The most reliable reference for straightening a tilted photo is a sea horizon line. In the absence of a natural horizon, you can often use either horizontal or vertical architectural features—or structural elements such as signposts and flag poles—as long as they are not pictured in perspective or too obviously affected by lens distortion.
The Crop tool’s grid overlays can guide you in framing a balanced composition. The Rule Of Thirds overlay divides the image horizontally and vertically into three equal parts; according to this rule, objects look more balanced when aligned with the lines and intersections of the grid.
Use the Mark For Protection brush to define areas in the image that you wish to protect from a scaling operation. Use the Mark For Removal brush to define any areas that you want removed from the image; the Recompose tool will cut those areas before compressing others. Each of the Recompose brushes has an associated eraser.
The History panel lists every action performed in chronological order. To restore the file to an earlier state, select an action higher in the list. Until you make further changes, you can restore the image to a later state by selecting a step lower in the list.
Both healing tools blend pixels from one part of an image into another. Although the Spot Healing Brush tool enables you to remove blemishes more quickly than is possible with the Healing Brush, the Healing Brush can be customized and enables you to specify the source reference area, giving you more control.