But before we get started, it's important to know that Lightroom uses what's known as non-destructive editing. What that means is that your original images are safe. Any changes you make in Lightroom, including cropping or straightening, doesn't actually change the original image. Rather, editing in Lightroom it creates a set of instructions to be applied when you export the image to create a new version. So even though you can see the changes in Lightroom, rest assured that you're not damaging your original image.
If you want to be extra cautious, create a virtual copy of throte image you want to work on (right click on the thumbnail and choose Create Virtual Copy). You can then work on that copy without affecting the master copy.
Another thing that's important to know from the get-go is that cropping is not the same thing as resizing. I have a separate post explaining how to resize photos in Lightroom.
For the example below, I'm going to use a photo of The Townley Caryatid, a Roman statue that is on display just inside the main entrance of the British Museum in London. It was shot on a Nikon using Nikon's RAW format, NEH. But whether you're using RAW format photos, TIFF, PSD, or JPG, the process is the same.
Accessing the Crop Tool
All of the cropping and straightening is done in Lightroom's Develop Module. If you're in the Library Module or something else, just use the module menu at top right or simply press the D key.
Once in the Develop Mode, click on the Crop icon, which is the rectangle with dashes.
When you click on the Crop tool, two things will happen. In the image panel, overlaying the image, you'll get new guide lines and borders. In the tool panel at right, the Crop & Straighten options will drop down.
Here's a closeup of the Crop & Straighten options.
The quickest way to crop is to drag the guidelines to where you want them. You can either drag the corners or the sides (including top and bottom).
This is the quick and rough way to do it, but because you get to immediately visualize the results, it can also often be all you need to do.
If that gives you the result you're going for, great, but there's more you can do to get it just so.
Straightening an Image
You'll notice that the version above isn't straight. All the horizontal lines in the frame make that pretty obvious. There are a couple of ways you can straighten a photo in Lightroom.
The quick and rough way is to move your cursor to a corner of the crop guides, click and rotate. I've done an exaggerated version here.
I also have a more comprehensive guide to rotating images in Lightroom here.
Bonus Tip: Flipping Landscape to Portrait (or vice versa)
In this mode, you can also change the orientation of the crop. If you start with a landscape (horizontal) image, you can crop to a portrait (vertical) image. Again, grab a corner and drag to the diagonal opposite corner. The crop guides will switch orientation. Like this.
Straightening by dragging and the crop guides is quick and easy and works in many cases, but there's a much more precise way to do it: using the straighten tool. You'll find that with the icon that looks like a ruler:
You can also use the slider or even input precise numbers in degrees if you like. There's no really right way to do it--whatever gets you the result you're looking for. But in most cases, I find the ruler tool works best for me because I can line up elements in the frame that I want to be straight.
Changing the Aspect Ratio
In the examples above, I'm using the same aspect ratio as the original photo was shot at, which is 3:2. When I drag the crop guides, the cropped shape remains the same--only the size and position changes. But you might want to change the aspect ratio. To toggle that, use the padlock icon.
When the padlock is closed, it locks the aspect ratio. If you want to toggle it, just click on the padlock. The difference between locked and unlocked isn't as obvious as it could be, but here's what the two states look like. Another way to tell is that if you can't change the shape of the crop, it's locked.
You can also specify a particular aspect ratio by clicking on the dropdown menu next to the lock icon. You can also enter a custom aspect ratio if you're trying to make it fit for a particular purpose.
In this example, I'm changing it to a square aspect ratio. Once I choose 1x1 from the dropdown menu, the padlock will lock again because you've specified a particular aspect ratio.
But suppose I want an entirely freeform aspect ratio. Examples might be creating a wide, narrow banner for the header of a website. To do this, make sure the padlock is unlocked and then just drag the guidelines.
If you want even more control over your crops, Lightroom has some very handy crop overlays that serve as guides. I have a post on Lightroom's crop overlays and how to use them here.
And that's about it for basic cropping and straightening. There are more advanced options for what amounts to straightening in 3D that use one of Lightroom's other tools, but I'll tackle that in another post.