The Amazing, Incredible Crop Tool
The basics of the Crop Tool in Photoshop are fairly straightforward. After dragging a rectangle over your image, you press enter and the image is cropped as you specified. Resolution doesn't change; the dimensions of your image just get smaller and whatever no longer fits is gone. But the Crop Tool is so much more versatile than just this that it seemed like a good idea to spend some time on it this week.
If you want to create a specific sized image, you can fill in the Width and Height in the options bar and the cropping rectangle will be bound to that aspect ratio as you drag it. If you want a specific Resolution for your result, you can specify that too and your image will be resampled using the default Interpolation method specified in your Photoshop preferences. If you only fill in some of these values, Photoshop will try to avoid changing the remainder. The Crop Tool will consider the resolution to be more important than height or width so if it must choose between altering dimensions and resolution, it will be the dimensions that get changed. Height and width can be entered in inches, centimeters, pixels or any units that Photoshop recognizes. Resolution can be entered in either pixels per inch or per centimeter.
After you finish dragging the crop box and let go of the mouse, the part that will get cut off becomes masked to help you visualize the result. By default, this will be shown as 75% opacity black overlaying the image but you can change this in the Crop Tool options bar. Or, if you don't want to mask this area at all, uncheck the Shield option and the cropping rectangle will appear much the way the standard rectangular marquee tool does.
If you want to tweak the crop area at this point you can grab one of the handles on the cropping rectangle and move it in or out as needed. Be aware that if you have already entered both height and width, the aspect ratio of the crop box will be fixed and you will only be able to adjust the corner handles. If you click inside the crop box you can move the entire thing around the image to where you want it.
What if you want to crop at an angle? No problem. After dragging a cropping rectangle, hold your mouse just outside of one of the corners and the cursor will change to a rotation icon allowing you to twist the box around its center. Or, if you'd prefer to rotate around some other point, just click on the center marker and drag it where you want it first.
As if that weren't enough, you can also correct perspective while cropping. First, drag a cropping box over your image approximately where you want it. Then click the Perspective checkbox on the right-hand end of the options bar. This will then permit you to drag each handle on the crop box independently. If you line each edge of the box up with something that should be vertical or horizontal, you can then use the handle in the middle of each box edge to pull that edge outward in order to size the box as desired. When ready, press Enter to commit your crop as you normally would. As a bit of fine print, the Perspective correction option will be disabled if your document contains any type layers.
Photoshop comes with a small selection of presets for cropping that you can access by clicking on the down arrow to the right of the Crop icon in the options bar. If you often need to create images in sizes not listed, you can add your own choices to this list. To do so, first fill select the Crop Tool and fill in the options as desired. Then click on the down arrow to get the existing Crop Tool presets. Finally, click on the small triangle in the upper right corner of the resulting menu and enter a name for your preset. Presets live in the file called "Crop and Marquee.tpl" in your Presets\Tools folder in the Photoshop program directory if you want to copy them from one machine to another.
The Crop Tool can also crop one image to the size of another. With the image you want to base your crop on in front, click on the "Front Image" button in the Crop Tool options bar. Then click on the image you actually want to crop to bring it to the front and have at it. If you only want some of the values the same as your source image, just change the others as needed before dragging the cropping rectangle over your image.
If you accidentally enter the height and width values in the wrong boxes, there's a button that looks like two arrows pointing back and forth between the two input boxes that allows you to easily swap these values. There's even a "Clear" button at the far end of the options bar to quickly reset all cropping options and blank out the height, width and resolution boxes. If you start making a crop and change your mind, just press the Escape key to cancel out completely.
Normally when you crop, the portion that gets cut off is thrown away completely. If you'd prefer though, the Crop Tool has yet another trick up its sleeve that will let you hide the cropped portion rather than actually cutting it off. To access it, first create your cropping rectangle, and then toggle the Cropped Area option on the left-hand end of the options bar from "Delete" to "Hide." After doing so, your file will actually be larger than the document size. The hidden portion won't be visible (that makes sense, doesn't it), nor will it print. You can later use Image >> Canvas Size to expand the canvas and expose the hidden portion. Or, if you drag or nudge a layer it will fill in with information from the hidden area. Hiding is not available if your document has only a Background layer.
There are other ways to crop in Photoshop, but the true Swiss Army knife of cropping is the appropriately named Crop Tool. Learning to use it can save you time as it combines the functions that would normally need to be done one at a time using several different tools.