To find a single photo in a large image library, you generally have to remember when the photo was taken. But iPhoto ’09 has come up with a better way. With a little input from you, Apple’s photo-management program can now detect the faces within your photos, letting you quickly sift through your image library, searching by content rather by date.

When you first install iPhoto ’09, or upload a batch of new photos, the program goes to work detecting where the faces are in your pictures. The process can take a while for large libraries; in the meantime, you’re free to work on other things.

Once iPhoto is finished, you can start assigning names to detected faces. Select a picture of a person from your library and click on the Name icon. Click on the Unnamed field below a face and type in the name. As you type, iPhoto will suggest matches from Address Book or from other faces you’ve named. If there are multiple people in the image, you can tab between the name fields to identify everyone.

As you assign names, iPhoto learns from your input and attemps to put names to faces on its own. To help it along, click on Faces under Library in the Source list. You’ll get a corkboard with entries for every person you’ve identified. Double-click on a person’s name, and you will see two areas. The top includes photographs of the person you’ve identified. Beneath a grey bar are other photos of people that iPhoto has identified as being of the same person. Click on the Confirm Name button to begin the verification process. Every time you confirm a correct instance or reject a wrong guess, the application gets a little smarter.

Those are the basics; but there are plenty of hidden features and speedy shortcuts for making the process easier.

As you type a name, iPhoto will suggest people from Address Book or people you’ve previously identified.

Corkboard shortcuts
Waving the mouse over a picture on the corkboard and scrubbing through dozens of shots of Aunt Jane can be a lot of fun, but the effect is sometimes hard to control. For a more relaxed experience, simply park the mouse over the corkboard picture and use the right and left arrow keys to view one thumbnail at a time. If you want to see the entire composition rather than the just a close-up of the face, hold down the alt key.

Once you find a shot you really like, press the spacebar to set the shot as the key image for the corkboard. See a bad picture? As you’re scrubbing through images on the corkboard, you can go straight to Edit mode for a photo by Control-clicking and choosing Edit from the contextual pop-up menu.

Quick confirmation
You can confirm iPhoto’s guesses without actually switching to Confirm Name mode. Just drag thumbnails from below the grey line to the list of identified thumbnails to add them quickly to the collection.

Set it straight
Telling iPhoto when it guessed incorrectly is helpful, but even more useful is letting iPhoto know who the person in the photograph really is. With iPhoto 8.0.2 or later, you can Control-click on a thumbnail in Confirm Name mode to assign a different name to a person who’s been incorrectly named.

Quickly create smart albums
Once you’ve identified your friends and family, you can combine face recognition with smart albums for some serious organising horsepower.

To build a smart album that finds photos of your cousin Jimmy, for example, select File New Smart Folder. Set the pull-down menus to read Face Is Jimmy. To group Jimmy’s wife and kids in the same album, click on the plus sign (+) and create a similar condition for each person. Make sure you choose Match Any to get the collection you want. (If you choose Match All, you will only get images tagged with the entire family.)

But there’s an even faster way to create a smart album: simply drag the person’s thumbnail from the Faces corkboard to the Library pane. iPhoto will build a new smart album containing all photos tagged with that person’s name.

To build a smart album that searches for multiple people, C-click on each person and drag them to the Library pane. You can modify the album further by control-clicking on it and choosing Edit Smart Album.

Find unnamed faces
Want to find out quickly whom you’ve missed? To build a smart album that displays all of the faces in your library that have yet to be identified, set the menus to read Face Is Unnamed. Each time you identify one of the unnamed people in a photo, it will immediately disappear from the smart album.

iPhoto learns by its mistakes. If it incorrectly identifies two of your friends as being the same person, don’t just reject the mistaken image. Instead, let iPhone know who this is for future reference

Back up your faces database
Once you’ve invested the time to tag your photographs, we recommend protecting your work from database corruption by backing up your Faces database files.

Begin by finding your iPhoto Library folder (by default it’s in the Pictures folder). Control-click on the iPhoto Library file and choose Show Package Contents from the contextual pop-up menu. In the resulting Finder window, look for two files: face_blob.db and face.db. Select them both and copy them to another location. Now, if your Faces database becomes corrupted, you can pull out the corrupted files and replace them with the ones you backed up.

Keep in mind, however, that you can’t transfer these database files to other iPhoto libraries. And if your hard drive crashes, your backup won’t be of much use to you.

iPhoto ’09 is fairly temperamental when it comes to reconstituting its Faces corkboard; not even iPhoto’s Burn command preserves these connections. The only sure-fire method for preserving the entire library – including all of the work you’ve invested – is to copy the entire iPhoto library to a separate hard drive. But if the database inside iPhoto goes bonkers, you may be able to restore your sanity by replacing the corrupted database files.

By the way, if you ever want to start fresh with your Faces database, you can use this trick to remove the database files from your iPhoto Library: Quit iPhoto, remove face_blob.db and face.db from the package contents, and restart the application. iPhoto will start anew with its face-detection process – useful if you discover you have a lot of wrong identities, or you regret adding pet names.