Perspective distortion can be corrected in PTLens. For example, when photographing a tall building you can tilt the camera up to include the building and correct perspective distortion during post processing in PTLens. It can also be corrected while taking the picture by shifting the camera lens independent of the sensor. Keep the camera's sensor parallel to the building and shift the lens vertically to minimize perspective distortion.
It is simple to shift a lens vertically on a view camera. Just turn a few knobs and shift the lens board. For 35mm cameras special lenses have been built that include a shifting mechanism. Due to their complex movements these lenses are often expensive. However professional architectural photographers often prefer their quality over perspective correction during post processing. Popular shift lenses for the 35mm format include Canon's TS-E series, the Olympus Zuiko shift lenses, and the PC-Nikkor shift lenses.
Although shift lenses correct perspective distortion, they can also exhibit barrel distortion, vignetting, and chromatic aberration. Corrections for these anomalies are done radially from the center of the image and only apply when the lens is not shifted. To correct images taken when the lens is shifted an alternative strategy is used.
The following is a composite of 12 images taken with Canon's TS-E 24mm f/3.5 lens shifted to its extremes in all directions. The composite image was calibrated and the correction for complex distortion is seen below.
To correct a single image resize the canvas in Photoshop to match the field of view of the composite correction in the PTLens database. Then move the image within the canvas to correspond to the amount of lens shift and correct the result with PTLens. After correction crop to remove excess canvas. The process can be done in a few mouse clicks utilizing a Photoshop action included in the PTLens distribution.
The following image was taken with the Samyang 24mm tilt-shift lens with the lens shifted to the left. The Landscape version was selected in the Action palette of Photoshop and the Play button pressed..
The action pauses and directs you to reposition the image within the expanded canvas.
Using the Move tool the image was positioned to the left since the lens was shifted to the left.
Pressing the Play button to continue, the action invokes PTLens.
After choosing the OK button in PTLens the final results are displayed in Photoshop.
For further information open PTLens and choose Help > PTLens Help > Advanced > Shift Lenses.