I'm trying hard not to be curmudgeonly about this. Lightroom is a different tool and I expect differences from Aperture. I'll just come out and say it. Lightroom's stacks are very, very silly.

What Are Stacks?

In case you're not familiar... what are stacks? Stacks group images into a set of (assumedly) related images. Each stack has a stack pick. I use stacks to focus my attention on the better shots and they help keep my collections less cluttered. Stacks are also handy to group an original image with a version edited in a plug-in.

Both Aperture and Lightroom support stacks. In Aperture, once a set of images are stacked they are stacked everywhere. If you stack images in a project, the stacks reflect in any albums or smart albums containing those images. And the reverse. Stack images in an album, they are also stacked in the project.

Your Boy Is Diff'rent, Ms. Gump

Lightroom is... different. As I migrate projects from Aperture into Lightroom, I am taking care to maintain my stacks (and associate images with "cooked" adjustments with their originals). I've gone to some trouble to do so (details in my free Migrating From Aperture To Lightroom book). After importing photos, I rebuild the stacks in the "Previous Import" group in the Catalog panel. That created stacks in the folder where my photos were stored. I was very surprised when I created a Collection of all the photos, and the stack information was not in the Collection. 

Lightroom stacks are a Folder-based concept. If you create a stack in a Folder, it's not reflected in a Collection. I have found you can create stacks "localized" to whatever container they were created in. But stacks in one container are entirely separate from another. Create a stack in a Collection and they are only in the collection. Stacks are disjoint.

Another kicker is stacks created in Collections are not visible in Collection Sets. That one just blows my mind. Especially since Lightroom modules are designed around the concept of a Collection. Stacks are disjoint to say the least.

My Approach

My "workaround" is culling images in the Folder first (standard practice), create a Collection of the culled images, and do my stacking in the Collection. Long term, I will work in Collections not Folders, so that's where I'll spend the time organizing. My Folders will just be a stream of images (there's only so much stacking a guy can do :).

One other quirk... round trips to external plug-ins. Lightroom will automatically stack an image edited with a plug-in - which is great. However, what I've experienced is the auto-stacking is done in different places depending on how you launch the plug-in. Launching plug-ins from Photo > Edit In… give me my desired stacking behavior. The round-tripped image is stacked with my original in both the Collection and the Folder. Nice.

If I launch a plug-in from File > Edit In… the auto-stack happens in the Folder, not the Collection. Worse, the new image file isn't added to the Collection. Ouch. UPDATE: Jim (see comments) pointed out the auto-stack in the Folder also happens when launching plug-ins through Plugin Extras.

An Alternative Approach

If you have a 1-to-1 relationship of Folders to Collections, you can create your stacks in the Folder first. Then, drag and drop the Folder into the Collections area. A new collection is created with the same name as your folder and the stacks are maintained. This doesn't work for me. I have collections that span folders.

Adobe, Are You Listening?

Adobe… if you would do two things, it would make stacks a lot less clunky.

1. Be consistent in how externally edited images are auto-stacked.
2. Reflect stacks created in Collections in Collection Sets.

I can workaround #1. I'd really like #2. Want extra credit? Make stacks consistent across all containers, whether they be created in a Folder, Collection or Collection Set. And add a toggle to display only Stack Picks in Smart Collections.

And for you Lightroom users out there… I'd be thrilled to be wrong about how stacks work. Are there settings or preferences I haven't found that make this "better"? If I can get auto-stacking of externally edited photos reflecting in my collections, I'll be much happier.