Stacks feature heavily in the promotional material for Aperture, so I leaped upon them as soon as I had the application installed on my hard drive. The first thing I did was to autostack on import. That was problematic because I had to undo many of the stacks later. I had found that the rating system and the stacking system interact in some surprising ways, sometimes to the point of having images vanish in front of my eyes.

The other thing I did was to stack things (manually) that should never have been stacked. Then I could no longer see or find the stacked images that were "hidden" behind the pick. Another way to "lose" images.

The real reason for stacking is to simplify the management and display of images 1) where a number of interchangeable alternatives exist and 2) to defer image selection to the point of context.

So don't use stacks to group images together for any other reason. They are for fine-tuning, not for grouping arbitrary images. Use Albums and Brown Folders to make arbitrary groups. If you shoot sports, for instance, where only one image of a burst shows the impact of one thing against another, then stacks are not for you. That one image is not interchangeable with the others shot at the same time. But if you have four great pictures of the same person undergoing an impact during the same game, then do stack them. In that way you can defer your selection to the point at which the images are used. The images are interchangeable in that context.

Lets look at stacks in action. To use stacks, first import some images:

Make a selection:

Hit command K, and voila! a stack is born:

Look carefully at those three graphics. The first has six separate images. The second has six separate images with a thick line around the primary selection and thin lines around the rest. When you create the stack, it is that primary selection that goes to the left and is determined to be the "pick" of the stack. That's handy and will save you some effort.

The third graphic shows the stack created and open. The background is now dark around the six images in the stack. The 6 on the pick shows how many members there are; clicking on the 6 or hitting shift S gives this, a closed stack:

If you open a stack and select a member it looks like this:

Doing a Select Parent (Edit menu, option shift E) will create yet another display that shows the pick selected and the whole stack selected:

Within a stack the images can have any order; and contrary to many expectations, the order has nothing to do with the rating given to each member:

If you close a stack with rated images then the pick is the rating for the whole stack. The others are ignored:

This means that if you have images filtered to, say, two stars or better, and you change the pick to one star or promote another stack image that already has one star to the current pick, then the whole stack disappears without warning. Images that are in the stack are "protected" by the rating of the pick. So you can have a stack with lots of rejects, but the filtering doesn't make them invisible if it is set to one star or better.

And this is a problem if your images are autostacked on import. All those rejects don't disappear with filtering. So a better workflow is to find the rejects first and delete them, then autostack. But then autostacking is not as effective because there are gaps in the images.

You can still find images that are rejected if they are in stacks. Look at this Smart Album:

At the bottom I have checked Ignore stack groupings. That allows the filter to look inside the stacks. So there is a wholesale way of deleting the rejects where ever they may be.