Tilting the easel

Is mainly used for correcting those converging verticals in pictures of buildings. The reason you get sloping upright lines is because the camera has not been held vertical to the view.  View cameras (large format) are the only cameras that can keep the film plane vertical and look slightly upwards at the same time with out this distortion. You can buy a shift lens which has the same ability or do it in Photoshop.

When enlarging you negative you can correct this by using a tiling easel.  Simply lift the easel by the appropriate amount and rest it on a block. Remember that you need to lift the easel on the side that has the converging lines. When doing this you should set the smallest possible aperture providing the greatest depth of field. When making the enlargement, part of the picture may be lost, it is just a case of cropping the picture to make it square. After processing the exposure may be different between the top and bottom this can be corrected by dodging to even up the overall exposure.

Related posts:

Scheimpflug's rule.