You will want to spend some time preparing your photos for printing by cropping them to the appropriate aspect ratios for prints. But what if you, like me, want to use the same photos for a book, which uses a 4 x 3 aspect ratio? Follow these steps for a solution.
To prepare photos for printing:
Make a new album, and add the photos that you want to order prints of.
Switch to the album, and edit each photo as desired, other than cropping.
In organize mode, select all the photos () and choose Duplicate from the Photos menu () to make copies (see "Duplicating Photos" in Chapter 4, "Editing Photos," for details).
Select just the copies in your album (again, "Duplicating Photos" in Chapter 4, "Editing Photos," explains this).
Drag them to the Source pane to create a new album of just the copies (append the word "Prints" to its name to be clear), and then return to the previous album and delete the copies from it.
You now have two albums with identical photos; one with the originals, the other with copies. Use the original album to create your book, and crop the photos in the "Prints" album for ordering prints.
Go through the photos in the "Prints" album again, this time cropping each to the desired aspect ratio.
If you're printing photos in different sizes, manually group them by size in the album. That makes keeping track of them in the Order Prints window easier.
Now you're ready to order the prints (see the next page).
Pixels and Prints
You may have noticed that iPhoto reports how large your photo is in pixels in the Information pane. But how does that match up with print sizes that you order? You mostly don't have to care, since iPhoto displays a low-resolution warning icon when a photo doesn't have enough pixels to print well at the desired size. For reference, here are the pixel sizes at which iPhoto starts adding the warning icon, given both in terms of the 4 x 3 aspect ratio of uncropped photos and with the appropriate cropping for the size at which you want to print.
Note that these are the minimum recommended resolutions. The larger your photos and the more they exceed these minimums, the better the final quality.
For wallet-sized prints (about 2.4" x 3.4"), you need at least 450 x 337 pixels (when cropped to the above aspect ratio, keep the long side above 450 pixels).
For a 4" x 6" print, you need at least 900 x 675 pixels (when cropped to 4 x 6, keep the long side above 900 pixels).
For a 5" x 7" print, you need at least 1050 x 788 (when cropped to 5 x 7, keep the long side above 1050 pixels).
For an 8" x 10" print, you need at least 1600 x 1200 (when cropped to 8 x 10, keep the short side above 1200 pixels).
For 16" x 20" prints, you need at least 2132 x 1600 (when cropped to 16 x 20, keep the short side above 1600 pixels).
For 20" x 30" prints, you need at least 2400 x 1800 (when cropped to 2 x 3, keep the long side above 2400 pixels).