Yesterday I did a try to correct perspective with PS. I shot my wide angle in a quite tricky way (not only pitched to the subject but also rotated) so that I could check how easy is to use PS for perspective correction in extreme cases.
My goal was to achieve with a pitched extreme wide angle exactly the same perspective we would have achieved with a tilt-shift lens shifted keeping the direction of observation perpendicular to the plane the object is contained, just to find out if shifted lenses can be properly substituted by wide angle+software correction (leaving aside quality loss due to interpolation of course).
Please be aware then that I was not looking for the best looking result, just a solution that is totally math consistent with the projection the tilt-shift lens would have achieved when facing the same situation and being shifted.
The result was OK (I insist, leaving aside the loss of quality due to interpolation and the logical need of crop):
I did 3 steps:
1. Rotation correction so the image was aligned with the plane in which the tilt of the wide angle was to be corrected.
2. Use the tool "perspective" to correct perspective in order to get parallel lines where lines should be parallel (sides of the stereo loudspeaker)
3. Correct scale in Y-axis so that proportions in it are correct (I seeked for a perfect circle for the loudspeakers).
Now the bad news: to achieve that was terribly painful:
1. The lens correction tool could have worked fine fine but I could not use it since the maximum vertical perspective correction (100) was not enough for my extreme image.
2. I used then the "perspective" tool, and it worked, but was really difficult to achieve parallel lines since it starts to behave curve when high correction values are applied.
3. The final stage, Y-scaling, could properly be done thanks to the 1x1 proportions of the loudspeakers. If the scene had not such references, how should we ensure we are obtaining the only mathematically correct solution?
4. Any other tool but "perspective" (free transform, distort, scale,...) were simply not valid even in the basic task of obtaining a symmetrical left-to-right horizontal correction or at least I didn't manage to make them work for my purpose.
Questions:
1. What is your procedure in PS to correct perspective? (please bear in mind my goal: obtain mathematically the same projection as would have achieved with a tilt-shift lens pointed 90º to the target and shifted to accomodate the field of view)
2. What other software is good for perspective correction? I am specially interested in a good interpolation achieved if possible in only one step, wich I am sure is conceptually possible since only one parameter is really playing into the game: the angle with respect to the subject in which we tilted the wide angle lens when shooting the scene (assuming we did no camera rotation as I did in my example).
Thanks
