If you’re outputting photos for clients, chances are they’re going to want them in standard sizes so they can easily find frames to fit their photos. If that’s the case, you’ll find this technique handy, because it lets you crop any image to a predetermined size (like 5×7″, 8×10″, and so on).
Let’s say our image measures roughly 17×11″, and we want to crop it to be a perfect horizontal 10×8″. First, press the C key to get the Crop tool, and up in the Options Bar on the left, you’ll see Width and Height fields. Enter the size you want for the width, followed by the unit of measurement you want to use (e.g., “in” for inches, “px” for pixels, “cm” for centimeters, “mm” for millimeters, etc.). Next, press the Tab key to jump over to the Height field and enter your desired height, again followed by the unit of measurement.
Click within your photo with the Crop tool and drag out a cropping border. You’ll notice that as you drag, the border is constrained to a horizontal shape, and once you release the mouse button, no side points are visible—only corner points. Whatever size you make your border, the area within that border will become a 10×8″ photo.
After your cropping border is onscreen, you can reposition it by moving your cursor inside the border (your cursor will change to an arrow). You can now drag the border into place, or use the Arrow keys on your keyboard for more precise control. When it looks right to you, press Return (PC: Enter) to finalize your crop, and the area inside your cropping border will be 10×8″. (I made the rulers visible by pressing Command-R [PC: Ctrl-R], so you could see that the image measures exactly 10×8″.)