Shift lenses allow the skewing of perspective as the camera sees it and are often accompanied with a tilt feature, as seen in the popular tilt-sift lenses. Shift helps with how lines converge or diverge in a scene by moving the lens elements to one side of camera body opening (as most shift lenses can rotate 180 degrees, this shift can be up, down, left, right and several areas in between). Shift lenses are helpful in architectural photography because they allow lines to be kept straight when they tend to want to converge or diverge with distance or the angle of the camera to the subject.
Another use for a shift lens is an aid to stitched or panoramic photos. Shifting a lens in this sense will help reduce the amount of curve normally seen in stitched images taken at wide angle. While a computer can adjust for this curve, I want to show the advantage of not having to ‘fix things in post', even though it can be done quite well by some.
Let me show you an example of a building in downtown Bellevue, Washington. The first two images are taken with a Canon 7D and a Canon TS-E 24mm F3.5 L II (courtesy of BorrowLenses.com) and are each composed of five images stitched together with PhotoMerge in Photoshop CS5. I am including the full image initially so you may see the difference in coverage area. The name of the building has been smudged at request of the property owner (as well as license plates), otherwise the photo is unedited. Click on the photos for a full sized download if you like (Warning: they are 11MB).
Images shot with no shift.
Image shot with a shift up.
Now to take things a bit further and show how shifting perspective in the computer affects the image. The next two images have been cropped to show the same area in the center of the frame and then, using the Perspective adjustment in Photoshop CS5, adjusted so the building lines are straight.
The image with no shift.
Glancing at the smaller 600 pixel high images, can you spot a difference? Let me give you a zoom on the Jeep to the right. (Click on image for an 1800 pixel wide version, not so large)
The Jeep in the non-shifted image.
And the Jeep in the shifted version.
In the version without the shift moved up to help correct perspective, the result is a squishing of objects near the bottom of the frame while compensating for the top in post production. This can be adjusted also with the Transform tool by skewing the image down, thus elongating the image, bringing the cars back to normal size. Yet all of this transforming, while possible, moves further and further away from the original content of the image (in the matter of quality). You may also notice a slight warp to the pillars in the non-shifted version due to perspective change.
The upside of using a shift lens for this type of work is there will be less work later in the computer. Certainly a Photoshop expert can take my photos and manipulate them to near perfection, but that is beside the point. As with most things in life, the best way to proceed to is to get it right the first time if at all possible. Using a shift lens helps remove computer work later on.
The downside to using a tilt-shift lens for this type of work? The price tag. The Canon TS-E 24mm F3.5 L II I used is regularly $2100USD on Amazon.com although, as previously noted, renting such a lens from the likes of BorrowLenses.com (here is that lens listed on their site), or another rental company, can save a bundle if you have a single project or two to complete in a short time frame.