See this little button? It's called the time saver button. Actually, it's called the “Ruler Tool” in Photoshop — but it's a time saver nonetheless. You can use this tool to automatically calculate the right amount of rotation to get your horizon straight. Not only does it calculate the value, it will place that value into the “Arbitrary Rotation…” dialog box.
HOW TO ACCESS THE RULER TOOL
- On your tools palette, click and hold on the “Eyedropper Tool” to reveal a drop-down menu that holds the ruler.
- OR press (I) to activate the tool group and (Shift+I) to cycle through the tools in that group.
HOW STRAIGHTEN WITH THE RULER TOOL
- Make sure your ruler is the active tool, as described above.
- Click (and hold) where you want to start your horizon, drag out a line that represents where straight should be, and release the mouse button.
- Adjust your line by dragging the endpoints or the entire line, and repeat until you're satisfied with your new horizon.
- On your menu, go to “Image >> Rotate Canvas >> Arbitrary…”, and the angle will be filled in for you — just hit OK.
- Now crop the image down and recompose, that's it.
This also works with vertical alignments — Photoshop just takes anything over 45 degrees as being intended for vertical. Adobe Camera RAW (ACR) users can bypass all of this by using the built-in straighten tool, which straightens and crops simultaneously. Why this isn't part of Photoshop is beyond me.