Previews are one of the most misunderstood and notoriously undocumented things in Aperture. A couple of weeks ago I wrote a post about what you can and cannot do when you have referenced masters that are off-line, as in all of your masters are on external drives and your library with previews happens to be on your laptop while you are away. Previews make some of the functionality that I mentioned possible. Understanding how to manage previews within Aperture is essential to effectively managing storage, performance, and functionality. Unfortunately most of the documentation supplied with Aperture focuses on Aperture preferences and really does not connect the dots on the rest of the preview picture.
Let's start at the beginning, in the preferences window preview tab. Here there are a couple of options that are pretty self explanatory. The options that are important are "New projects automatically generate previews", "Share previews with iLife and iWork", and "Limit preview size".
The "New projects" option has nothing to do with turning previews on and off for projects already in Aperture, it merely does exactly what it says - sets the preview options from projects created going forward (after changing it) to either on by defualt or off by default at a project by project level. You can turn it on and off until you are blue in the face and it will do nothing to previews already out there. More on this in a second. "Share previews" pretty much makes the OS X media browser connect with images within Aperture that have previews, there is not any great reason that I can think of to turn this off, if you know one please share it. When turned on you can get at the JPG previews from pretty much any other Application running in OS X, not just iLife and iWork. "Limit preview size" in combination with the quality setting is pretty important depending on your intended use of the previews as well as managing the size of the actual Aperture Library. I happen to use a 1920 pixel limit because that happens to be the full resolution of HD flat panels and projectors that I routinely display my images on when traveling with my laptop - very convienient without having to resort to lugging around a ton of RAW files or PSDs with me that I am not currently working on.
Moving on to the next peice of the preview puzzle take any selection images from any view, incuding the all photos view, in both the right click context menu or the image menu take a look at "Update preview(s)" and "Delete preview(s)". These will either create JPG preivew images or get rid of them respectively with no regard to the "New projects…" option setting in preferences. There is also a shortcut for either generating or deleteing previews for an entire project. You can get to this shortcut by looking at the context menu for a project in the inspector.Â These two simple menu items in combination with the next (and probably the most overlooked preview related function) are the keys to beating Aperture previews into submission.
The next and probably most important piece of preview generation functionality happens to be hidden in a place that few rarely venture. Take a look at the drop down menu up in the corner of the project pane in the inspector. Down at the bottom of the menu there is the magical "Maintain previews for this project" option. If you select this Aperture will generate previews for every image verison in the project. Aperture will also generate previews every time you make an adjustment to an image or lift and stamp a bunch of adjustments. This is the option that gets set on new projects if you check "New projects automatically generate previews" in the global Aperture preferences. Many Aperture users are driven to the brink of insanity by trying to turn previews off by turning the "New projects…" option in preferences on and off to no avail. If you are on a laptop or other machine that doesn't have unlimited resources you may want to turn this off while you are working on your RAW images it will free up a lot of computing juice for the task at hand. You can even set this on or off for every project in your library by clicking on the "Library" in the project inspector - pretty cool huh.
So what to do with all this new found preview goodness. Let's pretend that your Aperture previews are in a state of chaos and you want to start over from scratch. Easy - Turn off the "New projects…" option in Aperture preferences, click on the Library item - use the gear menu to un-check "Maintain previews for all projects", and last but not least go to the all photos view - select all the images - use delete previews. Now everything is clean and fresh, just like new. Now using your new found tools you can choose what projects exactly you want to maintian previews for or control them down to the image level and don't let Aperture generate them for you - I personally hate having Aperture startup processes to generate previews while I am trying to do image adjustments - especially on my laptop. Now you can generate previews for just certain albums or smart albums if you wish. You can do pretty crazy OCD kind of things as well by generating previews using different "Limit preview size" settings but there is no great way to automatically do this.
One word of warning. If previews are off Aperture will slow down when doing certain things to certain images that have no preview because it may have to render what you are looking at from the RAW master but that is what "Quick Preview" is for. The point of this article is not that previews are bad or that having Aperture automatically deal with them is bad, it is that knowing how to manage them for the task at hand is good. In reality I get rid of previews for old projects and turn off the manage previews for project - images that I care about show up in smart albums that i manually generate previews for. I also turn of the automatic generation of previews for a project while I am actively working on that project in terms of adjustments and/or using an external editor - especially on my laptop.
As usual feel free to question or contribute.