The easiest way to do this is to set a fixed cropping size, say 1500 x 1000 pixels (out of my full camera raw image of 5184 x 3456). I just don't see how to do this, and the references you provided don't address this situation. Can you advise further?
The simple truth is, you can't. The best you can do is to set the the Info Overlay to show the cropped pixel dimensions. If you then try to get the crop box to an exact size, you will soon drive yourself around the bend. The best you can do is to settle for a "near miss", like 1506x1004, and then at export resize to 1500x1000.
This is a two step process in Lightroom
1. Crop to desired aspect ratio
2. Export to the exact pixel dimensions you want
Actually, I think the way it is, it's perfect.
IMHO, when cropping one should not have to think about resolution and stuff - it's more an 'artistic' process of getting the best out of the photo.
In export, LR exports the best resolution (what is left after cropping) of your work or you may limit/set a desired pixel dimension.
The reason I would like an exact pixel dimension is that I am preparing frames to be used in a time-lapse sequence. I want to center my subject (in this case, the moon is my subject), and not have it "jitter" because the crop area changes from frame to frame (I will also need to ensure it is positioned accurately). I like the idea of doing it in camera raw via the xmp metadata so that I can later come back and make other adjustments. Once the images are exported, I fear that I would lose much of this flexibility. I thought Lightroom had Photoshop-like tools that would allow me to do these things. The Camera Raw editor doesn't have this crop ability either it seems.
Any suggestions for other camera raw metadata editors?
You might want to search this forum and/or search the internet for suggestions on how to use Lightroom for time-lapse photography.
You can set a crop frame aspect and cropped dimensions for one image, that you are happy with, and then Sync or Copy/Paste that onto all the other images.
Cropping happens in terms of a percentage shown of the overall image frame (visually), not in terms of any particular units (numerically) - not inches of course, but not pixels either (* see below). However if you sync (or copy/paste) a certain crop from one image onto another AND these two started out with the same overall pixel dimensions, then the cropped dimensions will correspondingly match too, by default.
Either the positioning of these crops needs to be constant, or else varying, within the various imported pictures. One could imagine two time-lapse projects: one where the moon travels across the sky while the scene stays still, and another where the moon is kept still in the frame while the scene wheels around. In the former case, if the camera is on a tripod then it is a simple matter to crop the whole sequence in one. In the latter case, the crop frame one synced between the frames in a common position, would then need to be manually moved or nudged suitably in each one (but without changing its size).
All of these identically sized crops would then naturally Export in a consistent manner, using the same Export settings (best not to impose any file size limit here, since that may in the event get implemented differently for one or another frame).
(*) If you import a fullsize original JPG, and crop it within Lightroom to show just someone's face, that will report particular cropped dimensions - but these are incidental, in that they do not themselves define the crop. Because: if you downsize a second copy of this original JPG and import that too, and then sync the Lightroom crop from the first picture onto the second version - that too will show just the face, in exactly the same way pictorially. But it will report different cropped dimensions; which will be equally incidental. The crop AFAICT uses coordinates defined as fractions of the total imported image width/height (when not cropped).