Modern image editing programs are packed with amazing adjustment tools that provide complex algorithms for radically altering the color, tone, and even content of an image. In spite of all this amazing technology, you'll often find that the single tool that can do the most to turn a bad image into a keeper is your crop tool. For the humble crop tool allows something quite amazing: with it you can re-compose your shot after you've taken it.
Composition is the art of arranging the elements of your scene so as to help guide the viewer's eye through your image. When you crop, the relationship of the elements in your scene changes, and as that relationship changes, the viewer reads your image in a different way.
Do a basic crop
Many times, you crop simply to remove extra image, not to perform a radical re-composition of your image.
Consider this simple snapshot portrait of two people. While the people look fine, there's an excessive amount of headroom above them. We don't really need to see the lamps or ceiling of the building, so a simple crop will tighten up the image, bring better attention to our subjects, and make them fill the frame more. If we print this image at a smaller size, such as 5-by-7, the people will end up larger in the final print, than if we leave it alone.
On occasion, people respond to cropping out background by saying, "But I wanted to show the room." Most of the time, if you want a picture of a room, or statue, or building, or background element of some kind, you should take a picture of that thing. If you want a portrait, that's a separate picture.
Don't go too small
When you crop, you reduce the total number of pixels in your image. If you ultimately intend to print your image, then you'll need to be careful about how much you crop. Crop too much, and you'll have an image that doesn't have enough resolution to make a good print.
If you're printing to an inkjet photo printer, you ideally want around 240 ppi (pixels per inch) at your given print size. You can probably get away with a resolution as low as 180 ppi, but much lower than that and you'll see a marked softening of your image. Fortunately, since most cameras these days pack a huge quantity of pixels, you can crop a lot before you get down to an image that's too small for viable printing.
Consider the aspect ratio
The aspect ratio is the ratio of an image's width to its height. Most digital SLRs have an aspect ratio of 3:2, while most point-and-shoots have a 4:3 aspect ratio. When you crop, you can choose to preserve the original aspect ratio, or crop freely to a different aspect ratio that works better for the image.
For example, a landscape image might be better-served if you crop it to a very wide, short aspect ratio, to accentuate its sweeping view. In fact, you'll often shoot such images with the idea of cropping them later, since it's not possible to get the crop that you want in camera.
There are times when you might need to fit a picture into a specific aspect ratio. For example, when you're printing an image to fit a store-bought frame, which probably conforms to a standard size, such as 4-by-6-inches (3:2 aspect ratio) or 5-by-7-inches (4:3 aspect ratio).
Most crop tools, including Photoshop's, iPhoto's, Lightroom's and Aperture's, allow you to constrain a crop to a particular aspect ratio or, in the case of Photoshop, to a specific size. When a crop tool is constrained, the crop rectangle that you drag out will automatically have the proportions that you specify.
After you've cropped to a specific aspect ratio, you still might need to resize the image to the specific length or width that you want (the other dimension will come automatically). In the case of Photoshop, if you specify particular dimensions, then your crop rectangle will be constrained to those proportions, and Photoshop will automatically set the size properties of your image so that you don't have to size it by hand after you crop.
Crop for better composition
When cropping, think about the same compositional ideas that you think about when shooting. Your goal is to help the viewer understand what the subject of your image is. Any extra information can be tossed aside.
Remember that you can trust your viewer to understand quite a bit. For example, consider a common picture of a figure on the beach. We don't really need a lot of that sky, water, and sand to understand that this woman is on a beach. If we crop a lot of that out, we bring more focus to her.
When cropping landscapes, you'll want to think carefully about where you want the horizon. In this image, I was struck by a sense of Shiprock sticking up out of the great plain upon which it sits. As shot, it's a little overpowered by sky, though.
I could crop with the horizon right in the center, which would work pretty well, but if I crop with the horizon a little lower, I reduce the amount of foreground that is visible, and increase the sense of the scale of the rock formation. At the same time, I've reduced the overbearing sky, and made Shiprock the dominant form on the horizon.
Like the rest of your photo skills, cropping is something you will continue to practice and improve at throughout the rest of your photo career. As you crop, pay attention to what does and doesn't work, and note if you have certain bad habits—such as leaving too much headroom in the frame, or not filling the frame with only what matters. Over time, you'll probably find that this understanding begins to impact your shooting, and that you come home with fewer images that require cropping.
[Macworld senior contributor Ben Long is the author of Complete Digital Photography, fifth edition (Charles River Media, 2009).]