If you are planning to print a photo or display it on your Desktop, you should crop it using an appropriate aspect ratio. (See "Understanding Aspect Ratios" in Appendix A, "Deep Background," for details.) Even if you don't plan to print a photo, cropping extraneous detail can improve an image.
To crop a photo:
Select the desired portion of the image, using a constrain setting if you plan to use the image for display or printing.
Click the Crop button in the edit pane or the image-editing window's toolbar, or -click and choose Crop from the contextual menu.
iPhoto deletes the fogged area of the picture, leaving just what you had selected (Figure 4.21 and Figure 4.22).
Figure 4.21. To crop an image, select the desired portion and then click the Crop button. Here I've cropped out all the irrelevant background around the insect.
Figure 4.22. As you can see, cropping this image improves it immensely.
Pressing and releasing no longer toggles between the "before" and "after" views when you're cropping in iPhoto.
If your selection rectangle is very close to one of the standard aspect ratios, it's best to use the standard aspect ratio in case you want to print the image later.
When you crop a photo, you remove pixels from it. So if you crop a 1600 x 1200 pixel photo (1,920,000 pixels) down to 1200 x 900 (1,080,000 pixels), you've removed almost half the image. Thus, if you print the original and the cropped version at the same size, the original will be of a much higher quality. Heavy cropping is one reason why iPhoto shows a low-resolution warning icon when you're creating books or prints. For details, see "Understanding Resolution" in Appendix A, "Deep Background."
Adjust Shooting Style
When taking pictures, you usually want to fill the frame with the scene, but if you plan to order prints of all your photos, you might want to include a little extra space on the edges to allow for cropping to a print aspect ratio. See Appendix B, "Taking Better Photos," for more tips!